MCR-TRGB: A Multiwavelength-covariant, Robust Tip of the Red Giant Branch Measurement Method

Abstract

We present a new method to measure colors and magnitudes of the tip of the red giant branch (TGRB) in multiple bandpasses simultaneously by fitting an n-dimensional Gaussian to photometry of candidate tip stars. We demonstrate that this method has several advantages over traditional edge detection, particularly in regimes where the TRGB magnitude is strongly color dependent, as is the case in the near-infrared (NIR). We apply this method to a re-reduction of a set of optical and NIR Hubble Space Telescope data originally presented in Dalcanton et al. (D12). The re-reduction takes advantage of the increased depth and accuracy in the NIR photometry enabled by simultaneous reduction with higher-resolution optical data in crowded fields. We compare three possible absolute calibrations of the resulting apparent TRGB measurements, one adopting the same distance moduli as in D12 and two based on predicted TRGB absolute magnitudes from two widely used, modern sets of model isochrones. We find systematic offsets among the model absolute calibrations at the ∼0.15 mag level, in line with previous investigations. The models also have difficulty reproducing the optical-NIR color-magnitude behavior of our measurements, making these observations a useful benchmark for future improvements. ^* Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.