HST/ACS observations of the old and metal-poor Sagittarius dwarf irregular galaxy

We have obtained deep color-magnitude diagrams (CMDs) of the Sagittarius dwarf irregular galaxy with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. The new diagrams show for the first time the star formation history of SagDIG. The young main-sequence is traced down to m(F606W) similar or equal to 27.5, well separated from the older He burning blue loop stars. The wide color range spanned by the blue and red super-giants is consistent with Padua evolutionary models with Z similar to 0.0004. A well-defined red giant branch indicates that the galaxy is dominated by a stellar population older than 1 Gyr, for which we have estimated a metallicity in the range [Fe/H] = - 2.2 to - 1.9 depending on the adopted reddening. The identification of several C stars on a luminous asymptotic giant branch, as well as the presence of a conspicuous red clump with an elongated structure, are indicative of an extended star formation epoch between 1 and 10 Gyr ago. Perhaps most importantly, we have identified a small population of stars on a genuinely old red horizontal branch at mF606W = 25.70 +/- 0.05, which sets the first epoch of star formation in SagDIG back to similar to 9-10 Gyr ago. This shows that SagDIG has been able to maintain a very low mean metallicity over a 10 Gyr life time, and that all Local Group dwarf galaxies share an ancient (= 10 Gyr) initial episode of star formation. Thus, at the moment, IZw18 remains the only exception to this general trend, favoring the so-called young galaxy scenario where a very low metallicity galaxy is not necessarily old. However, a re-analysis of ACS observations of IZw18 suggests that an older RGB population may be present also in IZw18. In order to address the interplay between stars and Inter-Stellar Medium, we compare the distribution of atomic hydrogen with that of stellar populations of various ages. The youngest stars are found in clumps located: (i) near to, but not coincident with, the highest column densities of HI, and (ii) in tails reminiscent of propagating star formation. We find, however, little correlation between stellar populations older than similar to 1 Gyr and the HI. In particular we fail to find any generation of stars that preferentially lie within the large HI hole. We therefore exclude the possibility that the ring-like HI distribution is the result of multiple supernova events. Alternative scenarios have to be explored for the ring-like HI morphology of SagDIG, which amongst dwarf irregular galaxies, is not unusual.