MCR-TRGB: A Multiwavelength-Covariant, Robust Tip of the Red Giant
  Branch Measurement Method
  We present a new method to measure colors and magnitudes of the tip of the
red giant branch 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. We apply this method to a re-reduction of
a set of optical and near-IR HST data originally presented in Dalcanton et al.
(2012). 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 (Williams et al. 2014). We compare three
possible absolute calibrations of the resulting apparent TRGB measurements, one
adopting the same distance moduli as in Dalcanton et al. (2012), 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 ~0.1 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.
