Star Formation Histories from Hubbl E Space T El Escope Color-magnitude Diagrams of Six Fields of the Large Magellanic Cloud1

We present results on the analysis of background Ðeld stars found in Hubble Space T elescope Wide Field Planetary Camera 2 observations of six of the old globular clusters of the Large Magellanic Cloud. Treated as contaminants by the globular cluster analysts, we produce (V [I, V) color-magnitude diagrams (CMDs) of the Ðeld stars and use them to explore the LMCÏs star formation history. The photometry approaches V D 26, well below the turno† of an ancient (D14 Gyr) LMC population of stars. The Ðeld star CMDs are generally characterized by an upper main sequence broadened by stellar evolution, an old red giant branch, a prominent red clump, and an unevolved lower main sequence. The CMDs also contain a few visual di†erences, the most obvious of which is the smeared appearance of the NGC 1916 Ðeld caused by heavy di†erential reddening. More subtly, the base of the subgiant branch near the old turno† appears extended in V , and the red giant branch appears broad in V [I in four of the Ðelds but not in the NGC 1754 Ðeld. We use a maximum likelihood technique to Ðt model CMDs drawn from Bertelli et al. isochrones to the observed CMDs. We constrain the models by the age-metallicity relation derived from LMC clusters, test four initial mass function (IMF) slopes, and Ðt for the reddening, distance modulus, and star formation rate. We Ðnd that we can just resolve structure in SFR(t) with time steps of D0.15 in log (age), implying a resolution of D4 ] 108 yr at an age of 1 Gyr. For a Salpeter IMF, our derived star formation history for the NGC 1754 Ðeld is characterized by an enhanced star formation rate over the past 4 Gyr, qualitatively resembling that derived by others for a variety of LMC Ðelds. The remaining four Ðelds, however, appear to have had high levels of star formation activity as long as 5È8 Gyr ago ; these Ðelds lie in the LMC bar whereas the NGC 1754 Ðeld lies in the disk, suggesting that the inner regions of the LMC contain signiÐcantly more older stars than the outer regions. Examining the residuals of the models and observations, we Ðnd that the old red giant branches of the models provide a poor Ðt to the observations, which suggests an error in the model isochrones. The e†ect of the disagreement appears to be to underestimate the contribution of the old population.