The Star Formation History of NGC 1705: A Poststarburst Galaxy on the Verge of Activity

We infer the star formation history in different regions of the blue compactdwarf NGC 1705 by comparing synthetic color-magnitude diagrams with HST opticaland near-infrared photometry. We find that NGC 1705 is not a young galaxybecause its star formation commenced at least 5 Gyr ago. On the other hand, weconfirm the existence of a recent burst of star formation between 15 and 10 Myrago. We also find evidence for new strong activity, which started 3 Myr ago andis still continuing. The old population is spread across the entire galaxy,while the young and intermediate stars are more concentrated in the centralregions. We derive an almost continuous star formation with variable rate, andexclude the presence of long quiescent phases between the episodes during thelast ~1 Gyr. The central regions experienced an episode of star formation of\~0.07 Msun/yr (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago.This coincides with the strong activity in the central super star cluster. Wefind a rate of ~0.3 Msun/yr for the youngest ongoing burst which started ~3 Myrago. This is higher than in other dwarfs and comparable to the rate of NGC1569. The star formation rate of earlier episodes is not especially high andfalls in the range 10^{-3}-10^{-1} Msun/yr. The IMF is close to the Salpetervalue or slightly steeper.Comment: 34 pages, including 6 tables and 14 .ps figures (9 in colour), AJ in pres