Volume 633, January 2020
|Number of page(s)||11|
|Published online||07 January 2020|
Stellar 3D kinematics in the Draco dwarf spheroidal galaxy⋆
Kapteyn Astronomical Institute, University of Groningen, 9747 AD
Groningen, The Netherlands
2 Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
3 INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
4 Department of Physics and Astronomy, University of California Riverside, 900 University Ave., Riverside, CA 92507, USA
5 Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
Accepted: 12 November 2019
Aims. We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy.
Methods. By combining first-epoch Hubble Space Telescope observations and second-epoch Gaia Data Release 2 positions, we measured the proper motions of 149 sources in the direction of Draco. We determined the line-of-sight velocities for a sub-sample of 81 red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether, this resulted in a final sample of 45 Draco members with high-precision and accurate 3D motions, which we present as a table in this paper.
Results. Based on this high-quality dataset, we determined the velocity dispersions at a projected distance of ∼120 pc from the centre of Draco to be σR = 11.0−1.5+2.1 km s−1, σT = 9.9−3.1+2.3 km s−1 and σLOS = 9.0−1.1+1.1 km s−1 in the projected radial, tangential, and line-of-sight directions. This results in a velocity anisotropy β = 0.25−1.38+0.47 at r ≳ 120 pc. Tighter constraints may be obtained using the spherical Jeans equations and assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also based on the assumption that the 3D velocity dispersion should be lower than ≈1/3 of the escape velocity of the system. In this case, we constrain the maximum circular velocity Vmax of Draco to be in the range of 10.2−17.0 km s−1. The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only.
Conclusions. Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models to this dataset and with new datasets from upcoming Gaia releases.
Key words: galaxies: dwarf / Local Group / galaxies: kinematics and dynamics / proper motions / techniques: radial velocities
Full Tables 2 and 3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/633/A36
© ESO 2020
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