Stellar populations of nearby galaxies

This thesis presents a study of the resolved stellar populations of two nearby galaxies, M31 and M33. Using a combination of high resolution infrared and optical observations, I analyze the properties of individual stars in these galaxies' bulges and globular clusters, looking for clues that will shed light on how these and other galaxies formed and evolve.; After a brief discussion the issues of calibrating our HST-NICMOS observations, I describe the problems associated with measuring stellar luminosities in crowded extragalactic fields. I have developed a new technique for modeling such crowded observations, and show how these simulations allow one to quantify and correct for the effects of limited image blending.; Using these simulations I analyze HST-NICMOS observations of five of M31's most metal rich globular clusters. After statistically subtracting the field population, I estimate the cluster metallicities based on the slopes of their infrared giant branches. Using the infrared luminosity functions of the two least compact clusters I show that these clusters are similar to Galactic globular clusters.; Next I study the stellar populations present in 18 fields in M31, and address the decade-old controversy surrounding the presence of an intermediate age stellar population in the bulge of M31. I show that previous claims were based on inadequate resolution, causing the light of many stars to blend together and be misinterpreted as single, luminous stars. I show that the luminosity function in the bulge of M31 is consistent with that measured in the Galactic Bulge, and using fields with varying bulge to disk ratios, I argue for a single luminosity function independent of Galactocentric distance.; Finally, using infrared Gemini-North observations I also study the central region of M33. These measurements show that the underlying old stellar population is slightly more metal poor than in our galaxy, and that there have been episodes of star formation as recently as 0.5 Gyr ago. Going against the common belief of a universal luminosity function, I show that the LF has a significantly different slope than that measured in the bulge of our Galaxy. I also derive surface brightness profiles and use them to show that M33 is best fit by a three-component, core + bulge + disk model. Finally I look for radial variations in the stellar properties, but find that both the CMDs and LFs show no significant change from 3--10″.