The Dwarf Irregular Galaxy Sextans A. II. Recent Star Formation History

Abstract

We have used the Hubble Space Telescope to obtain photometric data of the resolved stars in the nearby (D = 1.4 Mpc) dwarf irregular galaxy Sextans A (DDO 75, A 1008-04). The data consist of WFPC2 images in 3 bands: F439W (1 hour), F555W (30 minutes), and F814W (30 minutes). We constructed very accurate color-magnitude diagrams (CMDs) in V and I down to a limiting magnitude of 26 in V. The CMDs show several clearly separated populations that align well with stellar evolution model predictions for a low metallicity system. We use the MS stars to calculate the star formation history (SF11) over the past 100 Myr. We also present a method for extracting the SF11 from a second population, the blue He-burning (HeB) stars, independent of the MS. These are stars in the bluest part of the so-called "blue-loop" phase. This is the first time these stars have been unambiguously identified in a low metallicity system. This method has the potential to determine the SF11 over the past 1 Gyr, although photometric errors in the present data limit the range to 600 Myr. We have combined the spatial density distribution of the blue HeB stars with the star formation rate (SFR) calculations to determine the behavior of the star formation in both space and time. In the past 50 Myr, Sextans A has had an average SFR (5000±1500 M_sun Myr^-1 kpc^-2 assuming a Salpeter IMF) that is 20 times that of the average SFR over the history of the galaxy (310M_sun Myr^-1 kpc^{-2 assuming an 11 Gyr lifetime). This current activity is highly concentrated in a young region in the Southeast roughly 25 pc across. This coincides with the brightest H II regions and the highest column density of H I. This one region contains half of all the current star formation activity within our field of view. Between the ages of 100 and 600 Myr, the star formation is roughly constant at 2000±500M_sun Myr-1 kpc-2, still well above the lifetime average for the galaxy. The fluctuations during this time are spatially correlated. There are regions with a factor of ±5 enhancement in SFR compared to the full field of view (25%-30% of the star formation at any given time) that come and go in different locations over time. These regions are 200-300 pc in diameter and last 100-200 Myr. We estimate the star formation efficiency of these regions to be ∼9%. There is an age progression of these regions, suggesting the star formation may be propagating through the galaxy.}