STELLAR POPULATIONS AND STAR FORMATION HISTORY OF THE METAL-POOR DWARF GALAXY DDO 68

We present the star formation history (SFH) of the extremely metal-poor dwarf galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys. With a metallicity of only 12 + log(O/H) = 7.15 and a very isolated location, DDO. 68 is one of the most metal-poor galaxies known. It has been argued that DDO 68 is a young system that started forming stars only similar to 0.15 Gyr ago. Our data provide a deep and uncontaminated optical color-magnitude diagram (CMD) that allows us to disprove this hypothesis. since we find a population of at least similar to 1 Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is similar or equal to 0.01 M-circle dot yr(-1), corresponding to a total astrated mass of similar or equal to 1.3 x 10(8)M(circle dot). Our photometry allows us to infer the distance from the tip of the red giant branch, D = 12.08 +/- 0.67 Mpc; however, to let our synthetic CMD reproduce the observed ones, we need a slightly higher distance, D = 12.65 Mpc, or (m - M)(0) = 30.51, still inside the errors of the previous determination, and we adopt the latter. DDO 68 shows a very interesting and complex history, with its quite disturbed shape and a long tail, probably due to tidal interactions. The SFH of the tail differs from that of the main body mainly for enhanced activity at recent epochs. likely triggered by the interaction.