3.3. Sextans A
Sextans A (DDO 75, A 1008-04) is a gas-rich dI galaxy with active star formation, which is located on the periphery of the Local Group at a distance of 1.44 Mpc. The CMD of Sextans A shows several clearly separated populations that align well with stellar evolution model predictions for a low metallicity system (Dohm-Palmer et al. 1997b). The SFH from the MS+BL stars was determined over the last 600 Myr. This is the first time a BL sequence has been clearly identified in a CMD. The distribution of the BL stars was combined with sfr calculations to determine the spatial variation of the star formation across Sextans A with time. The modelling concludes that in the past 50 Myr, Sextans A has had an average sfr that is ~ 10 times that of the average sfr over the history of the galaxy. This current activity is highly concentrated in a young region in the Southeast roughly 25 pc across. This coincides with the brightest HII regions and the highest column density of HI. This one region contains half of all the current star formation activity within the field of view. Between the ages of 100 and 600 Myr ago, the star formation has been roughly constant at slightly above the average value. There are regions (200-300pc across) with a factor of ~ 5 enhancement in sfr with a duration of 100-200 Myr.
Figure 6. Here we show the results for the HST / WFPC2 analysis of Sextans A (Dohm-Palmer et al. 1997b). On the left is the V-I, I CMD, 1 orbit exposure time per filter. On the right is the SFH from the MS+BL which best matches the data. See Dohm-Palmer et al. for more details.