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

Every quasar spectrum contains absorption-line signatures from the interstellar medium, disk-halo interface, and circumgalactic medium (CGM) of the Milky Way (MW). We analyze HST/COS spectra of 132 quasars to study the significance and origin of the SiIV absorption at $|v_{\rm LSR}|\leq100$ km s$^{-1}$. Within this velocity range, SiIV has an average column density of log$\bar{N}=13.50\pm0.17$ and its all-sky distribution does not significantly correlate with the Galactic latitude or longitude. The gas in the north predominantly infalls at $v\gtrsim-50$ km s$^{-1}$ while in the south no such pattern is observed. We find that the SiIV distribution cannot be explained by the commonly adopted disk-halo model of a flat slab co-rotating with the disk. We propose a two-component model to reconstruct the distribution with a global background and a plane-parallel disk-halo component. With block bootstrapping, we find $13.41\lesssim$ log$N_{\rm global}\lesssim13.59$ and log$N_{\rm DH, \perp}\lesssim12.89$ for the global and disk-halo components at $1\sigma$, respectively. 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 extended CGM, our order-of-magnitude mass estimate finds a total mass of $\sim(3.7-5.6)\times10^{10} M_{\odot}(\frac{R}{160\ {\rm kpc}})^2$ for gas at warm ionized phases, consistent with CGM studies of $\sim L^*$ galaxies at low redshifts. Our analyses show that there is likely to be a large amount of gas at $|v_{\rm LSR}|\leq100$ km s$^{-1}$ hidden in the MW's CGM. Along with this work, we make our continuum-normalized spectra publicly available as the COS Quasar Database for Galactic Absorption Lines (COS-GAL).

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