tw1979_final_report_ref_correct

Astrophysics Group, School of Physics, University of Bristol c ESO 2015
May 13, 2015
Hunting for satellites of the most massive galaxy clusters
Thomas Wigg
H. H. Wills Physics Laboratory, University of Bristol, Tyndall Ave, Bristol BS8 1TL, UK.
e-mail: tw1979@my.bristol.ac.uk
ABSTRACT
Galaxy clusters accrete the majority of their mass through mergers with comparable mass satellite clusters, but these
lower mass satellites are often dicult to identify from their X-ray signal alone, especially at higher redshifts. Satellite
clusters were searched for in the vicinity of a sample of 11 X-ray detected clusters from the XMM-XXL cluster survey in
the redshift range 0.5 z 0.9 found in the overlap region of the VIPERS and CFHTLS-W1 galaxy catalogues. Two
selection techniques were used to create subsets of galaxies from the two catalogues likely to be part of the conrmed
clusters and any surrounding structure, and potential satellites were identied by looking for overdensities in the
combination of these selections. Sources from the XXL catalogue thought to originate from the satellites were selected
and mass estimates calculated based on their X-ray ux. The validity of any identied systems was then investigated
through comparison with similar systems from the Millennium Simulations and an additional mass estimate calculated
based on the stellar ux of each satellite and its more massive companion. Using light cones from the Millennium
Simulations the occurrence rate of cluster systems detected with apparent satellites and the number of these associated
satellites around each cluster were compared to expected rates predicted by the computer models. One conrmed
and ve potential satellites around four of the XXL clusters were identied, all of which have estimated masses and
separations from their companion cluster deemed reasonable. The rate of clusters with companion satellites and the
number of satellites per cluster were both greater in simulations than real observations but this was attributed to
limitations of the catalogues used, issues identifying weaker galactic overdensities and ambiguity in selecting satellites
in the simulations. No single piece of evidence suggests the satellite candidates found are not true clusters, but follow-up
observations are required to conrm their presence.
Key words. Galaxies: clusters: general - galaxies: clusters: intracluster medium - galaxies: distances and redshifts -
X-rays: galaxies: clusters - cosmology: dark matter.
1. Introduction
Only in recent years have attempts to reconcile observations
of the universe with a single model for the architecture of
the universe, and the growth of structure within it, been
successful. This is termed the concordance model and re-
quires a at geometry of the observable universe, implying
that the total energy density is close to the critical density
required for the universe to become closed. As it is under-
stood today, the universe is composed mainly of two domi-
nant components: non-baryonic dark matter and a form of
dark energy, where the gravity of the former is responsible
for structure formation and the latter is responsible for the
accelerated expansion of the universe. The mean density of
baryonic matter corresponds to around only 15% of the to-
tal matter in the universe (Voit 2005) and this matter is
only observable because it has been drawn into the deep
potential wells created by concentrations of dark matter,
where a small fraction has condensed into stars and galax-
ies.
Clusters of galaxies are important in testing the under-
lying cosmological model, as according to the concordance
model they are the largest and most recent gravitationally-
relaxed objects to form (as structure grows hierarchically)
and they trace the large-scale distribution of matter in the
universe. Structure formation is driven by gravity and in
the early universe regions where a uctuation caused the
density to slightly exceed the mean density became gravi-
tationally bound. These regions eventually decoupled from
expansion and collapsed upon themselves, entering a state
of virial equilibrium where the mean speeds of the compo-
nent particles are approximately half of the escape veloc-
ity (Voit 2005). Density perturbations in the concordance
model have greater amplitudes on smaller length scales and
due to this small, sub-galactic objects decoupled rst, col-
lapsed and virialised. The scale of the virialisation increased
until clusters of galaxies, with masses between 1013
and 1015
times that of the Sun (1013
M −1015
M ) were formed: this
is termed hierarchical clustering. Press and Schechter de-
veloped an analytic formalism for this process of structure
formation in their landmark 1974 paper (Press Schechter
1974).
With improvements to equipment over the past few
decades, the catalogue of known clusters is continuously
increasing and with it more information is being garnered
from observations of clusters than ever before. This, coupled
with recent advances in computational technology, means
the ability to develop models via simulations and semi-
analytic techniques and subsequently test said models with
observations has become an imperative reality.
However, technical limitations still pose an issue in clus-
ter detection: X-ray cluster surveys suer from inherently
Page 1 of 17

low count rates and poor signal-to-noise, meaning detec-
tion of lower mass clusters becomes increasingly dicult
at greater redshifts, whilst cluster identication from opti-
cal galaxy surveys suers from signicant projection eects.
Clusters accrete the majority of their mass through merg-
ers with comparable mass satellite clusters which are drawn
into the potential well of their more massive companion
(Lidman et al. 2013). Looking back into the early universe,
when few clusters were fully-formed, many clusters should
be present with satellites in their vicinity, but due to the
limitations of current observatories it is rarely possible to
identify these lower mass satellite clusters around the more
massive clusters detected on the sky; this issue provides the
motivation for this project.
Using photometric techniques to select galaxies from
the CFHTLS-W1 catalogue likely to belong to the larger-
scale structure around detected clusters from the XMM-
XXL cluster survey and using redshift information from
the spectroscopic VIPERS galaxy catalogue, overdensities
in the number of these galaxies in the vicinity of massive
clusters are identied, indicative of the satellites one ex-
pects to see. Potential X-ray sources from the full XXL
catalogue thought to originate from the satellites are then
identied and from this mass estimates of the potential clus-
ters are calculated. A variety of techniques are then used to
check whether the cluster systems identied seem reason-
able, notably utilising data from the Millennium Simula-
tions to investigate satellite characteristics and occurrence
rates.
This paper is split into four further sections: Section 2
covers the observable properties of clusters (Ÿ2.1) and de-
scribes the processes by which they grow (Ÿ2.2), along with
a collation of cluster catalogues and growth curves produced
using the Millennium Simulations which comprises the rst
work of this paper (Ÿ2.3). Section 3 describes the hunt for
the satellites of massive clusters conducted by making selec-
tions of structure in spectroscopic (Ÿ3.1) and photometric
(Ÿ3.2) galaxy catalogues and the combination of these tech-
niques to identify overdensities in these galaxies indicative
of clustering (Ÿ3.3). Section 3 continues with attempts to
identify X-ray signals from the potential satellites and from
these make an estimate of their mass (Ÿ3.4) and concludes
with eorts to determine the validity of the potential cluster
systems discovered using the Millennium Simulations and
comparisons between the masses calculated from the X-ray
ux of potential cluster sources and estimated from the ra-
tio of stellar mass between the satellites and their massive
companion (Ÿ3.5). Section 4 discusses the sources of pos-
sible uncertainty (Ÿ4.1), the signicance of the discovered
systems (Ÿ4.2) and the future of satellite detection with ref-
erence to the scheduled eROSITA observatory (Ÿ4.3), with
section 5 summarising the project.
Throughout this paper, a at cold dark matter cosmol-
ogy with ΩΛ = 0.70 , Ωm = 0.30 and H0 = 70km
-1
Mpc
-1
is
assumed.
2. Background and preliminary work
2.1. Observable properties of galaxy clusters
In order to select clusters (or groups) of galaxies from the
observed galaxy distribution, one must dene some selec-
tion criteria. In 1958, Abell selected 1,682 galaxy clusters
from the Palomar Sky Survey (Abell 1958) according to
two selection criteria: the rst put a constraint on the min-
imum density of the cluster and the second required that
the cluster contained a sucient amount of galaxies. These
were termed the compactness and richness criterion respec-
tively. Abell also classied a cluster as regular if its galaxy
distribution was roughly circularly symmetric and irregular
otherwise.
Stars make up only a small fraction of the total bary-
onic mass of a cluster, with substantially more contained
in hot gas which emits in the X-ray and is visible in the
microwave through the Sunyaev-Zel'dovich eect, allowing
cluster detection in these frequency ranges as well as the
optical. The baryonic mass of stars and hot gas in a clus-
ter only makes up approximately one eighth of the total
mass (Allen et al. 2002), which means gravitational lensing
is also an extremely useful probe of massive structures like
clusters.
2.1.1. Optical
Charles Messier and William Herschel (Messier 1781; Her-
schel 1785) were among the rst to identify concentrations
of galaxies using optical observations; they observed galax-
ies in the Virgo and Coma clusters. Optical techniques have
improved over the past two centuries, culminating in the
denitive cluster catalogues of George Abell and collabo-
rators (Abell 1958; Abell et al. 1989). Abell's catalogues
contain most of the known nearby galaxy clusters and are
the basis for much of our modern understanding of clusters.
Optical observations are useful as the cluster luminos-
ity scales with cluster mass, generally adhering to the lumi-
nosity distribution proposed by Schechter (1976), with the
number of galaxies in luminosity range dL about L propor-
tional to L−α
exp(−L/L∗), with α ∼ 1 (e.g. Balogh et al.
2001). This relationship is important as it is often more
useful to compare cluster masses, which dictate many of
the physical processes in the cluster, rather than cluster
luminosities.
Once a cluster has been optically identied, obtaining
the radial velocities vr of the cluster galaxies helps deter-
mine the cluster mass. As the velocity distribution of a re-
laxed cluster is expected to be approximately Gaussian in
velocity space, cluster candidates can be conrmed if their
velocities fall within a chosen number of standard devia-
tions of the mean dispersion, dependent on the stringency
of the cluster member selection. Zwicky (1933, 1937) was
the rst to measure the one-dimensional velocity dispersion
σ1D of a cluster, nding σ1D ∼ 700kms
-1
for the Coma clus-
ter. This provided some of the rst evidence for the presence
of dark matter in the universe, as the inferred mass from
this velocity dispersion along with his estimated cluster ra-
dius required substantially more mass than was observed in
stars.
In his 1937 paper, Zwicky also proposed that cluster
masses could be measured through the gravitational lensing
of background galaxies. The technique did not become prac-
tical for another sixty years but is now one of the primary
methods for measuring cluster mass. Chwolson in 1924 and
Einstein in 1936 were the rst to propose that, if a back-
ground star were precisely aligned with a massive object,
the gravitational eect of this mass would deect the path
of the star's photons resulting in a circular ring of light, cen-
tred on the deector (Chwolson 1924; Einstein 1936). Mea-
suring the weak-lensing distortion of any single galaxy is
Page 2 of 17

Thomas Wigg 2015 | Hunting for satellites of the most massive galaxy clusters
nearly impossible, but the mass of a cluster can be inferred
by looking at the distortion of an entire eld - this technique
is demonstrated in Bartelmann Schneider (1999).
2.1.2. X-ray
With only around one tenth of the universe's baryons re-
siding in stars in galaxies, there is a signicant amount
of matter located in intergalactic space. These inter-
galactic baryons are generally very dicult to observe,
but when compressed by the deep gravitational potential
wells of clusters, the intracluster gas is heated to X-ray
emitting temperatures, releasing photons through thermal
bremsstrahlung. This gas, which corresponds to approxi-
mately 15% of the cluster mass, has a temperature which
correlates to the depth of the potential well and from this
the total mass of the galaxy cluster can be calculated - the
physics of this is described in Sarazin (1988). It was follow-
ing the detection of the ionised iron line FeXXVI by the
Ariel-V satellite (Mitchell et al. 1976) that the usefulness
of X-ray emission of the intracluster medium (ICM) as a
probe of the gravitational potential within the cluster (and
therefore the gravitating mass) was realised. The calcula-
tion for this is presented by Fabricant, Lecar and Gorenstein
(Fabricant et al. 1980).
Galaxy clusters are identied from the X-ray signal of
their ICM through a reduction pipeline - an algorithm se-
lects source candidates based on whether their characteris-
tics are indicative of cluster emission. However, with the in-
herently low count rates and signal-to-noise associated with
cluster detection, at greater distances it becomes increas-
ingly dicult to identify the extended sources of galaxy
cluster ICM amidst a universe of X-ray bright point sources
such as active galactic nuclei (AGN). Cluster X-ray lumi-
nosity scales with mass (Piaretti et al. 2010), leading to
lower mass clusters being biased against at higher redshift,
as they are the rst to fall below either the ux limit or
pipeline conrmation limit for a given survey.
2.1.3. Microwave (Sunyaev-Zel'dovich eect)
The hot intracluster medium can also be observed through
its eect on the cosmic microwave background (CMB).
The CMB itself has a nearly perfect blackbody spectrum
(Mather et al. 1990) and soon after the discovery of this
background radiation it was theorised that the spectrum
would be distorted by Compton upscattering of the CMB
photons by the intergalactic gas (Weymann 1965, 1966).
Sunyaev Zel'dovich (1970, 1972) predicted that the CMB
photons would indeed be upscattered to higher energy by
the ICM and this eect is now known as the Sunyaev-
Zel'dovich (S-Z) eect.
Birkinshaw (1991) demonstrated some of the rst S-Z
detections of clusters, but these were marginal. However,
in the following decade, many clusters were detected with
high signicance (Birkinshaw 1999; Carlstrom et al. 2000).
Several S-Z surveys, including the South Pole Telescope
(SPT, Carlstrom et al. 2009) survey, the Atacama Cos-
mology Telescope (ACT, Fowler et al. 2007), and Planck
(Tauber et al. 2010), are actively ongoing, providing the
rst S-Z-selected cluster catalogues (e.g. Vanderlinde et al.
2010; Menanteau et al. 2010).
Using S-Z techniques to identify clusters is advantageous
as, due to the fact that the observed photons are from the
CMB, they are redshift-independent. However, X-ray ob-
servations of S-Z-detected clusters are still important for
many reasons. The X-ray properties allow a better calibra-
tion of the S-Z signal and yield the calibration of the scaling
relations needed for cosmological studies with S-Z-selected
cluster samples. X-ray observations also allow the testing of
the selection function of S-Z surveys and verication of new
S-Z cluster candidates. Furthermore, they are essential for
statistical analyses of the S-Z data (Piaretti et al. 2010).
2.2. Growth of clusters
Due to the hierarchical nature of structure growth and the
self-similarity of galaxy clusters, growth of structure on
vastly dierent scales occurs in much the same way. Mergers
not only appear to be the dominant channel for mass growth
of a galaxy or cluster's dark matter halo but also the stellar
mass growth, both directly through galaxy-galaxy mergers
and through the accretion of potentially star-forming gas
(Fakhouri et al. 2010; Lidman et al. 2013). Lidman et al.
(2013) dene a major merger as 0.25 µ∗
1, where µ∗
is
the mass ratio between the satellite and its more massive
companion. If the orbital energy is suciently low, close en-
counters between two systems can lead to a merger. How-
ever, galaxies in clusters are unlikely to merge as their en-
counter speed is generally much larger than their internal
velocity dispersion (Mo et al. 2010). There is one important
exception: through dynamical friction, galaxies lose energy
and momentum which causes them to `sink' towards the
centre of the potential well. Provided the dynamical fric-
tion time is suciently short, the galaxy will eventually
reach the cluster centre and merge with the central galaxy
residing there. This process is called galactic cannibalism.
2.2.1. Satellite clusters
According to self-similarity, satellite clusters are bona de
clusters which appear structurally identical to the most
massive clusters they are bound to (Neumann Arnaud
2001), albeit consisting of fewer galaxies and of lower mass.
Due to the nature of merger-based growth, satellite clusters
must be present in the vicinity of some massive clusters,
especially in earlier periods of cosmic history when few, if
any, galaxy clusters had cleared the surrounding universe
of dust and smaller clusters to become fully-formed. Clus-
ter systems in the process of merging have been observed
and detailed in numerous papers (Hagino et al. 2015; Kato
et al. 2015; Storm et al. 2015; Zhang et al. 2015 to list but
a few of the most recent), though little investigation has
been performed into the presence of discrete satellites, still
independent from the massive clusters to which they will
eventually infall.
This is due to the fact that the emission from a lower
mass satellite will likely be considerably smaller than that
observed from its more massive companion. In the case of
the emission from the satellite's ICM, the X-ray ux ob-
served is a function of its mass (Piaretti et al. 2010), mean-
ing that for more distant clusters, where the cluster emis-
sion itself is near the ux limit of the observations which
detect it, the X-ray signal from any satellite clusters will
Page 3 of 17

likely fall below the threshold to be conrmed as clusters
of their own volition.
Detection of these satellites is important, not only to
conrm theoretical models detailing the growth of clusters,
but also to plot the large-scale structure, of which clusters
are intrinsic components in forming the laments and voids
which trace the largest arrangement of matter in the uni-
verse.
2.2.2. Simulations
N-body simulations representing the dark matter haloes
of galaxies and clusters, occasionally combined with semi-
analytic descriptions, have been increasingly used over the
past two decades to investigate the hierarchical forma-
tion of large-scale structure in the universe (e.g. Navarro
et al. 1997; De Lucia Blaizot 2007; McBride et al. 2009;
Fakhouri et al. 2010; Laporte et al. 2013). Only recently,
however, have the theoretical expectations and observations
of cluster growth come into excellent agreement (Laporte
et al. 2013).
Fakhouri et al. (2010) utilise the Millennium (Springel
et al. 2005) and Millennium-II (Boylan-Kolchin et al. 2009)
to construct merger trees of dark matter haloes and quan-
tify their merger rates and mass growth rates from between
z = 0 and z = 15 for over ve orders of magnitude in the
descendant halo mass (1010
M M0 1015
M ). The
Millennium-II simulation has the same number of particles
but 125 times better mass resolution and the new data base
provides 7.5×106
dark matter haloes (each containing more
than 1000 simulation particles) between redshift 0 and 15,
adding to the 11.3 × 106
haloes (between z = 0 and z = 6)
from the Millennium simulation.
To compute the total mass growth rate of a halo of
given mas M0 at time t, Fakhouri et al. (2010) follow the
main branch of its merger tree and set ˙M = (M0 − M1)/t,
where M0 is the descendant mass at time t and M1 is the
mass of its most massive progenitor at time t − ∆t. The
plot showing the mean value of ˙M as a function of z, taken
from Fakhouri et al. (2010), can be seen in Fig. 1. They
nd the mass accretion rates shown in Fig. 1 to be well t
by equations (8) and (9) of McBride et al. (2009) with only
the coecients needing minor adjustments. They nd that
the updated t, shown by the dashed lines in Fig. 1, for the
mean growth rates of haloes of mass M at redshift z can
be written
˙M
mean
= 46.1M yr−1 M
1012M
1.1
×(1 + 1.11z) Ωm(1 + z)3 + ΩΛ (1)
2.3. Collation of cluster catalogues and analyses of cluster
mass growth
The work of Piaretti et al. (2010) sought to homogenise,
to an overdensity of 500
1
, many previous ROSAT All Sky
1
Cluster limits are dicult to dene, so in order to make cluster
properties comparable the region with an edge corresponding to
a certain overdensity, where the density of matter is a given
factor times the critical density, is normally used.
Fig. 1. Fakhouri et al. (2010) - Mean mass accretion rate of
dark matter on to haloes as a function of redshift from the two
Millennium simulations (solid curves). Halo masses ranging from
1010
to 1014
M are plotted. The dashed curves show the
accurate t provided by Equation (1). The right-hand side of
the vertical axis labels the mean accretion rate of baryons, Mb,
assuming a cosmological baryon-to-dark matter ratio of 1/6.
Survey-based (NORAS, REFLEX, BCS, SGP, NEP, MACS
and CIZA) and serendipitous (160SD, 400SD, SHARC,
WARPS and EMSS) cluster catalogues into a single Meta-
Catalogue of X-ray Clusters of galaxies: the MCXC. The
MCXC comprises of 1743 clusters, with redshift z, stan-
dardised 0.1 − 2.4 keV band luminosity L500, total mass
M500 and radius R500 provided. Being the largest catalogue,
all subsequent catalogues were homogenised to be compa-
rable to the MCXC according to Piaretti et al. (2010).
In addition to the clusters contained in the MCXC,
clusters from the XMM-XXL (Pierre et al. 2011), XMM-
LSS (Clerc et al. 2014) and XMM-XCS (Mehrtens et al.
2012) - all X-ray surveys utilising the XMM-Newton tele-
scope - were also added to the catalogue. In order to ho-
mogenise the surveys with the MCXC, it was necessary to
transform the luminosities into the correct (0.1 − 2.4 keV)
band, as the LSS and XXL catalogues were published in the
0.5 − 2.0 keV band and the XCS catalogue in the bolomet-
ric (0.01 − 100 keV) band. To do this, a correction factor,
which is simply the ratio between the uxes in each band,
is required. This factor is X-ray temperature and redshift
dependent, and a table of the correction factors was gen-
erated using the APEC emission spectrum in xspec. An
abundance of 0.3 was used.
As well as the mentioned X-ray clusters, two S-Z-
selected catalogues - the all sky Planck-SZ (Planck Collab-
oration 2013) and 720deg
2
South Pole Telescope SPT-SZ
(Reichardt et al. 2012) - were also included. The Planck
catalogue contains 861 conrmed clusters, with 178 con-
rmed as new clusters, mostly through follow-up observa-
tions whilst the SPT catalogue consists 158 optically or
infrared conrmed clusters, with 117 new discoveries. Both
Page 4 of 17

the Planck and SPT luminosities were already in the correct
band to be included in this catalogue.
To ensure the masses in the four catalogues used are
comparable, equation (2) of Piaretti et al. (2010) was used
to covert the L500 of the XMM-LSS and XMM-XCS to
cluster masses M500. This equation reads
h(z)−7/3 L500
1044ergs−1
= C
M500
3 × 1014M
α
(2)
where log(C) = 0.274, α = 1.64 and h(z) is the ratio of
the Hubble constant at redshift z to its present value, H0
i.e. h(z)2
= Ωm(1 + z)3
+ ΩΛ. Note that this is an example
of an X-ray scaling relation; references with a more exten-
sive derivation (e.g. Giodini et al. 2013) and papers tackling
scaling relations in the optical and microwave bands (Cza-
kon et al. 2014) are readily available.
After considering the work of Fakhouri et al. (2010), it
was decided that obtaining a form of equation (1) which
gives the change in mass ∆M of a given cluster of mass M
between the look-back times t1 and t2, corresponding to a
redshift change of z(t1) to z(t2), would be useful. Before
numerical integration was necessary, the simplest form of
this equation can be written
∆M = −
z(t2)
z(t1)
ΩΛ
Ω0
(1 + 1.11z)
(z + 1)
Ωm(z + 1)3 + ΩΛ
Ω0(z + 1)3 + ΩΛ
dz
(3)
Fig. 2 shows the distribution of the MCXC, XMM-XXL,
XMM-LSS, XMM-XCS, Planck-SZ and SPT-SZ clusters on
a plot of mass against redshift. Equation (3) was numeri-
cally integrated over all M(z) and z using the trapezium
rule to produce contour plot beneath the cluster points on
Fig. 2, where a line of constant colour traces the likely mass
history of a cluster based on the work of Fakhouri et al.
(2010).
It is clear from this that at intermediate redshifts, few
clusters have been detected with masses 1014
M . This
does not correlate to the actual existence of lower mass
clusters however, and is simply a comment on the technical
limitations of current detection techniques. As mentioned,
it is for this reason that is has not yet been possible to
detect many low mass satellites around massive clusters -
a problem discussed in the following section and addressed
in the remainder of this paper.
3. Hunting for satellite clusters
The nature of hierarchical clustering insists that at higher
redshift (z 0.5), where clusters are rarely fully-formed,
more massive clusters be surrounded by lower mass satellite
groups and clusters which will eventually merge with the
host. The statement by Lidman et al. (2013) and Fakhouri
et al. (2010) that major mergers provide the dominant chan-
nel of mass growth of massive clusters means that satel-
lite clusters of comparable (but lower) mass to the host
should be present around clusters in the early universe.
Self-similarity requires these be clusters of their own vo-
lition, with a present ICM and associated X-ray emission.
As discussed earlier, unidentied satellite clusters are in-
trinsically too low a mass to be conrmed from their X-ray
signal alone (see Ÿ2.1.2). In this paper, potential satellites
were identied by searching for overdensities in the number
of observed galaxies around conrmed clusters.
Determining the spatial distribution of objects in the
Universe when observing from our xed position on Earth
is inherently dicult: without some form of additional in-
formation, the three-dimension cosmos exists only as a two-
dimensional projection on our sky. Because of this, some
means of determining or inferring the three-dimensional dis-
tribution of galaxies is necessary to identify clustering. Two
methods for identifying this clustering were used in this in-
vestigation. The rst relies on using the spectroscopically
determined redshifts of a sample of galaxies to plot their
true spatial distribution. The second involves using pho-
tometric data from a dierent sample of galaxies to make
relevant colour cuts about a cluster's red sequence to iso-
late galaxies that were likely formed at the same epoch as
the cluster. Both of these techniques are described in more
detail in the subsequent subsections.
It is advantageous to combine these techniques in or-
der identify satellites which fall below the selection thresh-
old of any one observational technique, and as such the
areas surveyed using each method must necessarily over-
lap. This hugely restricts the available data, as only ar-
eas of sky with deep-eld X-ray, spectroscopic galaxy and
photometric galaxy data are usable. One such overlap be-
tween the the VIMOS Public Extragalactic Redshift Survey
(VIPERS), Canada-France-Hawaii Telescope Legacy Sur-
vey (CFHTLS) and the XXL-North cluster catalogue is
utilised in this paper.
The XXL Extragalactic Survey (Pierre et al. 2011;
Pierre XXL Consortium 2014) is a recent X-ray cluster
survey utilising the XMM-Newton observatory. It contains
336 likely clusters (188 of which are are very good cluster
candidates) with 0 z 2.24 split between two survey ar-
eas. The Northern catalogue utilised in this investigation is
centred on the CFHTLS-W1 area (RA: 2h23m, Dec: -4 deg
30') and covers an area of 25deg
2
. A total of 26,555 X-ray
sources were detected in the Northern eld of the XXL sur-
vey. Galaxy clusters demonstrate the largest X-ray source
extent, or core radius, as the emission from the ICM is dif-
fuse. Likely clusters are classied from this extent and the
corresponding likelihood of that being the true extension,
with a lower limit on both set to dene very good class 1
(C1) cluster candidates and possible class 2 clusters (C2).
A minimum extension of 15 arcseconds is imposed for both
C1 and C2 sources, with lower likelihood limits of 5 and 33
dening the two classes respectively (Pacaud et al. 2006).
The rst VIPERS public data release (PDR-1; Garilli
et al. 2014), comprises 57,204 spectroscopic measurements
of galaxies with iAB 22.5 and 0.5 z 1.2. This in-
vestigation uses the catalogue of 30,523 galaxies from the
incomplete W1 eld (centred at RA: 02h26m00.0s, Dec: -04
deg 30'00), with a surveyed area of 7.932deg
2
at the time
of the rst data release. Objects chosen for spectroscopic
follow-up to be included in VIPERS were selected from the
CFHTLS-Wide catalogue.
CFHTLS (Hudelot et al. 2012) is the combination of
a large fraction (∼50%) of Canada and France's dark
and grey telescope time from mid-2003 to early 2009 and
comprises two components: CFHTLS-Deep and CFHTLS-
Wide, with the latter being comprised of 4 contiguous in-
Page 5 of 17

∆ XMM-XCS ∇ XMM-LSS ❏ XXL ❍ MCXC + SPT-SZ × PCCS-SZ
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
z
1
10
100
1000
M500(z)[1012
solarmasses]
1
10
10
2
103
M0/[1012
solarmasses]
Fig. 2. Plot of mass M500/1012
M against redshift z showing the clusters collated from the MCXC, XMM-XXL, XMM-LSS,
XMM-XCS, Planck-SZ and SPT-SZ catalogues. The contour plot below shows the likely mass accretion of a cluster, with lines of
constant colour representing how the mass M(z) of a cluster of mass M0 today has changed since z = 2.
dependent patches covering a total area of ∼155deg
2
. The
CFHTLS-W1 area selected for this investigation is reduced
to contain the 1,034,605 objects for which 3.6µm and 4.5µm
band magnitudes are available. This is due to the nature of
the photometric cuts made in this investigation, described
in detail in Ÿ3.2.
The VIPERS area is the smallest of the three survey
areas used and consequently is the limiting area for this
project. Within this area, the 11 XXL clusters (of which 6
are C1) with 0.5 z 0.9 were selected for investigation,
with this restriction necessary due the fact that the density
of VIPERS galaxies drops to an unusable level at redshifts
greater than this.
3.1. Isolating structure using spectroscopic redshifts
If the redshifts of a sample of galaxies are known, it al-
lows the unique opportunity of plotting the precise three-
dimensional distribution of these galaxies in space, with a
redshift directly correlating to a spatial distance away from
Earth. This method provides the most direct way of identi-
fying clusters of galaxies, with overdensities in the distribu-
tion resulting from the true grouping of galaxies in space,
rather than being inferred from a selection proxy (as with
the photometric technique used in Ÿ3.2).
For each of the selected clusters, an initial redshift cut
in the VIPERS catalogue of ±0.1 about the cluster red-
shift in a square area of 0.25deg
2
, when possible centred
on the cluster, was made with a view to identify any ex-
tended structure. For clusters where galactic overdensities
were obvious, a manual selection of the structure was made
to produce a subset of galaxies that appear to belong to
the same, larger-scale structure as the conrmed XXL clus-
ter. For clusters where no obvious galactic clustering was
present, a cut in redshift of ±0.02 was made about the clus-
ter - this range was chosen as, for the clusters where galactic
overdensities were identiable in the redshift-RA plane, any
observable structure was contained in this cut.
An example of a selection of obvious structure can be
seen in the spatial distribution of galaxies shown in Fig. 3,
with evident clustering around n0286, a class 1 cluster from
the XXL-N catalogue. Whilst the selection was made man-
ually, the validity of this method is supported by the his-
togram: the selection of galaxies made is contained within a
strong, single peak in the distribution, suggesting that the
majority of the galaxies selected are in fact part of the larger
structure the cluster occupies. Of the 11 clusters examined,
6 were situated in regions of galactic overdensity such that
is was possible to select its likely containing structure.
3.2. Identifying likely clustering using photometric techniques
All rich galaxy clusters contain a population of passively
evolving, early-type elliptical galaxies which show a strong
linear relationship between their colour and magnitude.
This relation, termed the red sequence, demonstrates an in-
credibly low scatter and appears extremely homogeneous,
both within clusters and between them (Bower et al. 1992).
The stellar population which makes up the red sequence
appears to be formed at high redshift (z 2) (Gladders
Page 6 of 17

32.0 32.1 32.2 32.3 32.4
0.65
0.70
0.75
0.80
0.85 XXL cluster (n0286)
Selected subset
Excluded galaxies
z
RA (Deg)
0.66 0.68 0.70 0.72 0.74 0.76 0.78 0.80 0.82 0.84
0
5
10
15
Count
z
Selected subset
Excluded galaxies
Fig. 3. Left: spatial distribution of VIPERS galaxies (blue and orange) in the RA-z plane within a Dec. cut of ±0.25deg about
C1 XXL cluster n0286 (black circle). Orange points show the manually selected subset of galaxies assumed to be part of the
large-scale structure in which n0286 is present. Right: Distribution of VIPERS galaxies in redshift contained within a 0.25deg2
square annulus in RA and Dec. about the position of the C1 XXL cluster n0286. The manually selected subset of galaxies about
the cluster is shown.
Yee 2000), which is to be expected in cold-dark-matter
dominated scenarios of hierarchical structure formation, as
present-day clusters are associated with the most extreme
initial overdensities, which were the rst to collapse.
Other than their homogeneity, there are many other
observational reasons that make the red sequence an at-
tractive target for cluster identication. Elliptical galaxies
can be morphologically selected with a high level of con-
dence due to their core-dominated compact brightness pro-
les (Abraham et al. 1994; Gladders et al. 1998). In addi-
tion to this, elliptical galaxies have been shown to dominate
the bright end of the cluster luminosity function (Sandage
et al. 1985; Barger et al. 1998), meaning in ux limited
surveys they are the most readily observed. Cluster ellip-
ticals are also brighter at greater redshift (Schade et al.
1997; van Dokkum et al. 1998), which is to be expected of
stellar populations which are passively evolving. According
to the morphology-density relation (Dressler et al. 1997),
the radial distribution of elliptical galaxies in regular, cen-
trally concentrated clusters is also more compact than other
morphological types, providing a higher contrast against
the background. Even in irregular clusters with no well de-
ned centre, ellipticals still trace the densest cluster regions
as the morphology-density relation holds true, albeit with
lesser signicance (Dressler et al. 1997).
The Cluster Red Sequence (CRS) method of cluster de-
tection is described in detail by Gladders Yee (2000),
and identies clusters computationally as an overdensity of
galaxies on the sky, which correlates to an overdensity in
the colour-magnitude plane consistent with a red sequence
of early-type galaxies. The CRS method is practically un-
aected by projection eects for two reasons: the rst is
that as cluster ellipticals are the oldest stellar populations
in the universe, they are as red or redder than any other
galaxies at a given redshift. The second utilises the fact
that, with properly chosen optical lters (straddling the
4000Å break), the cluster red sequence has been shown to
be as red or redder than any other galaxies at a given red-
shift and all lower redshifts - a fact which means the CRS
method does not accumulate signicant foreground noise,
with background contamination also being minimised as
structures at higher redshift will generally be most signi-
cant at yet redder colours. Gladders Yee (2000) also note
that a cluster's red sequence can be used as a remarkably
precise redshift indicator, given a coherent enough popula-
tion of early-type galaxies.
The consequences of the previous statements mean that
any clusters that were formed at the same point in cos-
mic history (i.e. at the same redshift) should demonstrate
a unique red sequence, which means that a cluster and any
potential satellite clusters at approximately the same red-
shift should show a very similar red sequence. This is the
motivation for the photometric technique of satellite cluster
identication used in this section of the paper: by identify-
ing the red sequence of a conrmed XXL cluster and reduc-
ing the CFHTLS-W1 catalogue by making a restriction in
colour corresponding to the cluster's red sequence, it should
be possible to identify surrounding structure, despite there
being no direct redshift information available. Gladders
Yee (2000) states that the passively evolving, early-type
galaxies which make up the red sequence are present out to
a redshift of at least 1.3, which means this method is valid
for all clusters chosen for this investigation.
It was thought that the most rigorous way to create a
subset of galaxies from the CFHTLS-W1 catalogue, from
which to attempt to identify a red sequence, would be
achieved by calculating the virial radius of the XXL clus-
ter in question and selecting all galaxies which fall within
the angular region of sky corresponding to this radius. For
the purposes of this investigation, it was assumed that the
virial radius can be approximated to the radius which cor-
responds to an overdensity of 200 times the critical density,
R200 (Kravtsov 2013). In order to calculate R200 of the XXL
clusters, it was rst necessary to convert the given masses
Page 7 of 17

(which are to an overdensity of 500, M500) to an overdensity
of 200 (M200); the details of this conversion can be found in
Hu Kravtsov (2003). Assuming the cluster is spherically
distributed, the equation for the required radius,
R200 =
3M200
800πρc
1
3
(4)
where is ρc is the critical density, can be derived.
The angular separation on the sky corresponding to each
cluster's virial radius was then calculated given the clus-
ter's redshift (Wright 2006), and from this is was possible
to create a subset of galaxies for each cluster which fall
within the projected virial radius. The colour (rAB − iAB)
against i-band magnitude (iAB) was plotted for each of the
galaxies in each subset, but attempts to identify any red se-
quence in this data were futile: there was simply too much
noise. This is due to the fact that selecting all the galax-
ies within a projection of the virial radius on the sky will
include huge numbers of foreground and background galax-
ies, with largely dierent colours and magnitudes, which
will pollute the true red sequence of the cluster. Progress
was made by utilising the earlier statement that, according
to the morphology-density relation, the early-type galaxies
responsible for the red sequence occupy the densest clus-
ter region(s). Assuming the clusters we are investigating
are centrally concentrated, the early-type galaxies should
inhabit the central core of the cluster. Using this informa-
tion, a circular region corresponding to a projection of 30%
of the virial radius of the cluster on the sky was instead
used to select the subset of galaxies from which attempts
to identify the the red sequence would be made, under the
assumption that this would greatly increase the proportion
of galaxies in each selection which are true cluster ellipti-
cals.
This selection technique proved much more eective,
with possible red sequences identied for 8 of the 11 clus-
ters. To make a selection of galaxies using the identied red
sequence, a cut of (rAB −iAB)RS ±0.1 about the estimated
centre of the red sequence was taken, and all galaxies out-
side of this colour cut excluded. It was considered that this
method of selection assumes a perfectly horizontal red se-
quence, but this was deemed acceptable for two reasons.
Firstly, in Gladders Yee (2000) it is shown that the slope
of the red sequence for sequence colours of rAB −iAB 1.5
is very shallow and as all the clusters for which red se-
quences were identied had sequence colours which fall be-
low this value, a horizontal approximation was considered
valid. The second is a comment on the quality of the data
- because of the inevitable projection eects suered even
with a reduced selection annulus, the red sequences still re-
mained tricky to identify and any attempts to determine
the slope of the sequence were unsuccessful.
Fig. 4 shows the galaxies selected from within the pro-
jection of 30% of the virial radius of cluster n0286. Note that
a magnitude cut of iAB 23 has been made to remove any
faint background galaxies that are unlikely to be part of
the cluster or its surrounding structure. The red sequence
can be identied with some condence as a line of higher
density in the colour-magnitude plane, with a restriction
in colour containing the red sequence shown. Fig. 5 shows
this selection in context with the other galaxies contained
in the 0.25deg
2
area surrounding n0286. The nal selection
of galaxies made using a magnitude cut of iAB 23 and
restriction in colour of 1.25 rAB − iAB 1.45 is shown
and it is evident that this technique signicantly reduces
the population of galaxies being investigated.
19.5 20.0 20.5 21.0 21.5 22.0 22.5 23.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
RS selection
Galaxies within 30% VR
r-i
i
Fig. 4. Galaxies from the CFHTLS-W1 catalogue selected from
within the projection of 30% of the virial radius of XXL cluster
n0286 on the sky, with a magnitude cut of iAB 23 performed.
Orange points show the selected red sequence, contained in a
cut of 1.25 rAB − iAB 1.45.
16 18 20 22 24 26
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0.25deg
2
galaxies
Galaxies in 30% VR
RS members
Colour/mag. cut
r-i
i
Fig. 5. Galaxies from the CFHTLS-W1 catalogue in a 0.25deg2
area around XXL cluster n0286. Dark grey points show a subset
of these galaxies in a selection with a magnitude restriction of
iAB 23 and colour restriction of 1.25 rAB −iAB 1.45. The
selection of galaxies made from within the projection of 30% of
the virial radius of n0286 on the sky, along with the identied
red sequence both shown in Fig. 4, are also displayed.
As mentioned, by making a cut in colour about a clus-
ter's red sequence, it was hoped that it would be possible to
isolate not only the population of early-type galaxies from
within the cluster, but also any populations of passively
evolving ellipticals contained in the cores of nearby cluster
structures, as these must have formed at the same time as
the cluster's red sequence members.
Page 8 of 17

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.0
0.5
1.0
1.5
2.0
1.2 1.4
0.0
0.5
0.25deg
2
galaxies
RS members
Double colour/mag. cut
3.6m-4.5m
r - i
Fig. 6. Galaxies from the CFHTLS-W1 catalogue in a 0.25deg2
area around XXL cluster n0286. Dark grey points show a subset
of these galaxies in a selection with a magnitude restriction of
iAB 23 and a double restriction in colour of 1.25 rAB −
iAB 1.45 and −0.2 3.6µmAB − 4.5µmAB 0.2. The subset
of galaxies selected to be the cluster's red sequence is also shown.
The area of interest is shown for clarity.
However, it is likely that a number of foreground and
background eld galaxies, which happen to have colours
within the relatively broad range of (rAB−iAB)RS ±0.1, will
enter the selection and pollute the data. To further isolate
galaxies which are part of the cluster and any surrounding
structure, an additional cut in colour in the 3.6µm - 4.5µm
band was made to isolate galaxies with a similar spectral
energy distribution as the red sequence members.
Fig. 6 shows how this selection was performed by plot-
ting the colour of the identied red sequence galaxies in the
3.6µm - 4.5µm band against the colour in the rAB − iAB
band and identifying any corresponding grouping in the for-
mer; a range of 0.4 in the 3.6µm - 4.5µm band was used to
make a second restriction in colour. This range often did
not include all galaxies from the original selection, which
suggests those galaxies are not truly part of the cluster's
population of early-type galaxies as the red sequence should
be a ubiquitous presence in all colour bands (Gladders
Yee 2000). The 0.4 cut was chosen as it was the range over
which any obvious grouping in the 3.6µm - 4.5µm band ex-
isted. Whilst this is a more generous restriction than the
one used in the original red sequence selection, this was
deemed necessary as identifying any sort of central value
for the sequence in the second band was impossible with
such a limited number of galaxies. As such, with a smaller
band, it is possible that true sequence galaxies may be ex-
cluded, reducing the quality of the data. It is clear from Fig.
6 however, the eectiveness of the double colour restriction
in reducing the number of galaxies being investigated.
Applying this selection method to all clusters for which
a red sequence was identiable produced a subset of galax-
ies within a 0.25deg
2
area around each cluster which have
a similar spectral energy distribution to those selected as
early-type galaxies from the XXL cluster. The advantages
of this selection are twofold for identifying satellite clusters:
in the case of passively evolving galaxies such as these, they
were very likely to have formed at the same cosmic epoch
and therefore exist at the same redshift as the cluster and,
as is the nature of galaxies of this morphological type, they
are likely to inhabit the central regions of nearby clusters.
The validity of this method was supported by the fact that,
for all clusters for which a red sequence was identiable, a
strong overdensity in the number of galaxies in each selec-
tion was present at the locations of the clusters in the XXL
catalogue, dened by their respective X-ray sources, sug-
gesting that the restrictions imposed are in fact selecting
galaxies which are part of the cluster structure.
3.3. Combining spectroscopic and photometric cuts to
identify cluster systems
Fig. 7 shows how the combination of both subsets of galax-
ies from the VIPERS (Ÿ3.1) and CFHTLS-W1 (Ÿ3.2) cata-
logues can be combined to plot the number density of galax-
ies as a two-dimensional histogram of the sky to identify
any clustering. Note that to avoid any double counts re-
sulting from galaxies contained in both the CFHTLS-W1
and VIPERS catalogues, a matching algorithm was used
to remove any duplicates. For the scale of clustering neces-
sary to be gravitationally bound and considered a satellite
cluster of its own volition, clustering in the subsets selected
using both methods would likely be evident, and for this
reason combining the two selection methods is advanta-
geous: any clustering found in both selections will show as
a strong overdensity, whilst any clustering in an individual
catalogue not present in the other, which is therefore un-
likely to correspond to a massive cluster, will not appear so
evidently. It is worth noting that, as there is no direct red-
shift information available for the CFHTLS-W1 catalogue,
any clustering has been inferred by proxy and condence
in its existence must be cautious. The combination of both
techniques has the additional advantage that, should clus-
tering in the VIPERS catalogue be present at a location
of clustering in the CFHTLS-W1 galaxies, this adds cre-
dence to assumption that the clustering is present and at
the redshift of the XXL cluster being investigated.
As is to be expected, for all of the clusters for which a
red sequence was identiable in the CFHTLS-W1 galaxies
and apparent structure selectable in the VIPERS catalogue,
strong overdensities in the number of galaxies were present
at the locations of the clusters from the XXL catalogue.
Fig. 8 shows the four of these clusters which also demon-
strated overdensities in the number of galaxies on the sky
in the proximity of, but separate from, the XXL cluster
location following the selections made in accordance with
Ÿ3.1 and Ÿ3.2. These nearby overdensities are indicative of
clustering, but two questions need to be answered before
they can be dened as bone de satellites of the four C1
XXL clusters shown. The rst asks whether the overdensi-
ties correspond to a true clustering of galaxies in space or
whether the grouping on the sky is a coincidental result of
the galaxy selections made. The second asks whether the
characteristics of the identied cluster are reasonable and
in accordance with current theoretical understanding of the
growth of clusters. Both of these questions are discussed in
detail in subsequent subsections.
Page 9 of 17

31.9 32.0 32.1 32.2 32.3 32.4
-4.7
-4.6
-4.5
-4.4
-4.3
CFHTLS-W1 galaxies
VIPERS galaxies
Cluster (n0286)
Dec(Deg)
RA (Deg)
31.9 32.0 32.1 32.2 32.3 32.4
-4.7
-4.6
-4.5
-4.4
-4.3
RA (deg)
Dec(deg)
0
1
2
3
4
5
6
7
8
9
10
Count
Fig. 7. Left: Distribution of galaxies on the sky in a 0.25deg2
area around class 1 XXL cluster, n0286 from the CFHTLS-W1
and VIPERS catalogues following selections made according to Ÿ3.2 and Ÿ3.1 respectively. CFHTLS-W1 galaxies are subject to a
magnitude restriction of iAB 23 and a double restriction in colour of 1.25 rAB −iAB 1.45 and −0.2 3.6µmAB −4.5µmAB
0.2. The subset of VIPERS galaxies was constructed using a manual selection of structure. Right: Number density on the sky of
the selected galaxies using a square bin of 0.025deg.
3.4. Identifying potential satellite cluster X-ray sources
As discussed earlier, self-similarity dictates that the satellite
clusters this report attempts to identify must be composed
in structurally the same way as the massive XXL clusters
in question. This means ∼15% of their mass must be con-
tained in the hot, X-ray emitting intracluster medium. As
previously explained, likely clusters were conrmed from
the XXL source catalog based on a lower limit exclusion
in both source extension and extension likelihood and the
most massive clusters in the redshift range we are investi-
gating have source characteristics such that they are iden-
tied as class 1 XXL clusters. Identifying clusters in this
way has its limitations however, as in order for a cluster
source to be extended the cluster must not only have a
large physical size but also a signal well above ux limit
for the observations. With both radius and X-ray ux di-
rectly correlated to the mass of the cluster, it is clear that
lower mass clusters - such as the satellites of massive clus-
ters being searched for in this investigation - will unlikely
demonstrate X-ray signals good enough to be conrmed as
clusters from X-ray observations alone. X-ray sources from
the ICM of the most massive satellites should still be de-
tectable though, and with the locations of potential satel-
lites determined from the overdensities observed in Ÿ3.3, it
should be possible to identify any X-ray sources in the vicin-
ity of these galactic overdensities with a view to estimate
the mass of the satellites from their X-ray ux.
By plotting the full XXL X-ray source catalogue in
the RA-Dec plane, it was possible to identify sources in
the vicinity of the potential satellites for three of the four
clusters around which strong galactic overdensities were
present. For each cluster, a selection of any sources in, or
very close to, the identied potential satellites were taken.
The extent of the selected sources in arcseconds was plot-
ted against their respective extension likelihoods and the
sources with properties most resembling clusters (those
with the greatest extent and extension likelihood) were se-
lected as the most promising candidates for ICM signal.
Fig. 9 shows the position of these sources relative to the C1
and C2 XXL clusters, along with all the sources in the XXL
catalogue. In the case of the apparently diuse satellite near
cluster n0286, three potential sources were identied. It can
be suggested from inspection of Fig. 7 that the overdensity
near cluster n0286 can be split into at least two indepen-
dent potential satellites. X-ray sources 1 and 2 are located
in the vicinity of the upper overdensity, whilst source 3 is
located in the lower.
Assuming the sources are located the same distance
away from Earth as the host cluster, the cluster redshift
from the XXL catalogue can be used to calculate the lu-
minosity distance to the potential satellites (Wright 2006).
From this the X-ray luminosity L500 can be calculated from
the X-ray ux of the selected sources. Using equation (2) it
is then possible to deduce the mass of the satellite cluster
from which the source is assumed to originate. The calcu-
lated masses M500 of the potential satellite X-ray sources
identied earlier are shown and discussed in Ÿ3.5.1.
3.5. Investigating the validity of potential cluster systems
Now that potential satellite clusters have been identied
from galactic overdensities and the masses of said clusters
estimated from the ux of X-ray sources in their vicinity, it
is important to check whether the systems discovered are
physically valid. Investigations using the Millennium Simu-
lations were used to determine whether the mass and sepa-
ration of the potential satellites is reasonable and to verify
whether the observed occurrence rates of massive satellites
are in line with simulated predictions, whilst a method of
mass estimation using stellar ux ratios was used as a self-
consistency check to determine whether the X-ray sources
are likely to originate from the assumed satellite clusters.
Page 10 of 17

35.6 35.7 35.8 35.9 36.0
-4.6
-4.5
-4.4
-4.3
-4.2
Count
RA (deg)
Dec(deg)
0
2
4
6
8
10
12
14
16
18
d
z=0.6308; d ~ 6.23Mpc
35.4 35.5 35.6 35.7 35.8 35.9
-5.2
-5.1
-5.0
-4.9
-4.8
-4.7
Count
RA (deg)
Dec(deg)
0
1
2
3
4
5
6
7
d
z=0.8327; d ~ 5.04Mpc
31.9 32.0 32.1 32.2 32.3 32.4
-4.7
-4.6
-4.5
-4.4
-4.3
d
RA (deg)
Dec(deg)
0
1
2
3
4
5
6
7
8
9
10
Countz=0.7556; d ~ 3.31Mpc
30.8 30.9 31.0 31.1 31.2
-5.2
-5.1
-5.0
-4.9
-4.8
Count
RA (deg)
Dec(deg)
0
1
2
3
4
5
6
7
8
9
d
z=0.5202; d ~ 4.60Mpc
Fig. 8. Number density on the sky of a selection of galaxies from the CFHTLS-W1 and VIPERS catalogue in a 0.25deg2
area
around four selected XXL clusters, chosen using the techniques described in Ÿ3.1 and Ÿ3.2. Clockwise from top left, the XXL
clusters shown are n0019, n0064, n0260, n0286. The locations of the X-ray sources from the XXL catalogue corresponding to each
cluster are circled in white and the distance d between each cluster and the nearest overdensity are shown for scale, with the value
noted at the top left of each plot. A square bin of 0.025deg was used.
3.5.1. Mass and separation of satellites from the Millennium
Simulations
The Millennium I and Millennium II Simulations are N-
body simulations which plot the movement and growth of
dark matter haloes at discrete redshift snapshots from the
early universe through to the present day (0 z 15).
The masses of the clusters are inferred from their halo mass
and the history of each halo and its constituent parts are
recorded. To check whether the estimated satellite masses
and the separations from their host cluster are reasonable,
the ve nearest mass haloes at the nearest redshift snapshot
(this was within zcluster ± 0.15 in all cases) were taken and
the mass and separation of the most massive satellite cluster
recorded. Tab. 1 summarises these results and compares
them to the observed mass and separation of the satellites
determined in previous sections.
In almost all cases does the estimated satellite mass fall
within the predicted range and in cases where they do not,
the observed characteristics are extremely close to those de-
termined from the Millennium Simulations. In the case of
the separations, all satellites fall well within the discovered
range and within one standard deviation of the mean. Due
to the fact that only the ve nearest mass clusters were
taken from the simulations, the results shown here are by
no means extensive enough to disregard the X-ray sources
for which the satellite mass does not fall within the deter-
mined range; in the case of n0064, the estimated satellite
mass is only slightly greater than the upper limit on the
range from the simulations. Considering n0286, compari-
son to the range deduced from the simulations must be
Page 11 of 17

Table 1. Comparison of observationally determined satellite (sat.) masses and separations (sep.) with those determined from the
Millennium Simulations. Potential satellite masses were estimated from the X-ray ux of sources assumed, but not conrmed, to
have originated from the satellite's ICM. For cluster n0286, the satellite masses from top to bottom were calculated from X-ray
sources 1, 2 and 3 respectively.
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
XXL sources
C2 clusters
C1 clusters
Satellite sources
n0064
n0260
n0286
1
n0286
2
n0286
3
Extension Likelihood
Extent(arcsec)
Fig. 9. X-ray sources from the full XXL catalogue with sources
corresponding to class 1 (blue) and class 2 (red) clusters indi-
cated. A solid line at an extent of 15 arcseconds denes a lower
limit for both cluster classes, with a solid line at an extension
likelihood of 5 and dashed line at 33 establish the limits for
C2 and C1 clusters respectively. X-ray sources located in the
vicinity of the galactic overdensities corresponding to potential
satellite clusters identied in Ÿ3.3 are shown.
cautious as we are certainly considering at least the second
most massive satellite and potentially the third, depend-
ing on whether the galactic overdensity corresponds to two
or three discrete satellites. Given more time, a mean and
range of characteristics for the second and third most mas-
sive satellites of the Millennium Simulation clusters would
also be investigated and compared to those implied from
observation.
3.5.2. Estimating the satellite mass from stellar ux
In general, redshift information is not readily available from
X-ray pointings alone, with optical follow-up often needed
to determine the redshift of a cluster with any condence.
Yu et al. (2011), among others, have investigated the pos-
sibility of determining the redshift spectroscopically using
the ubiquitous Fe-line complex in the X-ray spectrum of
a cluster's ICM signal. To reliably determine a redshift in
this manner, higher resolution and longer pointings than
are generally available are necessary. For this reason, no
redshift information is available for the potential satellite
sources discussed earlier, meaning it cannot be condently
asserted that the signal truly originates from the ICM of
the potential satellite. With the additional galaxy uxes
in the 3.6µm and 4.5µm bands that the CFHTLS-W1 cat-
alogue provides, the total galactic ux of the cluster and
satellite in each of the two bands can be determined. Es-
kew et al. (2012) provide the following relation between the
total 3.6µm and 4.5µm uxes in Jy (F3.6 and F4.5 respec-
tively) and the total stellar mass of the cluster M∗,
M∗ = 105.65
F2.85
3.6 F−1.85
4.5
D
0.05
2
M (5)
where D is the luminosity distance to the cluster in Mpc.
Given the quoted masses of the conrmed clusters from
the XXL catalogue and by assuming that the ratio of stel-
lar mass to total mass of a cluster is constant, it is possible
to reach a mass estimate of the satellite based on galactic
emission alone. By comparing this mass estimate with that
deduced from the identied X-ray sources provides an ef-
fective, albeit far from rigorous, consistency check for the
validity of the calculated X-ray mass of the satellites: if the
ratio of the inferred X-ray mass to 3.6µm/4.5µm mass is
far from unity, this would suggest the satellite's ICM is not
responsible for the X-ray signal.
To determine the galaxies to include within the selection
for the clusters and their satellites, it seemed most appro-
Page 12 of 17

Table 2. Comparison of the satellite masses estimated from the
X-ray ux of the XXL sources thought to originate in the satel-
lite and the masses estimated from a calculation of their stellar
mass based on their 3.6µm and 4.5µm ux - the ratio shown is
between the latter and former mass estimates respectively. For
cluster n0286, the satellite masses from top to bottom corre-
spond to X-ray sources 1, 2 and 3 respectively.
priate to use the virial radii. The virial radii of the satel-
lites were calculated according to the method described in
Ÿ3.2 using the masses inferred from the potential satellite
X-ray sources; this again serves as an additional check for
self-consistency. Using these calculated radii along with the
virial radii of the XXL clusters determined earlier, a se-
lection of all the galaxies, already subject to the relevant
photometric cuts, inside the virial radius of each cluster and
its satellites was made. In the case of the satellites, the po-
sition of the X-ray source used to calculate its virial radius
was used as the cluster centre. Following this, the total ux
in both the 3.6µm and 4.5µm bands, calculated by summing
the individual ux from each of the galaxies, was used to
determine the stellar mass of the galaxies in the selection
for each cluster and their satellites using equation (5). An
estimate of the total satellite mass, based on the satellite's
and its host cluster's stellar mass was then calculated using
the ratio method described earlier.
Tab. 2 shows the resulting total mass estimates for the
satellites based on the X-ray ux of the sources thought
to originate from the satellite's ICM and the stellar mass
ratios independently, along with a ratio of these values for
comparison.
The fact that the resulting ratios do not fall close to
unity appears unfortunate for the continued assumption
that the satellites' ICM is responsible for the X-ray sources
used to calculate an estimate for their mass. However, due
to the sheer number of points at which error on the mass
estimate based on the stellar mass can be accumulated
throughout the calculations and conversions, a result of this
mass estimate being ∼ 9% to ∼ 321% of the mass calcu-
lated from the X-ray signal may actually be an encouraging
result. Again, this section of the investigation should not
hope to conrm with any certainty that the X-ray sources
selected do come from the satellite cluster, but instead iden-
tify any sources for which the X-ray mass estimate and the
mass calculated from the 3.6µm and 4.5µm uxes dier by
such an extreme amount that the source can be rejected; in
this case none of them appears too unreasonable to exclude
at this point.
3.5.3. Expected observed satellite occurrence rates from the
Millennium Simulations
The previous subsections have been useful in determining
whether the properties of the potential satellite clusters
discovered are reasonable. It is also of interest conrming
whether the occurrence rate of massive clusters and their
satellites detected is to be expected in the universe as it is
currently understood. Again, the Millennium Simulations
provide a unique opportunity to investigate the number of
these massive clusters with satellite companions that are
actually present in a universe modelled to represent ours.
Following the simulation of the growth of dark matter
haloes, twenty-four 1deg
2
light cones representing a view
of the sky from redshift 0 were produced and individual
galaxies were `painted' onto each halo in each cone us-
ing computer models of cluster structure. These provide a
unique opportunity of exploring a simulated universe from
a xed viewpoint, such as observations of the real universe
are made from Earth.
For each of eight light cones chosen at random a redshift
restriction of 0.5 z 0.9 was made and a subset of any
galaxies located in a containing cluster with a central virial
mass of greater than 1014
M was created. This limit was
chosen as the massive clusters chosen from the XXL survey
around which potential satellites were identied were of a
mass of this order of magnitude or greater. Once these most
massive clusters were identied, a further subset of galaxies
with central virial mass greater than 1013
M contained in a
0.25deg
2
square area of `sky' within a redshift cut of ±0.02
about the cluster centre was taken. An area of this size
was chosen as it corresponds directly to the area about
each XXL cluster within which satellites were searched for
whilst this redshift range was selected as any identiable
structure from the spectroscopic selections made in Ÿ3.1
was contained within this cut. A cuto mass of 1013
M
was used as this was the order of the lower limit on the
cluster mass estimated from the X-ray sources thought to
originate from the potential satellites. The RA-Dec plane
for each subset was then plotted and the number of satellite
clusters around each 1014
M cluster were recorded.
Tab. 3 shows a summary of the results from the sample
of eight light cones. This part of the investigation enabled
two important insights: rst a test of the rate of detection of
massive clusters from the XXL survey and second a check of
whether the number of satellites identied in this report is
reasonable. On the rst count, a total of 36 clusters of mass
1014
M were identied, corresponding to a mean number
of 5 massive clusters per square degree, in the stated red-
shift range of 0.5 z 0.9. Compare this to the 11 XXL
clusters in the ∼8deg
2
overlap with the VIPERS region and
it is clear there is a discrepancy between the number pre-
dicted by the simulations and what is actually observed.
This suggests that the cluster catalogue produced by XXL
is not extensive, even for the most massive end of the clus-
ter population, for which their X-ray signal should be most
easily classied. This also supports the assertion that there
are still many unclassied cluster signals in the XXL source
catalogue, which was the assumption used when selecting
potential satellite X-ray signals. On the question of the fre-
quency of satellite systems, it quickly became apparent that
the number of satellites per cluster in the simulations far
exceeded that which we observed on the sky, with a mean
number of 5 satellites per cluster found across the eight
Page 13 of 17

Table 3. The total and average number per square degree of
massive ( 1014
M ) clusters from a sample of eight 1deg2
light
cones produced from the Millennium Simulations. The average
number of satellite ( 1013
M ) clusters observed in a 0.25deg2
area of `sky' about each massive cluster is also shown.
cones. With 1 conrmed (see Ÿ4.2) and 5 likely satellites
identied around 4 of the 11 XXL clusters investigated, the
results from the simulations may seem irreconcilably dier-
ent. However, when identifying potential satellites, only the
greatest overdensity around each XXL cluster was investi-
gated. Looking back to Fig. 8, it is clear there is more than
one area of overdensity present in the vicinity of most of the
clusters, each of which could correspond to an additional
satellite cluster. This highlights an issue in the method and
suggests that it would advantageous to return to each of the
chosen clusters and investigate these further overdensities
in the same way as the potential satellites identied in order
to more eectively compare the rate of satellite occurrence
with observation. This does not, however, account for the
fact that around every one of the 36 massive clusters in the
simulations was there at least one satellite. Of the clusters
for which it was possible to identify a red sequence and com-
bine this selection with the VIPERS subset, of which there
were 6, 4 showed apparent satellites in their vicinity. Whilst
this rate is markedly lower than the simulations, with such
a small sample size of XXL clusters it is dicult to make
any further comment of whether the simulations and ob-
servations truly contradict. Note this section includes the
count of the C2 (n0211) satellite found in the vicinity of C1
cluster n0219, discussed in Ÿ4.2 for more accurate compar-
ison between the observations and simulations.
4. Discussion
4.1. Sources of uncertainty and potential improvements
The nature of this project means that areas of potential er-
ror incursion are numerous. With regard to the galaxy cat-
alogues used, the area of sky surveyed is not continuous for
either catalogue, with the VIPERS region split into many
distinct rectangular regions with signicant gaps between.
This means there are times when combining selections that
an area of interest falls in or near a gap in the VIPERS
range. The sampling rate for neither survey is complete,
with the sampling rate of the VIPERS catalogue uctuat-
ing around a redshift-variable value of ∼ 40% (Guzzo et al.
2014). Both of these factors mean that the view of sky inves-
tigated, whilst still very useful in the search for structure,
is incomplete and as such there is the potential for unseen
systems.
Despite this, the redshift information provided by the
VIPERS catalogue is incredibly precise and provides one
of the most useful tools in the search for cluster sys-
tems: it is not only possible to identify clustering in the
three-dimensional distribution of VIPERS galaxies, but to
conrm the presence of true galactic overdensities in the
CFHTLS-W1 catalogue which are only inferred by selec-
tion.
When making selections in the CFHTLS-W1 catalogue
by locating the red sequence, a horizontal cut in colour was
made. This was discussed earlier, with a comment made on
the diculty of identifying the red sequence, let alone its
slope, when any selection of cluster members was inherently
polluted by eld galaxies. Gladders Yee (2000) use the
CRS (cluster red sequence) algorithm to identify the red se-
quence computationally. It would be interesting to attempt
the identication of the red sequences of the XXL clusters
investigated in this paper in the CFHTLS-W1 catalogue
using the CRS algorithm. If successful, it would provide a
more rigorous method of identication and would allow se-
lections to be made about the slope of the sequence, though
the ratio of sequence galaxies to eld galaxies in the cluster
region may prove problematic.
The nature of binning galaxies when combining the
CFHTLS-W1 and VIPERS selections means that, should
a satellite cluster straddle the border between two or more
bins, it will appear less overdense than if the cluster were
central in a bin, meaning the potential for missed identi-
cation of true satellites was present. Another issue faced
was the fact that x-y binning has the eect of smoothing the
data along those principal axes. This means that identifying
whether larger satellites are truly extended or in fact multi-
ple discrete clusters and locating any lamentary structure
is inherently dicult. The eect of this was minimised by
plotting the number density of galaxies using a variety of
bin sizes and identifying overdensities in each case.
When attempting to identify X-ray sources originating
from the potential satellites, the obvious issue is that with-
out any redshift information about the source, there is no
way of knowing whether it is truly the signal from the clus-
ter's ICM or a chance projection along the line of sight. The
masses estimated from the sources seem encouraging, but
the majority of the sources in the region of sky investigated
are in an X-ray ux range that would produce a reason-
able estimated cluster mass using equation 2. For this rea-
son, calculating an estimate for the mass is more useful in
identifying sources which are obviously too bright to orig-
inate from a cluster and can therefore be excluded. How-
ever, when considering the position of the potential satellite
sources in the extent-extension likelihood plane (see Fig. 9),
their proximity to the conrmation threshold, especially in
the case of the three sources identied for n0286, suggests
they are extended sources and is further evidence in sup-
port of the assumption they are cluster signals. It is worth
noting however, the uncertainty in X-ray-based cluster de-
tection; with such a wide variety of X-ray sources in the
universe, many of which displaying emission similar to that
of a cluster, conrmation through other detection methods
is often required to classify a cluster with condence. The
XXL sources identied as class 2 clusters have a ∼50% pos-
sibility of in fact originating from an AGN (Pierre XXL
Consortium 2014).
The ability to compare the characteristics of the po-
tential satellites with the properties of similar cluster sys-
tems from the Millennium Simulations is advantageous, but
the small sample size means that excluding any anomalous
systems would only have been possible should the satellite
mass estimate and/or separation from its massive compan-
ion have fallen well outside the range recorded from the sim-
Page 14 of 17

ulations; in the case of all systems investigated, the proper-
ties of the potential satellites fall in or very near this range
and are therefore deemed reasonable. Given more time, a
more extensive list of similar systems from the simulations
would have been created such that any anomalous observa-
tions could be more easily identied.
Estimating the satellite mass using the mass ratios cal-
culated from the stellar ux was intended to provide a useful
self-consistency check for the selected potential satellite X-
ray sources and whilst, again, it allows the identication of
any sources which are obviously too bright, inherent aws
in the method mean that error is inccured at almost every
step. As mentioned, the sampling rate for the CFHTLS-W1
catalogue is not complete, and further to this, only galaxies
for which 3.6 and 4.5 micron uxes are recorded were avail-
able. In the case of the ratio method used, this will only
leave the satellite mass estimate unaected if the ratio of
galaxies for which 3.6/4.5 micron data is available to the
total number of galaxies is identical for the satellite and its
massive companion: an assumption which will inevitably
incur a varying amount of error for each of the identied
systems. The same is true when considering the photomet-
ric restrictions made in Ÿ3.1: the ratio is only consistent
provided the proportion of the number and ux of galax-
ies selected between the satellite and conrmed cluster is
indicative of the true ratio of these quantities. The eect
of background counts of eld galaxies projected along the
line of sight when selecting cluster members was also ne-
glected, which will cause an over-estimation of the satellite
mass. It would be more rigorous to determine and subtract
an average background ux from the total stellar ux of
the clusters before calculating their mass. However, with
the other sources of error incurred and the relatively low
background galaxy count, this will be of negligible eect.
When determining the number of massive clusters and
companion satellites from the Millennium Simulation light
cones, deciding clusters to include in each count posed an is-
sue. When using the simulations the information about each
galaxy's containing structure, and the mass of the cluster
of which it is a member, is readily available; this is evi-
dently not the case for real observations. For this reason,
only satellites in the simulations with galaxy densities in
excess of around ve per square arcminute were counted,
corresponding to the lower limit of the bin density seen for
the potential satellites identied. Despite this, this makes
no correction for the fact that the Millennium Simulations
are an extensive galaxy population, not subject to any of
the survey sampling rates or selections made from the cata-
logues in this investigation. This goes some way to explain-
ing the increased number of massive clusters with compan-
ion satellites recorded from the simulations. This nal part
of the investigation would be improved by making appro-
priate corrections for the selection eects intrinsic of real
observations.
4.2. Conrming potential satellites and a comment on
method
This investigation aimed to identify satellite clusters of
more massive, X-ray detected clusters; has this aim been
fullled? The project combined many techniques for dis-
covering satellites and determining the validity of the po-
tential cluster systems identied, none of which alone is
enough to conrm the presence of the satellites discovered.
Many of the satellite properties determined are also subject
to large potential error or operate under bold assumption
and are therefore of use only to exclude obviously anoma-
lous systems. However, with all the information provided
by these techniques, it can be stated that no single piece of
evidence contradicts the claim that 5 satellite clusters have
been discovered around 3 conrmed XXL clusters, and the
combination of all of the determined properties and com-
parison with simulated expectations alludes to the true ex-
istence of the potential systems identied. This is especially
true for cluster n0286, whose three identied satellites all
have potential X-ray sources with extent above and exten-
sion likelihood very close to the XXL cluster conrmation
threshold. In order to conrm with any certainty the pres-
ence of these satellites, longer X-ray pointings or follow-up
observations to determine the velocity dispersions of the
member galaxies are required.
A nal comment on the method of galaxy selection is
provided by the cluster system in Fig. 10. The subset of
CFHTLS-W1 galaxies shown was selected as usual by iden-
tifying the red sequence of the class 1 XXL cluster n0219,
which has a mass of M500 = 12.8 × 1013
M located at red-
shift z = 0.6265. When the corresponding double colour
cut was made, a strong overdensity at the location of class
2 XXL cluster n0211, with a mass of M500 = 7.56×1013
M
at redshift z = 0.6090, a distance of ∼ 6.44Mpc from n0219
was identied. A selection of VIPERS galaxies in redshift
was made with a slightly larger range to encompass both
clusters and their likely surrounding structure, with strong
overdensities apparent at the positions of the two clusters
also. Returning to the Millennium Simulations, identifying
the ve nearest mass haloes to that of n0219 and recording
the mass and separation of the most massive satellite, the
cluster system shown in Fig. 10 is of reasonable properties
to suggest that the two clusters will eventually undergo a
merger. At the very least, it can be said that this system is
exactly of the type this investigation has been attempting
to identify. This supports the fact that the satellite systems
being searched for do exist and that it is the technical lim-
itations of the observations that restrict their discovery -
cluster n0211 happens to have source characteristics such
that it can be identied from the X-ray signal of its ICM
alone. More encouragingly though, it also supports the va-
lidity of the red sequence method, not only to identify the
cluster itself, but also any surrounding satellites formed at
the same cosmic epoch.
4.3. Future
With such a limited area of sky as was available for this
investigation, attempting any sort of statistics regarding
occurrence rates and characteristic properties of satellite
systems is dicult: only 11 XXL clusters in the redshift
range were examined and of these only 3 (4 including
n0219) show evidence of likely companion satellites. The
extended ROentgen Survey with an Imaging Telescope Ar-
ray (eROSITA) is a revolutionary new observatory due to
launch in 2016, designed to probe the entire X-ray sky with
unprecedented spectral and angular resolution out to red-
shifts z 1: mapping in the soft band (0.5-2keV) will be
twenty times more sensitive than the ROSAT all-sky survey
whilst in the hard band (2-10keV) it will provide the rst
ever true imaging survey of the sky at that energy (Merloni
et al. 2012). The mission has many goals, most relevant of
Page 15 of 17

37.0 37.1 37.2 37.3 37.4 37.5
-4.9
-4.8
-4.7
-4.6
-4.5
Count
RA (deg)
Dec(deg)
0
2
4
6
8
10
12
14
16
z
1
=0.6265, z
2
=0.6090; d ~ 6.44Mpc
d
Fig. 10. Number density on the sky of a selection of galaxies
from the CFHTLS-W1 and VIPERS catalogues in a 0.25deg2
area around around XXL clusters n0219 (class 1; left) and n0211
(class 2; right). The selections were made according to Ÿ3.1 and
Ÿ3.2 using n0219 as the reference cluster. The locations of the
X-ray sources from the XXL catalogue corresponding to each
cluster are circled in white and the distance d between the two
clusters is shown for scale, corresponding to a physical separa-
tion of ∼ 6.44Mpc. A square bin of 0.025deg was used.
which is the predicted identication of ∼ 105
new galaxy
clusters.
The all-sky nature of eROSITA's planned survey has
many distinct advantages. With the entire sky surveyed to
a depth comparable to XXL, the geometry of large-scale
structure in the universe will be accurately determined and
with the predicted discovery of so many new clusters, pro-
ducing rigorous statistics about the distribution of matter
and its containing systems will, for the rst time, become a
possible reality. With over 40,000deg
2
of sky mapped (com-
pared to the ∼ 8deg
2
of this investigation), eROSITA will
also nd intrinsically rare objects. With its improved reso-
lution, eROSITA will also be able to better identify cluster
signals from the population of AGN and other X-ray sources
in the universe. For these reasons, eROSITA will allow the
rst real possibility of comparing the large-scale distribu-
tion of matter in the universe with modelled predictions
such as those provided by the Millennium Simulations.
eROSITA should not only detect many systems like that
of n0219/n0211 shown in Fig. 10, but also clusters whose
satellites currently fall below the detection threshold of cur-
rent observations. This will allow some insight into the dis-
tribution of mass around larger clusters and also allow cal-
culations of the percentage of mass contained in observable
cluster satellites to be made. The method of satellite iden-
tication developed in this investigation will not be made
redundant however, as utilising a combination of techniques
such as in this paper will prove valuable in situations when
a single observational technique is insucient to conrm
the presence of a cluster. However, it is a time-consuming
method which requires good coverage in both optical pho-
tometric and spectroscopic galaxy, and deep eld X-ray ob-
servations. With minor improvements made according to
the issues identied, the techniques described in this paper
should still provide an eective means of satellite cluster
identication when X-ray observations alone are insucient
to conrm their existence, but in the case of combining it
with data from the eROSITA observatory, it will likely have
to be restricted to points of interest as applying this tech-
nique to the whole sky is not practically valid.
5. Summary
The project utilised a sample of 11 clusters from the XMM-
XXL cluster survey in the redshift range 0.5 z 0.9
found in the overlap region of the VIPERS and CFHTLS-
W1 galaxy catalogues. Using the spectroscopic redshift in-
formation of the VIPERS catalogue and photometric data
of the CFHTLS-W1 catalogue to create a subset of galaxies
about each cluster likely to be part of the conrmed cluster
or any surrounding undetected clusters, potential satellites
were identied by looking for overdensities in the number
of galaxies when combining these selections. Sources from
the full XXL catalogue thought to originate from any satel-
lites were selected and mass estimates calculated based on
their X-ray ux. The validity of any identied systems was
then investigated through comparison with similar systems
from the Millennium Simulations and an additional mass
estimate calculated based on the stellar ux of each satel-
lite and its more massive companion - the latter also acting
as a check of self-consistency of the satellites' potential X-
ray sources. Finally, using light cones from the Millennium
Simulations the occurrence rate of cluster systems detected
with apparent satellites and the number of these associated
satellites around each cluster was compared to expected
rates predicted by the computer models.
In total, 5 potential satellites around 3 of the XXL clus-
ters in addition to a C2 satellite of a more massive C1
cluster were identied, all of which have masses and sep-
arations from their companion cluster deemed reasonable
following comparison with the simulations. The masses es-
timated from the stellar ux were subject to large errors
and as such provided little use other than to identify any
extremely anomalous properties, of which there appeared to
be none. Finally, the rate of clusters with companion satel-
lites and the number of satellites per cluster both appeared
much greater in the simulations than real observations -
this was attributed to the incomplete sampling rate of the
galaxy surveys used, the diuse nature of satellites in the
simulation and selection eects intrinsic of the techniques
employed. Follow-up observations would be required to con-
rm the presence of the potential satellites, as this was not
possible with the information gathered in this investigation
alone; though no single piece of evidence suggests the satel-
lite candidates found are not true clusters.
Little rigorous statistical insight has been possible with
such a small sample size, however the launch of the all-sky
eROSITA X-ray observatory should provide a more than
sucient population of galaxy clusters to investigate and
allow truly eective comparison between simulation and
observation.
Acknowledgements. I would like to thank my partner, J. Ider
Chitham, for his continuous input to the project, along with Prof
M. Bremer, Dr B. Maughan, Miss K. Husband and Dr M. Taylor for
guidance throughout the duration of the investigation.
Page 16 of 17

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