2019ApJ...871...35Z

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| ≤ 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_SiIV>=(3.8±1.4)×10¹³ cm^–2. At | b| ≳30^○, N_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_SiIV-b distribution: a plane-parallel component N_DH^⊥^ 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^⊥^=1.3-0.7_^+4.7×10¹² cm^–2 and N_G=(3.4±0.3)×10¹³ 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≳4.7×10⁹ M_☉(C_f/1)(R/75kpc)²(f_Si IV_/0.3)-1^(Z/0.3Z☉_)^–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| ≤ 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).