Probing the Southern Fermi Bubble in Ultraviolet Absorption using
  Distant AGNs
  The Fermi Bubbles are two giant gamma-ray emitting lobes extending
55$^{\circ}$ above and below the Galactic Center. While the Northern Bubble has
been extensively studied in ultraviolet (UV) absorption, little is known about
the gas kinematics of the southern Bubble. We use UV absorption-line spectra
from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to
probe the southern Fermi Bubble using a sample of 17 background AGN projected
behind or near the Bubble. We measure the incidence of high-velocity clouds
(HVC), finding that four out of six sightlines passing through the Bubble show
HVC absorption, versus six out of eleven passing outside. We find strong
evidence that the maximum absolute LSR velocity of the HVC components decreases
as a function of galactic latitude within the Bubble, for both blueshifted and
redshifted components, as expected for a decelerating outflow. We explore
whether the column-density ratios SiIV/SiIII, SiIV/SiII and SiIII/SiII
correlate with the absolute galactic latitude within the Bubble. These results
demonstrate the use of UV absorption-line spectroscopy to characterize the
kinematics and ionization conditions of embedded clouds in the Galactic Center
outflow.
