The Warm Gas in the MW: A Kinematical Model

Abstract

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 {\it HST}/COS archival data, which contains 186 AGN 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 $z_0$ 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 $r_0\approx 12.5\pm 0.6$ kpc. The difference of density distributions over $r_{\mathrm{XY}}$ and $z$ directions indicates that the warm gas traced by \ion{Si}{4} 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 $\log (M(50\mathrm{kpc})/M_{\odot })\approx 8.1$ (assuming $Z\approx 0.5Z_{\odot }$), and a $3\sigma$ upper limit of $\log (M(250\mathrm{kpc})/M_{\odot })\approx 9.1$ (excluding the Magellanic system). Kinematically, the warm gas disk is nearly co-rotating with the stellar disk at $v_{\mathrm{rot}}=215\pm 3\mathrm{km}{\mathrm{s}}^{-1}$, which lags the midplane rotation by about $8\mathrm{km}{\mathrm{s}}^{-1}\mathrm{kp}{\mathrm{c}}^{-1}$ (within 5 kpc). Meanwhile, we note that the warm gas in the northern hemisphere has significant accretion with $v_{\mathrm{acc}}$ of $69\pm 7\mathrm{km}{\mathrm{s}}^{-1}$ at 10 kpc (an accretion rate of $-0.60_{-0.13}^{+0.11}{M}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$), while in the southern hemisphere, there is no measurable accretion, with an upper limit of $0.4M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$.

arXiv commentAccepted by ApJ

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