The QuaStar Survey: Detecting Hidden Low-velocity Gas in the Milky Way’s Circumgalactic Medium

Abstract

Abstract From our position embedded within the Milky Way’s interstellar medium, 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 LSR∣ ≲150 km s−1. To overcome this complication, the QuaStar survey applies a spectral differencing technique using paired quasar−star sight lines to measure the obscured content of the Milky Way’s CGM for the first time. We present measurements of the C iv doublet (λ λ1548, 1550), a rest-frame UV metal line transition, detected in Hubble Space Telescope/Cosmic Origins Spectrograph spectra of 30 halo-star/quasar pairs evenly distributed across the sky at Galactic latitudes ∣b∣ > 30°. The 30 halo stars have well-constrained distances (d ≈ 5–14 kpc) and are paired with quasars separated by <2.°8. We argue that the difference in absorption between the quasar and stellar sight lines originates primarily in the Milky Way’s extended CGM beyond ∼10 kpc. For the Milky Way’s extended, low-velocity CGM (∣v∣ <150 km s−1), we place an upper limit on the mean C iv column density of Δ log N LVCGM < 13.39 and find a covering fraction of f C IV , LVCGM ( log N > 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 C iv -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.