The star formation history of the large magellanic cloud

We present deep photometric observations of stars in three fields of the Large Magellanic Cloud (LMC) and interpret these data using synthetic color-magnitude diagrams (CMDs) and luminosity functions (LFs) generated from the overshoot models of Bertelli et al., Bressan et al., and Aparicio et al. We can successfully model the field CMDs and LFs with a star formation rate that experienced a large increase (4 +/- 0.5) x 10(9) yr ago. The precise age of this "burst" depends sensitively on the characteristics of the models. Classical (i.e., nonovershoot) models yield a burst age about 2 x 10(9) yr younger than the value we obtain. An initial mass function with slope of 2.35 (the Salpeter value) and a mean field star metallicity of [Fe/H] approximately -0.7 are consistent with the photometric data and LFs. Furthermore, our primary conclusion, that is, the possibility of a single burst of star formation, depends on the distance modulus adopted for LMC. If (m - M)0 = 18.4, we find evidence for a single burst throughout the LMC. If (m - M)0 = 18.6, this is no longer possible. Given the differences between the ages derived using overshoot and classical models, the age distribution we derive for LMC field stars older than about 1 x 10(9) yr is similar to that found by earlier studies using deep photographic observations of other fields throughout the LMC, and with the observed age distribution of intermediate-age and old LMC star clusters. This suggests that the star formation rate in the LMC was globally quite low during at least the first half of its lifetime, and that a major event triggered a substantial and relatively sudden increase in the star formation rate throughout the entire LMC which persisted for several 10(9) yr and even up to the present epoch in some parts of that galaxy.