Mapping Outflowing Gas in the Fermi Bubbles: a UV Absorption Survey of
  the Galactic Nuclear Wind
  Using new ultraviolet (UV) spectra of five background quasars from the Cosmic
Origins Spectrograph on the Hubble Space Telescope, we analyze the low-latitude
(|b|=20-30 degree) regions of the Fermi Bubbles, the giant gamma-ray emitting
lobes at the Galactic Center. We combine these data with previous UV and atomic
hydrogen (HI) datasets to build a comprehensive picture of the kinematics and
metal column densities of the cool outflowing clouds entrained in the Fermi
Bubbles. We find that the number of UV absorption components per sightline
decreases as a function of increasing latitude, suggesting that the outflowing
clouds become less common with increasing latitude. The Fermi Bubble HI clouds
are accelerated up to b~7 degree, whereas when we model the UV Fermi Bubbles
clouds deprojected flow velocities, we find that they are flat or even
accelerating with distance from the Galactic center. This trend, which holds in
both the northern and southern hemispheres, indicates that the nuclear outflow
accelerates clouds throughout the Fermi Bubbles or has an acceleration phase
followed by a coasting phase. Finally, we note the existence of several
blueshifted high-velocity clouds at latitudes exceeding ~30 degree, whose
velocities cannot be explained by gas clouds confined to the inside of the
gamma-ray defined Fermi Bubbles. These anomalous velocity clouds are likely in
front of the Fermi Bubbles and could be remnants from past nuclear outflows.
Overall, these observations form a valuable set of empirical data on the
properties of cool gas in nuclear winds from star-forming galaxies.
