Weighing the local dark matter with RAVE red clump stars

¹ Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
e-mail: olivier.bienayme@unistra.fr
² Mount Stromlo Observatory, RSAA, Australian National University, Weston Creek, ACT 2611 Canberra, Australia
³ Institute for Computational Astrophysics, Dept of Astronomy & Physics, Saint Marys University, Halifax, NS, BH3 3C3, Canada
⁴ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, PR1 2HE, UK
⁵ Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 0HA, UK
⁶ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁷ Sydney Institute for Astronomy, School of Physics A28, University of Sydney, Sydney NSW 2006, Australia
⁸ INAF Osservatorio Astronomico di Padova, 36012 Asiago ( VI), Italy
⁹ Department of Physics and Astronomy. University of Victoria, Victoria, BC. Canada V8P 5C2
¹⁰ Department of Physics, Macquarie University, Sydney NSW 2109, Australia
¹¹ Research Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia
¹² Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, UK
¹³ Department of Physics and Astronomy, Padova University, Vicolo dellOsservatorio 2, 35122 Padova, Italy
¹⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁵ Australian Astronomical Observatory, PO Box 915, North Ryde NSW 1670, Australia
¹⁶ Johns Hopkins University, Homewood Campus, 3400 N Charles Street, Baltimore, MD 21218, USA
¹⁷ Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia

Received: 26 June 2014
Accepted: 17 September 2014

Abstract

We determine the Galactic potential in the solar neigbourhood from RAVE observations. We select red clump stars for which accurate distances, radial velocities, and metallicities have been measured. Combined with data from the 2MASS and UCAC catalogues, we build a sample of ~4600 red clump stars within a cylinder of 500 pc radius oriented in the direction of the South Galactic Pole, in the range of 200 pc to 2000 pc distances. We deduce the vertical force and the total mass density distribution up to 2 kpc away from the Galactic plane by fitting a distribution function depending explicitly on three isolating integrals of the motion in a separable potential locally representing the Galactic one with four free parameters. Because of the deep extension of our sample, we can determine nearly independently the dark matter mass density and the baryonic disc surface mass density. We find (i) at 1 kpc K_z/ (2πG) = 68.5 ± 1.0 M_⊙ pc^-2; and (ii) at 2 kpc K_z/ (2πG) = 96.9 ± 2.2 M_⊙ pc^-2. Assuming the solar Galactic radius at R₀ = 8.5 kpc, we deduce the local dark matter density ρ_DM(z = 0) = 0.0143 ± 0.0011 M_⊙pc^-3 = 0.542 ± 0.042 Gev cm^-3 and the baryonic surface mass density Σ_bar = 44.4 ± 4.1 M_⊙pc^-2. Our results are in agreement with previously published K_z determinations up to 1 kpc, while the extension to 2 kpc shows some evidence for an unexpectedly large amount of dark matter. A flattening of the dark halo of order 0.8 can produce such a high local density in combination with a circular velocity of 240 km s^-1. It could also be consistent with a spherical cored dark matter profile whose density does not drop sharply with radius. Another explanation, allowing for a lower circular velocity, could be the presence of a secondary dark component, a very thick disc resulting either from the deposit of dark matter from the accretion of multiple small dwarf galaxies, or from the presence of an effective “phantom” thick disc in the context of effective galactic-scale modifications of gravity.