Revealing the Milky Way’s Hidden Circumgalactic Medium with the Cosmic Origins Spectrograph Quasar Database for Galactic Absorption Lines

Abstract

Every quasar (quasi-stellar object; QSO) spectrum contains absorption-line signatures from the interstellar medium, disk-halo interface, and circumgalactic medium (CGM) of the Milky Way (MW). We analyze Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectra of 132 QSOs to study the significance and origin of Si IV absorption at | {v}_LSR}| ≤slant 100 km s^-1 in the Galactic halo. The gas in the north predominantly falls in at -50≲ {v}_LSR}≲ 0 km s^-1, whereas in the south, no such pattern is observed. The Si IV column density has an average and a standard deviation of < {N}_Si{{IV}}}> =(3.8+/- 1.4)× {10}¹³ cm^-2. At | b| ≳ 30^\circ , {N}_Si{{IV}}} does not significantly correlate with b, which cannot be explained by a commonly adopted flat-slab geometry. We propose a two-component model to reconstruct the {N}_Si{{IV}}}-b distribution: a plane-parallel component {N}_DH}^\perp to account for the MW’s disk-halo interface and a global component {N}_{{G}} to reproduce the weak dependence on b. We find {N}_DH}^\perp ={1.3}_-0.7^+4.7× {10}¹² cm^-2 and {N}_{{G}}=(3.4+/- 0.3)× {10}¹³ cm^-2 on the basis of Bayesian analyses and block bootstrapping. The global component is most likely to have a Galactic origin, although its exact location is uncertain. If it were associated with the MW’s CGM, we would find {M}_gas,{all}}≳ 4.7× {10}⁹ {M}_⊙ ≤ft(\tfrac{{C}_f}{1}\right){≤ft(\tfrac{R}{75{kpc}}\right)}²{≤ft(\tfrac{{f}_Si{{IV}}}}{0.3}\right)}^-1{≤ft(\tfrac{Z}{0.3{Z}_⊙ }\right)}^-1 for the cool gas at all velocities in the Galactic halo. Our analyses show that there is likely a considerable amount of gas at | {v}_LSR}| ≤slant 100 km s^-1 hidden in the MW’s CGM. Along with this work, we make our QSO data set publicly available as the COS Quasar Database for Galactic Absorption Lines (COS-GAL).