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تسجيل مستخدم جديدThe QuaStar Survey: Detecting Hidden Low-Velocity Gas in the Milky Ways Circumgalactic Medium
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From our position embedded within the Milky Ways 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 Ways 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 Ways CGM for the first time. We present measurements of the CIV doublet ($lambdalambda$ 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 Ways extended CGM beyond $sim$10 kpc. For the Milky Ways extended, low velocity CGM ($|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 Ways CIV-traced CGM lies at low Galactic latitudes, or that the Milky Ways CGM is lacking in warm, ionized material compared to low-redshift ($z < 0.1$) star-forming galaxy halos.
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