THE RECENT STAR-FORMATION IN SEXTANS-A

We investigate the relationship between the spatial distributions of s tellar populations and of neutral and ionized gas in the Local Group d warf irregular galaxy Sextans A. This galaxy is currently experiencing a burst of localized star formation, the trigger of which is unknown. We have resolved various populations of stars via deep UBV(RI), imagi ng over an area with diameter similar to 5.'3. We have compared our ph otometry with theoretical isochrones appropriate for Sextans A, in ord er to determine the ages of these populations. We have mapped out the history of star formation, most accurately for times less than or simi lar to 100 Myr. We find that star formation in Sextans A is correlated both in time and space, especially for the most recent (less than or similar to 12 Myr) times. The youngest stars in the galaxy are forming primarily along the inner edge of the large H I shell. Somewhat older populations,,less than or similar to 50 Myr, are found inward of the youngest stars. Progressively older star formation, from similar to 50 -100 Myr, appears to have some spatially coherent structure and is mor e centrally concentrated. The oldest stars we can accurately sample ap pear to have approximately a uniform spatial distribution, which exten ds beyond a surface brightness of mu(B) similar or equal to 25.9 mag a rcsec(-2) (or, a radius r similar or equal to 2.'3). Although other pr ocesses are also possible, our data provide support for a mechanism of supernova-driven expansion of the neutral gas, resulting in cold-gas pileup and compression along the H I shell and sequential star formati on in recent times.