Deep Hubble Space Telescope imaging of Sextans A.: I.: The spatially resolved recent star formation history

We have measured stellar photometry from deep Cycle 7 Hubble Space Telescope /WFPC2 imaging of the dwarf irregular galaxy Sextans A. The imaging was taken in three filters: F555W (V; eight orbits), F814W (I; 16 orbits), and F656N (Halpha; one orbit). Combining these data with Cycle 5 WFPC2 observations provides nearly complete coverage of the optically visible portion of the galaxy. The Cycle 7 observations are nearly 2 mag more sensitive than the Cycle 5 observations, which provides unambiguous separation of the faint blue helium-burning stars (BHeB stars) from contaminant populations. The depth of the photometry allows us to compare recent star formation histories recovered from both the main-sequence stars and the BHeB stars for the last 300 Myr. The excellent agreement between these independent star formation rate (SFR) calculations is a resounding confirmation for the legitimacy of using the BHeB stars to calculate the recent SFR. Using the BHeB stars we have calculated the global star formation history over the past 700 Myr. The history calculated from the Cycle 7 data is remarkably identical to that calculated from the Cycle 5 data, implying that both halves of the galaxy formed stars in concert. We have also calculated the spatially resolved star formation history, combining the fields from the Cycle 5 and Cycle 7 data. The star-forming regions are found in three major zones of the galaxy. One of these zones is extremely young, consisting of only a single star-forming region that is less than 20 Myr old. Two of these zones are associated with high column density neutral gas, while the third, and oldest, is not. Our interpretation of this pattern of star formation is that it is an orderly stochastic process. Star formation begins on the edge of a gas structure and progressively eats away at the cloud, breaking it up and inducing further star formation. A more quantitative analysis of the star formation process must await a larger sample of galaxies with spatially resolved star formation histories to allow correlation studies with the physical properties of the galaxy.