Empirical Approaches to the Near-Infrared Tip of the Red Giant Branch

Abstract

The infrared tip of the red giant branch (IR-TRGB) is a powerful tool for measuring distances to galaxies in the local Universe. However, establishing its absolute "anchor" with sufficient precision and accuracy has proved challenging due to lingering sources of systematic uncertainty in both theoretical predictions and empirical calibrations. Here I describe three studies aimed at improving observational constraints on the IR-TRGB. First, I develop a computational method for self-consistently measuring TRGB magnitudes and colors, and the covariance thereof, in multiwavelength stellar photometry catalogs. Traditional detection methods either marginalize over color, or else rely on assuming a fidicial color dependence, both of which may result in different stellar populations dominating the TRGB signal at different wavelengths. Next, I explore two complementary approaches to reconciling observations of the IR-TRGB as measured with ground- and space-based instruments respectively. The former are impacted by absorption features in Earth's atmosphere, where the latter are not. Furthermore, there is an extremely limited range of magnitudes at which current facilities can achieve the requisite data quality from both locations. To this end, I derive transformation equations between respective photometric systems using synthetic photometry of observed stellar spectra, and then present initial results from a three-year observing campaign designed to directly compare space- and ground-based observations of bright RGB stars in the Magellanic Clouds.