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

Every quasar (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 SiIV absorption at $\mathrm{\mid }{v}_{\mathrm{L}\mathrm{S}\mathrm{R}}\mathrm{\mid }\le 100$ km/s in the Galactic halo. The gas in the north predominantly falls in at $-50\lesssim v_{\mathrm{L}\mathrm{S}\mathrm{R}}\lesssim 0$ km/s, whereas in the south, no such pattern is observed. The SiIV column density has an average and a standard deviation of $\langle N_{\rm SiIV}\rangle=(3.8\pm1.4)\times10^{13}$cm$^{-2}$\mathrm{.}At$|b|\gtrsim 30$degree,$N_{\rm SiIV}$doesnotsignificantlycorrelatewith$b$, which cannot be explained by a commonly adopted flat-slab geometry. We propose a two-component model to reconstruct the $N_{\mathrm{S}\mathrm{i}\mathrm{I}\mathrm{V}}$-$b$ distribution: a plane-parallel component $N_{\mathrm{D}\mathrm{H}}^{\perp }$ to account for the MW’s disk-halo interface and a global component $N_{\mathrm{G}}$ to reproduce the weak dependence on $b$. We find $N_{\mathrm{D}\mathrm{H}}^{\perp }=1.3_{-0.7}^{+4.7}\times {10}^{12}$ cm${}^{-2}$ and $N_{\rm G}=(3.4\pm0.3)\times10^{13}$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 $M_{\rm gas, all}\gtrsim4.7\times10^9\ M_{\odot} (\frac{C_f}{1})(\frac{R}{75\ {\rm kpc}})^2 (\frac{f_{\rm SiIV}}{0.3})^{-1}(\frac{Z}{0.3\ Z_{\odot}})^{-1}$ for the cool gas at all velocities in the Galactic halo. Our analyses show that there is likely a considerable amount of gas at $\mathrm{\mid }{v}_{\mathrm{L}\mathrm{S}\mathrm{R}}\mathrm{\mid }\le 100$ km s${}^{-1}$ hidden in the MW’s CGM. Along with this work, we make our QSO dataset publicly available as the COS Quasar Database for Galactic Absorption Lines (COS-GAL).