Resolved Near-Infrared Stellar Populations in Nearby Galaxies

TL;DR

This study uses near-infrared color-magnitude diagrams from HST to analyze stellar populations in nearby galaxies, revealing rapid NIR flux variations due to young stars and calibrating the NIR tip of the red giant branch for distance measurement.

Contribution

It provides new insights into NIR stellar population features, calibrates the NIR TRGB for distance estimation, and discusses the impact of star formation on NIR mass-to-light ratios.

Findings

Red core Helium burning stars significantly affect NIR flux in young populations.

NIR luminosity is not directly proportional to stellar mass in star-forming galaxies.

Empirical calibration of the NIR TRGB as a function of color.

Abstract

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.