UVIT observation of Milky Way satellite galaxy Reticulum II

Abstract

We present the UV photometry of one of the Ultra-Faint Dwarf (UFD) Milky Way (MW) satellite galaxies, Reticulum II, using images acquired with the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat. Reticulum II is a dark matter dominated, one of the most metal-poor and old (13.5 Gyr) satellite galaxies. High resolution spectroscopy of red giant branch (RGB) stars in the galaxy showed that the stars are metal-poor and highly enhanced in rapid neutron capture process (r-process) elements, indicating operation of r-process at a very early time in the galaxy. Understanding the stellar population of the galaxy will provide insights to the sites of r-process production and also clues to the formation of UFDs. Here, we present UV and optical color magnitude diagrams (CMD), of Reticulum II using samples selected based on Gaia data release-II proper motions and theoretical isochrone fitting. We identified eight members including the four new members detected for the first time in this study which adds to only 24 confirmed members. These new members are bright enough for follow-up high resolution spectroscopic studies that will be very valuable to probe the early chemical history of the galaxy. We identified three blue horizontal branch (BHB) stars and a possible red horizontal branch (RHB) star in Reticulum II. RHB stars are rare among ultra faint dwarf galaxies. This might indicate the presence of more than one epoch of star formation in Reticulum II, contrary to the earlier studies.