STAR FORMATION HISTORY AND CHEMICAL EVOLUTION OF THE SEXTANS DWARF SPHEROIDAL GALAXY

Abstract

We present the star formation history and chemical evolution of the Sextans\ndSph dwarf galaxy as a function of galactocentric distance. We derive these\nfrom the $VI$ photometry of stars in the $42' \\times 28'$ field using the SMART\nmodel developed by Yuk & Lee (2007, ApJ, 668, 876) and adopting a closed-box\nmodel for chemical evolution. For the adopted age of Sextans 15 Gyr, we find\nthat $>$84% of the stars formed prior to 11 Gyr ago, significant star formation\nextends from 15 to 11 Gyr ago ($\\sim$ 65% of the stars formed 13 to 15 Gyr ago\nwhile $\\sim$ 25% formed 11 to 13 Gyr ago), detectable star formation continued\nto at least 8 Gyr ago, the star formation history is more extended in the\ncentral regions than the outskirts, and the difference in star formation rates\nbetween the central and outer regions is most marked 11 to 13 Gyr ago. Whether\nblue straggler stars are interpreted as intermediate age main sequence stars\naffects conclusions regarding the star formation history for times 4 to 8 Gyr\nago, but this is at most only a trace population. We find that the metallicity\nof the stars increased rapidly up to [Fe/H]=--1.6 in the central region and to\n[Fe/H]=--1.8 in the outer region within the first Gyr, and has varied slowly\nsince then. The abundance ratios of several elements derived in this study are\nin good agreement with the observational data based on the high resolution\nspectroscopy in the literature. We conclude that the primary driver for the\nradial gradient of the stellar population in this galaxy is the star formation\nhistory, which self-consistently drives the chemical enrichment history.\n