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Missing CIV in the Milky Way's Galactic Atmosphere

The QuaStar Survey (Bish et al. 2021) pairs absorption-line measurements of quasars and galactic stars along similar lines of sight to constrain the contents of our galaxy's circumgalactic medium

Published onNov 28, 2022
Missing CIV in the Milky Way's Galactic Atmosphere

An all-sky map of low-velocity CIV in the Milky Way’s circumgalactic medium (CGM), from the QuaStar Survey of Bish et al. (2021). The survey probed 30 lines of sight. Boldly outlined circles with strong colors indicate detections of CIV associated with the CGM. Circles with faded colors indicate upper limits on CIV in the CGM. High-velocity HI clouds surrounding the galaxy are shown in grey.

This all-sky map from Bish et al. (2021) [1] shows a survey seeking CIV absorption arising in our galaxy’s circumgalactic medium (CGM). 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 contents of the Milky Way's circumgalactic CGM are poorly constrained at vLSR < 150 km s-1.

To overcome this complication, the QuaStar Survey [1] 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 (λλ\lambda\lambda1548, 1550A˚{\rm \AA}) detected in HST/COS spectra of 30 halo-star/quasar pairs evenly distributed across the sky at Galactic latitudes b>30|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.

The difference in absorption between the quasar and stellar sightlines appears to originate primarily in the Milky Way's extended CGM beyond ~10 kpc. We place an upper limit (NCIV < 1013.4 cm-2 ) on the mean CIV column density of the Milky Way's extended, low velocity (|v|< 150 km/s) CGM and detect CIV with NCIV < 1013.65 cm-2 along only 6 out of 30 lines of sight, representing a covering fraction of 20%. This incidence rate is 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.

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