Tracing the stellar component of low surface brightness Milky Way dwarf galaxies to their outskirts

I. Sextans^⋆

¹ Instituto de Astrofísica de Canarias (IAC), C/Vía Láctea, s/n, 38205 San Cristóbal de la Laguna, Tenerife, Spain
e-mail: lcicuend@iac.es
² Departamento de Astrofísica, Universidad de La Laguna, 38206 San Cristóbal de la Laguna, Tenerife, Spain
³ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK
⁴ Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁵ Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, NSW 2006, Sydney, Australia
⁶ Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
⁷ NRC Herzberg Institute of Astrophysics, Dominion Astrophysical Observatory, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁸ Kapteyn Astronomical Institute, University of Groningen, 9700AV Groningen, The Netherlands

Received: 26 June 2017
Accepted: 9 September 2017

Abstract

Aims. We present results from deep and very spatially extended CTIO/DECam g and r photometry (reaching out to ~2 mag below the oldest main-sequence turn-off and covering ~20 deg²) around the Sextans dwarf spheroidal galaxy. We aim to use this dataset to study the structural properties of Sextans overall stellar population and its member stars in different evolutionary phases, as well as to search for possible signs of tidal disturbance from the Milky Way, which would indicate departure from dynamical equilibrium.

Methods. We performed the most accurate and quantitative structural analysis to-date of Sextans’ stellar components by applying Bayesian Monte Carlo Markov chain methods to the individual stars’ positions. Surface density maps are built by statistically decontaminating the sample through a matched filter analysis of the colour-magnitude diagram, and then analysed for departures from axisymmetry.

Results. Sextans is found to be significantly less spatially extended and more centrally concentrated than early studies suggested. No statistically significant distortions or signs of tidal disturbances were found down to a surface brightness limit of ~31.8 mag/arcsec² in V-band. We identify an overdensity in the central regions that may correspond to previously reported kinematic substructure(s). In agreement with previous findings, old and metal-poor stars such as Blue Horizontal Branch stars cover a much larger area than stars in other evolutionary phases, and bright Blue Stragglers (BSs) are less spatially extended than faint ones. However, the different spatial distribution of bright and faint BSs appears consistent with the general age and metallicity gradients found in Sextans’ stellar component. This is compatible with Sextans BSs having formed by evolution of binaries and not necessarily due to the presence of a central disrupted globular cluster, as suggested in the literature. We provide structural parameters for the various populations analysed and make publicly available the photometric catalogue of point-sources as well as a catalogue of literature spectroscopic measurements with updated membership probabilities.