Stellar chemo-kinematics of the Cetus dwarf spheroidal galaxy^⋆,⋆⋆

¹ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: staibi@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
⁵ Excellence Cluster Origin and Structure of the Universe, Boltzmannstr. 2, 85748 Garching bei München, Germany
⁶ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
⁷ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁸ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁹ Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
¹⁰ Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
¹¹ GEPI, CNRS UMR 8111, Observatoire de Paris, PSL Research University, 92125 Meudon Cedex, France

Received: 11 May 2018
Accepted: 3 July 2018

Abstract

Context. The great majority of early-type dwarf galaxies, in the Local Group as well as in other galaxy groups, are found in the vicinity of much larger galaxies, making it hard to disentangle the role of internal versus external effects in driving their evolution.

Aims. In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of these systems, we study one of the few dwarf spheroidal galaxies of the Local Group found in isolation: Cetus. This system is of particular interest since it does not follow the Local Group morphology-density relation.

Methods. We obtained Very Large Telescope (VLT) FORS2 spectra (R ∼ 2600) in the region of the nIR CaII triplet lines for 80 candidate red giant branch (RGB) stars. The analysis yielded line-of-sight velocities and metallicities ([Fe/H]) for 54 bona fide member stars.

Results. The kinematic analysis shows that Cetus is a mainly pressure-supported (σ_v = 110_−1.3^+1.6 km s⁻¹), dark-matter-dominated system (M_1/2/L_V = 23.9_−8.9^+9.7 M_⊙/L_⊙) with no significant signs of internal rotation. We find Cetus to be a metal-poor system with a significant metallicity spread (median [Fe/H]= − 1.71 dex, median-absolute-deviation = 0.49 dex), as expected for its stellar mass. We report the presence of a mild metallicity gradient compatible with those found in other dwarf spheroidals of the same luminosity; we trace the presence of a stellar population gradient also in the spatial distribution of stars in different evolutionary phases in ancillary SuprimeCam photometric data. There are tentative indications of two chemo-kinematically distinct sub-populations, with the more metal-poor stars showing a hotter kinematics than the metal-richer ones. Furthermore, the photometric dataset reveals the presence of a foreground population that most likely belongs to the Sagittarius stream.

Conclusions. This study represents an important step forward in assessing the internal kinematics of the Cetus dwarf spheroidal galaxy as well as the first wide-area spectroscopic determination of its metallicity properties. With our analysis, Cetus adds to the growing scatter in stellar-dark matter halo properties in low-mass galactic systems. The presence of a metallicity gradient akin to those found in similarly luminous and pressure-supported systems inhabiting very different environments may hint at metallicity gradients in Local Group early-type dwarfs being driven by internal mechanisms.