Stellar populations in the Large Magellanic Cloud: Evidence for a significant number of older stars or a steeper IMF?

Abstract

We present deep photometry obtained with the HST in an outer LMC field. A well-defined main sequence is seen down to V > 26. We derive a luminosity function from the data and use it to constrain the IMF and the star formation history. We derive limits on the IMF slope, ɑ(with dN/∝dM^ɑ), from stars on the main sequence which are fainter than the oldest turnoff. For most choices of star formation history and metallicity, we derive slopes which are consistent the Salpeter (ɑ= -2.35) or local solar neighborhood IMF, although the preferred values are steeper. We can rule out IMF slopes shallower than -1.6 and steeper than -3.1 for the mass range 0.6≾M≾1.1 M_☉. Assuming a Salpeter IMF over the entire observed mass range, we derive star formation histories from the entire luminosity function, which covers the mass range 0.6≾M≾1.1 3 M_☉. We find that the luminosity function is inconsistent with the scenario in which the bulk of the field stars in the LMC are younger than 4 Gyr. Instead, we find that there must be a comparable number of stars older and younger than 4 Gyr. Our best model has a star formation rate which is roughly constant for 10 Gyr then increases by about a factor of three for the past 2 Gyr. Such a model is also roughly consistent with the distribution of stars in the color-magnitude diagram. Similar model parameters are derived if we adopt the Kroupa, Tout, and Gilmore solar neighborhood LMF instead of a Salpeter slope. Alternatively, we can fit the luminosity function with a predominantly young population if we use a steeper single power law IMF slope with ɑ ~-2.75 over the entire range of observed masses.