RESOLVED NEAR-INFRARED STELLAR POPULATIONS IN NEARBY GALAXIES

Abstract

We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the\nresolved stellar populations within 26 fields of 23 nearby galaxies (<4 Mpc),\nbased on F110W and F160W images from Wide Field Camera 3 (WFC3) on the Hubble\nSpace Telescope (HST). The CMDs sample both old dormant and young star-forming\npopulations. We match key NIR CMD features with their counterparts in optical\nCMDs, and identify the red core Helium burning (RHeB) sequence as a significant\ncontributor to the NIR flux in stellar populations younger than a few 100 Myrs\nold, suggesting that star formation can drive surprisingly rapid variations in\nthe NIR mass-to-light ratio. The NIR luminosity of star forming galaxies is\ntherefore not necessarily proportional to the stellar mass. We note that these\nindividual bright RHeB stars may be misidentified as old stellar clusters in\nlow resolution imaging. We also discuss the CMD location of asymptotic giant\nbranch (AGB) stars, and the separation of AGB sub-populations using a\ncombination of optical and NIR colors. We empirically calibrate the NIR\nmagnitude of the tip of the red giant branch (TRGB) as a function of color,\nallowing this widely adopted filter to be used for distance measurements. We\nfind a clear trend between NIR RGB color and metallicity. However, it appears\nunlikely that the slope of the NIR RGB can be used as a metallicity indicator\nin extragalactic systems with comparable data. Finally, we discuss scattered\nlight in the WFC3, which becomes significant for exposures taken close to a\nbright earth limb.\n