{
  "id": "1205.6864",
  "version": "v1",
  "published": "2012-05-31T00:59:24.000Z",
  "updated": "2012-05-31T00:59:24.000Z",
  "title": "The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass",
  "authors": [
    "Roeland P. van der Marel",
    "Mark Fardal",
    "Gurtina Besla",
    "Rachael L. Beaton",
    "Sangmo Tony Sohn",
    "Jay Anderson",
    "Tom Brown",
    "Puragra Guhathakurta"
  ],
  "comment": "38 pages, 4 figures, to be published in ApJ. Version with high resolution figures and N-body movies available at http://www.stsci.edu/~marel/M31 . Press materials, graphics, and visualizations available at http://hubblesite.org/newscenter/archive/releases/2012/20",
  "doi": "10.1088/0004-637X/753/1/8",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "We determine the velocity vector of M31 with respect to the Milky Way and use this to constrain the mass of the Local Group, based on HST proper-motion measurements presented in Paper I. We construct N-body models for M31 to correct the measurements for the contributions from stellar motions internal to M31. We also estimate the center-of-mass motion independently, using the kinematics of satellite galaxies of M31 and the Local Group. All estimates are mutually consistent, and imply a weighted average M31 heliocentric transverse velocity of (v_W,v_N) = (-125.2+/-30.8, -73.8+/-28.4) km/s. We correct for the reflex motion of the Sun using the most recent insights into the solar motion within the Milky Way. This implies a radial velocity of M31 with respect to the Milky Way of V_rad = -109.3+/-4.4 km/s, and a tangential velocity V_tan = 17.0 km/s (<34.3 km/s at 1-sigma confidence). Hence, the velocity vector of M31 is statistically consistent with a radial (head-on collision) orbit towards the Milky Way. We revise prior estimates for the Local Group timing mass, including corrections for cosmic bias and scatter. Bayesian combination with other mass estimates yields M_LG = M_MW(vir) + M_M31(vir) = (3.17 +/- 0.57) x 10^12 solar masses. The velocity and mass results imply at 95% confidence that M33 is bound to M31, consistent with expectation from observed tidal deformations. (Abridged)",
  "revisions": [
    {
      "version": "v1",
      "updated": "2012-05-31T00:59:24.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "milky way",
      "implied local group mass",
      "m31 velocity vector",
      "average m31 heliocentric transverse",
      "m31 heliocentric transverse velocity"
    ],
    "tags": [
      "journal article"
    ],
    "publication": {
      "journal": "The Astrophysical Journal",
      "year": 2012,
      "month": "Jul",
      "volume": 753,
      "number": 1,
      "pages": 8
    },
    "note": {
      "typesetting": "TeX",
      "pages": 38,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 1116597,
      "adsabs": "2012ApJ...753....8V"
    }
  }
}