The Recent Star Formation in Sextans A

We investigate the relationship between the spatial distributions of stellar populations and of neutral and ionized gas in the Local Group dwarf 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)C imaging over an area with diameter ∼5.′3. We have compared our photometry with theoretical isochrones appropriate for Sextans A, in order to determine the ages of these populations. We have mapped out the history of star formation, most accurately for times ≲100 Myr. We find that star formation in Sextans A is correlated both in time and space, especially for the most recent (≲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, ≲50 Myr, are found inward of the youngest stars. Progressively older star formation, from ∼50–100 Myr, appears to have some spatially coherent structure and is more centrally concentrated. The oldest stars we can accurately sample appear to have approximately a uniform spatial distribution, which extends beyond a surface brightness of μB ≃ 25.9 mag arcsec-2 (or, a radius r ≃ 2.′3). Although other processes 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 formation in recent times.