Volume 634, February 2020
|Number of page(s)||13|
|Published online||28 January 2020|
Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
2 Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
3 Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
4 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
5 European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
6 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
7 School of Natural Sciences, University of Tasmania, Private Bag 37 Hobart, Tasmania 7001, Australia
8 Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
9 GEPI, CNRS UMR 8111, Observatoire de Paris, PSL Research University, 92125 Meudon, Cedex, France
10 National Research Council, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
11 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
12 Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
Accepted: 28 November 2019
Context. Dwarf galaxies found in isolation in the Local Group (LG) are unlikely to have interacted with the large LG spirals, and therefore environmental effects such as tidal and ram-pressure stripping should not be the main drivers of their evolution.
Aims. We provide insight into the internal mechanisms shaping LG dwarf galaxies by increasing our knowledge of the internal properties of isolated systems. Here we focus on the evolved stellar component of the Aquarius dwarf galaxy, whose kinematic and metallicity properties have only recently started to be explored.
Methods. Spectroscopic data in the region of the near-infrared Ca II triplet lines has been obtained with FORS2 at the Very Large Telescope for 53 red giant branch (RGB) stars. These data are used to derive line-of-sight (l.o.s.) velocities and [Fe/H] of the individual RGB stars.
Results. We derive a systemic velocity of −142.2+1.8−1.8 km s−1, in agreement with previous determinations from both the HI gas and stars. The internal kinematics of Aquarius appears to be best modelled by a combination of random motions (l.o.s. velocity dispersion of 10.3+1.6−1.3 km s−1) and linear rotation (with a gradient −5.0+1.6−1.9 km s−1 arcmin−1) along a PA = 139+17−27 deg, broadly consistent with the optical projected major axis. This rotation signal is significantly misaligned or even counter-rotating to that derived from the HI gas. We also find the tentative presence of a mild negative metallicity gradient and indications that the metal-rich stars have a colder velocity dispersion than the metal-poor ones.
Conclusions. This work represents a significant improvement with respect to previous measurements of the RGB stars of Aquarius as it doubles the number of member stars already studied in the literature. We speculate that the misaligned rotation between the HI gas and evolved stellar component might have been the result of recent accretion of HI gas, or re-accretion after gas-loss due to internal stellar feedback.
Key words: techniques: spectroscopic / galaxies: dwarf / Local Group / galaxies: kinematics and dynamics / galaxies: abundances / galaxies: stellar content
Table A.2 and the averaged spectra are 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/634/A10
© ESO 2020
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