Comparative Analysis of TRGBs (CATs) from Unsupervised, Multi-Halo-Field Measurements: Contrast is Key

Abstract

The Tip of the Red Giant Branch (TRGB) is an apparent discontinuity in the color-magnitude diagram (CMD) along the giant branch due to the end of the red giant evolutionary phase and is used to measure distances in the local universe. In practice, the tip is often fuzzy and its localization via edge detection response (EDR) relies on several methods applied on a case-by-case basis. It is hard to evaluate how individual choices affect a distance estimation using only a single host field while also avoiding confirmation bias. To devise a standardized approach, we compare unsupervised, algorithmic analyses of the TRGB in multiple halo fields per galaxy, up to 11 fields for a single host and 50 fields across 10 galaxies, using high signal-to-noise stellar photometry obtained by the GHOSTS survey with the Hubble Space Telescope. We first optimize methods for the lowest field-to-field dispersion including spatial filtering to remove star forming regions, smoothing and weighting of the luminosity function, selection of the RGB by color, and tip selection based on the number of likely RGB stars and the ratio of stars above versus below the tip ($R$). We find $R$, which we call the tip `contrast', to be the most important indicator of the quality of EDR measurements; we find that field-to-field EDR repeatability varies from 0.3 mag to $\leq$ 0.05 mag for $R=4$ to 7, respectively, though less than half the fields reach the higher quality. Further, we find that $R$, which varies with the age/metallicity of the stellar population based on models, correlates with the magnitude of the tip (and after accounting for low internal extinction), i.e., a tip-contrast relation with slope of $-0.023\pm0.0046$ mag/ratio, a $\sim 5\sigma$ result that improves standardization of the TRGB. We discuss the value of consistent TRGB standardization across rungs for robust distance ladder measurements.