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

Abstract

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 $|v_{\mathrm{LSR}}|$ $\lesssim$ 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 Å, 1550 Å), 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 $\mathrm{\Delta log}{\mathrm{N}}_{\mathrm{LVCGM}}<13.39$ and find a covering fraction of $f_{\mathrm{CIV},\mathrm{LVCGM}}(\mathrm{logN}>13.65)=$ 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.

15 pages, 6 figures, 1 table. Accepted to ApJ

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• Discovery of the most luminous quasar of the last 9 Gyr
• A Component of the Smith High Velocity Cloud Now Crossing the Galactic Plane
• The Warm Gas in the MW: the Kinematical Model of C IV and its Connection with Si IV
• Introducing the Descriptive Parametric Model: Gaseous Profiles for Galaxies, Groups, and Clusters
• The Warm-Hot Disk-Halo Interface Below the Perseus Spiral Arm
• Hot accretion onto spiral galaxies: the origin of extended and warped HI discs
• Mainly on the Plane: Observing the Extended, Ionized Disks of Milky Way Analogs in IllustrisTNG
• Intergalactic Medium Tomography with the Sunburst Arc