{
  "id": "2002.06434",
  "version": "v1",
  "published": "2020-02-15T19:28:21.000Z",
  "updated": "2020-02-15T19:28:21.000Z",
  "title": "The Warm Gas in the MW: A Kinematical Model",
  "authors": [
    "Zhijie Qu",
    "Joel N. Bregman",
    "Edmund Hodges-Kluck",
    "Jiang-Tao Li",
    "Ryan Lindley"
  ],
  "comment": "Accepted by ApJ",
  "categories": [
    "astro-ph.GA"
  ],
  "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, $n(r_{\\rm XY})=n_0\\exp(-(r_{\\rm XY}/r_0)^{3.36})$, where $r_0 \\approx 12.5\\pm0.6$ kpc. The difference of density distributions over $r_{\\rm XY}$ 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 $\\log (M(50 {\\rm kpc})/M_\\odot)\\approx8.1$ (assuming $Z\\approx0.5Z_\\odot$), and a $3\\sigma$ upper limit of $\\log (M(250 {\\rm kpc})/M_\\odot)\\approx9.1$ (excluding the Magellanic system). Kinematically, the warm gas disk is nearly co-rotating with the stellar disk at $v_{\\rm rot}=215\\pm3\\rm~km~s^{-1}$, which lags the midplane rotation by about $8\\rm~km~s^{-1}~kpc^{-1}$ (within 5 kpc). Meanwhile, we note that the warm gas in the northern hemisphere has significant accretion with $v_{\\rm acc}$ of $69\\pm 7\\rm ~km~s^{-1}$ at 10 kpc (an accretion rate of $-0.60_{-0.13}^{+0.11}~M_\\odot\\rm~yr^{-1}$), while in the southern hemisphere, there is no measurable accretion, with an upper limit of $0.4~M_\\odot\\rm~yr^{-1}$.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2020-02-15T19:28:21.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "warm gas",
      "kinematical model",
      "density distribution",
      "way si iv-bearing gas",
      "column density line shape sample"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}