The Warm Gas in the MW: A Kinematical Model

We develop a kinematical model for the Milky Way Si IV-bearing gas to determine its density distribution and kinematics. This model is constrained by a column density line-shape sample extracted from the Hubble Space Telescope/Cosmic Origins Spectrograph archival data, which contains 186 active galactic nucleus sight lines. We find that the Si IV ion density distribution is dominated by an extended disk along the z-direction (above or below the midplane), I.e., $n{(z)={n}_{0}\exp (-(z/{z}_{0})}^{0.82})$ , where z₀ is the scale height of ${6.3}_{-1.5}^{+1.6}$ kpc (northern hemisphere) and ${3.6}_{-0.9}^{+1.0}$ kpc (southern hemisphere). The density distribution of the disk in the radial direction shows a sharp edge at 15-20 kpc given by, $n{({r}_{\mathrm{XY}})={n}_{0}\exp (-({r}_{\mathrm{XY}}/{r}_{0})}^{3.36})$ , where r₀ ≈ 12.5 ± 0.6 kpc. The difference of density distributions over r_XY and z directions indicates that the warm gas traced by Si IV is mainly associated with disk processes (e.g., feedback or cycling gas) rather than accretion. We estimate the mass of the warm gas (within 50 kpc) is $\mathrm{log}(M(50\mathrm{kpc})/{M}_{\odot })\approx 8.1$ (assuming Z ≈ 0.5 Z_⊙), and a 3σ upper limit of $\mathrm{log}(M(250\mathrm{kpc})/{M}_{\odot })\approx 9.1$ (excluding the Magellanic system). Kinematically, the warm gas disk is nearly co-rotating with the stellar disk at ${V}_{\mathrm{rot}}=215\pm 3\,\mathrm{km}\,{{\rm{s}}}^{-1}$ , which lags the midplane rotation by about $8\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{kpc}}^{-1}$ (within 5 kpc). Meanwhile, we note that the warm gas in the northern hemisphere has significant accretion with V_acc of 69 ± 7 $\mathrm{km}\,{{\rm{s}}}^{-1}$ at 10 kpc (an accretion rate of $-{0.60}_{-0.13}^{+0.11}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$), while in the southern hemisphere, there is no measurable accretion, with an upper limit of 0.4 M_⊙ yr^-1.