Talk at MUSE Science Meeting @ Leibniz-Institut für Astrophysik in Potsdam (AIP) presenting a recent Paper by P. Laursen et al. http://dx.doi.org/10.1088/0004-637X/766/2/124

1. On the (Non-)Enhancement of the Lyα
Equivalent Width by a Multiphase Interstellar
Medium
P. Laursen, F. Duval & G. ¨Ostlin
-
2013 ApJ 766 124
Christian Herenz
MUSE Science Meeting @ Leibniz-Institut f¨ur Astrophysik in Potsdam (AIP)
August 28, 2013

2. Introduction
• Lyα equivalent width
EWLyα =
λLyα
λLyα
F(λ) − F
≈λLyα
cont.
F
≈λLyα
cont.
dλ (1)
depends on galacitic parameters such as IMF and
metallicity ⇒ suggested as a probe for these quantities.
((in)famous EWLyα = 240 ˚A limit for “normal” stellar
populations)
• EWLyα ∼ escape fraction of Lyα photons relative to
continuum photons.
• Reonant nature of Lyα photons = no direct Lyα escape.
F
≈λLyα
cont. prone to dust extinction.

3. Notation
• EWin . . . galaxies intrinsic EW
• EWem . . . emitted EW
• EWobs = (1 + z) · EWem . . . observed EW
Boost: b =
EWem
EWin
Boost does not imply an increase in Lyα luminosity, rather a
reduction in continuum luminosity.

4. The Neufeld Scenario
Neufeld D. 1991, ApJ 370, L85

5. The Neufeld Scenario - Numerically Revisted
• “Brute Force” 3D
Monte-Carlo Lyα radiative
transfer simulations (
MoCaLaTA) of the Neufeld
Scenario on adaptive grid
with cells either cloud or
inter-cloud.
• Cell-Parameters: nH, nd, T
& vbulk
• Galaxy-Parameters: Ncl, rcl
& ncl = Ncl/Vcl ∝ Ncl/r3
gal.
• Related Quantitites: Z, fc
Typical Model: 103
− 105
clouds,
each with 102
− 103
cells

6. Fiducial Model
Fiducial Model:
Ncl = 6500,
rgal = 5 kpc,
nH = 1 cm−3,
Tcl = 104 K (WNM),
later TICM = 106 K (HIM)
Vary one Parameter at a time,
“realism” of those parameters is
considered later.

7. Fiducial Model Examined - I
fc = nclrgal.σcl =
3
4
Ncl
rcl
rgal.
2
τd = NHσd
Z
Z0
σd = 4 × 10−22
cm−2
Z0 0.25Z

8. Fiducial Model Examined - II
cloud velocity dispersion terminal outflow expansion velocity

9. Fiducial Model Examined - III
ICM neutral hydrogen density Probability of photon emitted
inside a cloud

10. Towards a Realistic Model
Approach
⇒ Define typical (unusual & unrealistic) ranges of parameters
to be expected in LAEs (using observational constraints, also
from nearby SBGs).
⇒ Randomly sample sets of parameters in these ranges
⇒ Set up model “galaxies” spanned in this multi-dimensional
parameter space P & run high-res RT
⇒ Likelihood map for b.

13. Discussion
• Neufeld Scenario allows for significant b only under very
special circumstances?
• Reports of “unusual” large Lyα EW do exist - how to
explain.
• Measurment Errors & Missinterpretations.
• Stellar Populations.
• Delayed escape of Lyα → (homogenous dust free
rgal. = 8kpc → t
Lyα
esc. ≈ 100 kyr)
• AGN activity
• Viewing Angle
• Cooling Radiation
• Star Formation Stochasticity (only becomes relevant
SFR 1 M yr−1
)
• Inhomogenous Escape (only if galaxies are resolved)

14. Summary & Conclusion
• Neufeld Scenario can be reproduced in RT simulations.
• However, as soon as more realistic “ingredients” are
introduced (random motions of clouds, expansion flows,
nHI,ICM ...) EWLyα boosts are unlikely in this scenario.
However, if
• Z Z
• nHI,cl much larger than typical
• Most Lyα photons launched in ICM (i.e. at edges of clouds)
• (almost) no “outflows” & no random motions
significant boosts could be reached.
• These explanations are deemed less likely than other
scenarios for observed high Lyα EWs.