Stellar Populations in the Large Magellanic Cloud from 2MASS

Abstract

We present a morphological analysis of the feature-rich 2MASS LMC color-magnitude diagram, identifying Galactic and LMC populations and estimating the density of LMC populations alone. We also present the projected spatial distributions of various stellar populations. The conditions that prevailed when 2MASS observed the LMC provided 10 σ limiting sensitivity for J 16.3, H 15.3, and Ks 14.7. Major populations are identified based on matching morphological features of the color-magnitude diagram with expected positions of known populations, isochrone fits, and analysis of the projected spatial distributions. 2MASS has detected a significant population of asymptotic giant branch (AGB) stars (104 sources) and obscured AGB stars (2 × 103 sources). The LMC populations along the first-ascent red giant branch (RGB) and AGB are quantified. Comparison of the giant luminosity functions in the bar and the outer regions of the LMC shows that both luminosity functions appear consistent with each other. The luminosity function in the central (bar) field has well-defined drop-off near Ks = 12.3 mag corresponding to the location of the RGB tip; the same feature is seen in the luminosity function of the entire LMC field. Isochrone fits for the corresponding giant branches reveal no significant differences in metallicities and ages between central and outer regions of the LMC. This may be evidence for strong dynamical evolution in the last several gigayears. In particular, the observed LMC giant branch is well fit by published tracks in the CIT/CTIO system with a distance modulus of μ = 18.5 ± 0.1, reddening EB-V = 0.15-0.20, metallicity Z = 0.004, and age 3-13 Gyr. Analysis of deep 2MASS engineering data with 6 times the standard exposure produces similar estimates.