Small typo correction. -1112348723 Assigned ID: 1746 ----- Attempting Submission 1 (Fri Mar 01 20:45:01 GMT 2013) ----- HST Phase I Proposal 1746 (LGIMF.apt) successfully submitted. ----- Attempting Submission 2 (Sat Mar 02 00:22:04 GMT 2013) ----- HST Phase I Proposal 1746 (LGIMF.apt) successfully submitted. ----- Attempting Submission 3 (Sat Mar 02 00:39:00 GMT 2013) ----- There are many theoretical reasons to expect that the stellar initial mass function (IMF) may not be universal across all types of galaxies and eras of star formation, but the empirical evidence for IMF variations is only now becoming compelling. Recent work using multiple complementary indirect methods is converging on the result that more massive, higher metallicity systems contain more bottom-heavy IMFs. Geha et al. (2013) and Kalirai et al. (2013) have just obtained similarly dramatic results with a much more direct and model-independent method: counting stars below the oldest main sequence turnoff in the closest galaxies to the Milky Way. Reliable IMF measurements using this technique are available in the literature for only four galaxies: the Milky Way, the Small Magellanic Cloud, and two ultra-faint dwarfs, which show extremely bottom-light IMFs. We propose to use identical analysis techniques on archival WFPC2, ACS, and WFC3 imaging to determine the IMF from 0.5 to 0.75 Msun in four additional galaxies: the Large Magellanic Cloud, Sagittarius, Ursa Minor, and Draco. These measurements will substantially improve our knowledge of the dependence of the IMF on different galaxy properties such as metallicity and mass, providing new constraints on the physics responsible for IMF variations. RESOLVED STELLAR POPULATIONS Default Small false false false false false SUPPORTED