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

Abstract

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 km s−1 in the Galactic halo. The gas in the north predominantly falls in at 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 cm−2. At , 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 –b distribution: a plane-parallel component to account for the MW’s disk-halo interface and a global component to reproduce the weak dependence on b. We find cm−2 and 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 for the cool gas at all velocities in the Galactic halo. Our analyses show that there is likely a considerable amount of gas at 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).