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 compact dwarf NGC 1705 by comparing synthetic color-magnitude diagrams with the Hubble Space Telescope optical and near-infrared photometry. We find that NGC 1705 is not a young galaxy, because its star formation commenced at least 5 Gyr ago. On the other hand, we confirm the existence of a recent burst of star formation between 15 and 10 Myr ago. We also find evidence for new strong activity, which started 3 Myr ago and is still continuing. The old population is spread across the entire galaxy, while the young and intermediate stars are more concentrated in the central regions. We derive an almost continuous star formation with variable rate and exclude the presence of long quiescent phases between the episodes during the last approximate to1 Gyr. The central regions experienced an episode of star formation of similar to0.07 M-circle dot yr(-1) (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago. This coincides with the strong activity in the central super-star cluster. We find a rate of similar to0.3 M-circle dot yr(-1) for the youngest ongoing burst, which started similar to3 Myr ago. This is higher than in other dwarfs and comparable to the rate of NGC 1569. The star formation rate of earlier episodes is not especially high and falls in the range 10(-3) to 10(-1) M-circle dot yr(-1). The IMF is close to the Salpeter value or slightly steeper.