Resolved Near-Infrared Stellar Populations in Nearby Galaxies
  We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the
resolved stellar populations within 26 fields of 23 nearby galaxies (<4 Mpc),
based on F110W and F160W images from Wide Field Camera 3 (WFC3) on the Hubble
Space Telescope (HST). The CMDs sample both old dormant and young star-forming
populations. We match key NIR CMD features with their counterparts in optical
CMDs, and identify the red core Helium burning (RHeB) sequence as a significant
contributor to the NIR flux in stellar populations younger than a few 100 Myrs
old, suggesting that star formation can drive surprisingly rapid variations in
the NIR mass-to-light ratio. The NIR luminosity of star forming galaxies is
therefore not necessarily proportional to the stellar mass. We note that these
individual bright RHeB stars may be misidentified as old stellar clusters in
low resolution imaging. We also discuss the CMD location of asymptotic giant
branch (AGB) stars, and the separation of AGB sub-populations using a
combination of optical and NIR colors. We empirically calibrate the NIR
magnitude of the tip of the red giant branch (TRGB) as a function of color,
allowing this widely adopted filter to be used for distance measurements. We
find a clear trend between NIR RGB color and metallicity. However, it appears
unlikely that the slope of the NIR RGB can be used as a metallicity indicator
in extragalactic systems with comparable data. Finally, we discuss scattered
light in the WFC3, which becomes significant for exposures taken close to a
bright earth limb.
