A deep survey of the Fornax dSph. I. Star formation history

Based on a deep imaging survey, we present the first homogeneous star formation history (SFH) of the Fornax dwarf spheroidal (dSph) galaxy. We have obtained two-filter photometry to a depth of B similar to 23 over the entire surface of Fornax, the brightest dSph associated with the Milky Way, and derived its SFH using a CMD-fitting technique. We show that Fornax has produced the most complex star formation and chemical enrichment histories of all the Milky Way dSphs. This system has supported multiple epochs of star formation. A significant number of stars were formed in the early universe; however, the most dominant population are the intermediate-age stars. This includes a strong burst of star formation approximately 3-4 Gyr ago. Significant population gradients are also evident. Similar to other dSphs, we have found that recent star formation was concentrated toward the center of the system. Furthermore, we show that the central region harbored a faster rate of chemical enrichment than the outer parts of Fornax. At the center, the ancient stars (age > 10 Gyr) display a mean metallicity of [Fe/H] similar to -1.4, with evidence for three peaks in the metallicity distribution. Overall, enrichment in Fornax has been highly efficient: the most recent star formation burst has produced stars with close to solar metallicity. Our results support a scenario in which Fornax experienced an early phase of rapid chemical enrichment, producing a wide range of abundances. Star formation gradually decreased until similar to 4 Gyr ago, when Fornax experienced a sudden burst of strong star formation activity accompanied by substantial chemical enrichment. Weaker star-forming events followed, and we have found tentative evidence for stars with ages less than 100 Myr.