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

Abstract

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 $|v_{\mathrm{LSR}}|\le 100$ km/s in the Galactic halo. The gas in the north predominantly falls in at $-50\lesssim v_{\mathrm{LSR}}\lesssim 0$ km/s, whereas in the south, no such pattern is observed. The SiIV column density has an average and a standard deviation of $⟨N_{\mathrm{SiIV}}⟩=(3.8\pm 1.4)\times 10^{13}$ cm${}^{-2}$. At $|b|\gtrsim 30$ degree, $N_{\mathrm{SiIV}}$ 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 $N_{\mathrm{SiIV}}$-$b$ distribution: a plane-parallel component $N_{\mathrm{DH}}^{\perp }$ 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 $N_{\mathrm{DH}}^{\perp }=1.3_{-0.7}^{+4.7}\times 10^{12}$ cm${}^{-2}$ and $N_{\mathrm{G}}=(3.4\pm 0.3)\times 10^{13}$ 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 $M_{\mathrm{gas},\mathrm{all}}\gtrsim 4.7\times 10^9{M}_{\odot }(\frac{C_f}{1})(\frac{R}{75\mathrm{kpc}}{)}^2(\frac{f_{\mathrm{SiIV}}}{0.3}{)}^{-1}(\frac{Z}{0.3Z_{\odot }}{)}^{-1}$ for the cool gas at all velocities in the Galactic halo. Our analyses show that there is likely a considerable amount of gas at $|v_{\mathrm{LSR}}|\le 100$ 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).

Published on ApJ. https://iopscience.iop.org/article/10.3847/1538-4357/aaf6eb/meta

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