THE DWARF IRREGULAR GALAXY SEXTANS-A .2. RECENT STAR-FORMATION HISTORY

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 b ands: F439W (1 hour), F555W (30 minutes), and F814W (30 minutes). We c onstructed very accurate color-magnitude diagrams (CMDs) in V and I do wn to a limiting magnitude of 26 in V. The CMDs show several clearly s eparated populations that align well with stellar evolution model pred ictions for a low metallicity system. We use the MS stars to calculate the star formation history (SFH) over the past 100 Myr. We also prese nt a method for extracting the SFH 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 ti me these stars have been unambiguously identified in a low metallicity system. This method has the potential to determine the SFH over the p ast 1 Gyr, although photometric errors in the present data limit the r ange 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. I n the past 50 Myr, Sextans A has had an average SFR (5000 +/- 1500 M. Myr(-1) kpc(-2) assuming a Salpeter IMF) that is similar to 20 times t hat of the average SFR over the history of the galaxy (similar to 310 M. 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 high est column density of H I. This one region contains half of all the cu rrent star formation activity within our field of view. Between the ag es of 100 and 600 Myr, the star formation is roughly constant at simil ar to 2000+/-500 M. Myr(-1) kpc(-2), still well above the lifetime ave rage for the galaxy. The fluctuations during this time are spatially c orrelated, There are regions with a factor of similar to 5 enhancement in SFR compared to the full field of view (25%-30% of the star format ion at any given time) that come and go in different locations over ti me. These regions are 200-300 pc in diameter and last 100-200 Myr. We estimate the star formation efficiency of these regions to be similar to 9%. There is an age progression of these regions, suggesting the st ar formation may be propagating through the galaxy. (C) 1997 American Astronomical Society. [S0004-6256(97)00712-7].