The document summarizes research probing the magnetized turbulence in the Fermi Bubbles through radio observations of Faraday rotation. Key points: - Researchers analyzed archival radio data and detected a signature of the bubbles at the shock boundary to the Galactic halo. - New observations were performed with the JVLA to confirm preliminary findings and investigate shock energetics, providing clues to the bubbles' origin from nuclear Galactic activity. - Analysis of the JVLA data is ongoing to characterize magnetized turbulence and compare results to expectations from an impulsive event versus continuous star formation over 10 million years.

1. Probing Magnetized Turbulence in the Fermi Bubbles
Kelsey Lund1,2, Christopher A. Hales2, Meng Su3
1 University of California San Diego, San Diego, CA: klund@ucsd.edu
2 National Radio Astronomy Observatory, Socorro, NM
3 Hong Kong University, Hong Kong
Fermi-LAT observations have revealed giant, sharply defined gamma-ray structures emanating from the Galactic center known as the Fermi bubbles. They extend ~50
degrees (~8.5 kpc) above and below the plane of the Milky Way. Their origin is uncertain but thought to be related to an energetic event such as accretion onto Sgr A* or
a burst of nuclear star formation. We analyzed archival radio measurements of Faraday rotation toward extragalactic sources and detected a signature of the bubbles at
the shock boundary to the Galactic halo. To confirm these preliminary findings we performed new radio observations with the Karl G. Jansky Very Large Array (JVLA).
We are finalizing analysis of these observations, and investigating the shock energetics of the bubbles and their implications for nuclear Galactic activity.
Abstract
Introduction
The Fermi Bubbles, originally discovered at gamma-
ray wavelengths2, share spatial correspondence with
signatures in microwave, x-ray, and radio
wavelengths. The large structures are suggestive of
past large-scale activities in the center of the Milky
Way.
JVLA Observation
Background
Preliminary Results
Current Status
References
Galactic Longitude (deg)Galactic Longitude (deg)
RMSofRotationMeasure(radm2)
RotationMeasure(radm2)
The origin of the Bubbles is currently unclear..
The evidence of a large-scale, hard spectrum gamma-
ray jet-like structure supports the idea that the
Bubbles were caused by accretion onto Sgr A*. This
would have caused a short but bright AGN phase
within the last 10 Myr.
In contrast, the spatial morphology of the Bubbles
near the Galactic center coincides with that of a star-
forming molecular gas ring. This, together with
analysis of polarized radio emission possibly tied to
the Fermi Bubbles, reveals the possibility of a
continuous star formation driven origin.
By mapping out turbulence
in the Bubbles’ magnetic
fields, we seek to
characterize the shock
energetics of the Bubbles to
infer their origin.
We observed 511 individual polarized extragalactic
radio sources (z~1) with lines of sight along the eastern
side of the northern Bubble. We gathered data over a
single observing period in May of 2016 using the
Jansky Very Large Array (L-band, 1-2 GHz). This
observation gives us roughly double the density of
polarized sources compared to the NVSS data, as well
as increased accuracy in the resulting rotation measures.
Fig. 1: Rotation measures from
NVSS Data
Analysis of the data obtained from the JVLA
observation is currently ongoing. We expect that the
dedicated observations will result in a catalog of
robust rotation measures that can be used to constrain
the Mach number associated with the outflows.
Our resulting catalog of rotation measures will be
used to characterize the magnetized turbulence. We
can then compare our results with the energetics we
would expect from an impulsive event compared
with a continuous process over ~10 Myr.
-Rotation Measure Synthesis:
-Brentjens, de Bruyn, 2005, A&A, 441: 1217-1228
-Fermi Bubble Detection:
-Su, Slatyer, Finkbeiner, 2010, ApJ, 724: 1044-1082
-NVSS Rotation Measure Synthesis:
-Taylor, Stil, Sunstrom, 2009, ApJ, 702: 1230-1236
Image:DurhamUniversity
Fig. 2: RMS of rotation
measures from NVSS Data
NVSS Data
We analyzed archival rotation measures of NVSS data
to plot the rotation measures and their variances near
the edge of the Fermi Bubbles. The values in both bins
are shown in equal number bins and correspond to a
rotation measure density of ~1 deg-2on the sky. NVSS
data was obtained at 1.4 GHz.
Rotation measures and their variances shown in
figures 1 and 2, respectively, both show a sharp jump
in their values at spatial coordinates corresponding to
the edge of the bubble.
We seek to use data obtained from our JVLA
observation to tighten constraints on these values with
our denser sampling and increased accuracy.