Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic
  Signature of the Milky Way's Biconical Nuclear Outflow
  Giant lobes of plasma extend 55 degrees above and below the Galactic Center,
glowing in emission from gamma rays (the Fermi Bubbles) to microwaves (the WMAP
haze) and polarized radio waves. We use ultraviolet absorption-line spectra
from the Hubble Space Telescope to constrain the velocity of the outflowing gas
within these regions, targeting the quasar PDS 456 (Galactic coordinates
l,b=10.4, +11.2 degrees). This sightline passes through a clear biconical
structure seen in hard X-ray and gamma-ray emission near the base of the
northern Fermi Bubble. We report two high-velocity metal absorption components,
at v_LSR=-235 and +250 km/s, which cannot be explained by co-rotating gas in
the Galactic disk or halo. Their velocities are suggestive of an origin on the
front and back side of an expanding biconical outflow emanating from the
Galactic Center. We develop simple kinematic biconical outflow models that can
explain these observed profiles with an outflow velocity of ~900 km/s and a
full opening angle of ~110 degrees (matching the X-ray bicone). This indicates
Galactic Center activity over the last ~2.5-4.0 Myr, in line with age estimates
of the Fermi Bubbles. The observations illustrate the use of UV absorption-line
spectroscopy to probe the properties of swept-up gas venting into the Fermi
Bubbles.
