The QuaStar Survey: Detecting Hidden Low-Velocity Gas in the Milky Way's
  Circumgalactic Medium
  From our position embedded within the Milky Way's interstellar medium (ISM),
we have limited ability to detect gas at low relative velocities in the
extended Galactic halo because those spectral lines are blended with much
stronger signals from dense foreground gas. As a result, the content of the
Milky Way's circumgalactic medium (CGM) is poorly constrained at $|v_{\rm
LSR}|$ $\lesssim$ 150 km s$^{-1}$. To overcome this complication, the QuaStar
Survey applies a spectral differencing technique using paired quasar-star
sightlines to measure the obscured content of the Milky Way's CGM for the first
time. We present measurements of the CIV doublet ($\lambda\lambda$ 1548 \r{A},
1550 \r{A}), a rest-frame UV metal line transition detected in HST/COS spectra
of 30 halo-star/quasar pairs evenly distributed across the sky at Galactic
latitudes $|b|>30^\circ$. The 30 halo stars have well-constrained distances
(d$\approx$5-14 kpc), and are paired with quasars separated by $<$ 2.8$^\circ$.
We argue that the difference in absorption between the quasar and stellar
sightlines originates primarily in the Milky Way's extended CGM beyond $\sim$10
kpc. For the Milky Way's extended, low velocity CGM ($|v|<$150 km/s), we place
an upper limit on the mean CIV column density of $\rm \Delta logN_{LVCGM} <
13.39$ and find a covering fraction of $f_{\rm CIV,LVCGM} (\rm logN>13.65)=$
20% [6/30], a value significantly lower than the covering fraction for
star-forming galaxies at low redshift. Our results suggest either that the bulk
of Milky Way's CIV-traced CGM lies at low Galactic latitudes, or that the Milky
Way's CGM is lacking in warm, ionized material compared to low-redshift ($z <
0.1$) star-forming galaxy halos.
