Using the Tip of the Red Giant Branch as a Distance Indicator in the
  Near Infrared
  The tip of the red giant branch (TRGB) is a well-established standard candle
used to measure distances to nearby galaxies. The TRGB luminosity is typically
measured in the I-band, where the luminosity has little dependency on stellar
age or stellar metallicity. As the TRGB is brighter at wavelengths redder than
the I-band, observational gains can be made if the TRGB luminosity can be
robustly calibrated at longer wavelengths. This is of particular interest given
the infrared capabilities that will be available with the James Webb Space
Telescope and an important calibration consideration for using TRGB distances
as part of an independent measurement of the Hubble constant. Here, we use
simulated photometry to investigate the dependency of the TRGB luminosity on
stellar age and metallicity as a function of wavelength (475 nm - 4.5 micron).
We find intrinsic variations in the TRGB magnitude to increase from a few
hundredths of a magnitude at 800-900 nm to ~0.6 mag by 1.5 micron. We show that
variations at the longer infrared wavelengths can be reduced to 0.02-0.05 mag
(1-2% accuracy in distance) with careful calibrations that account for changes
in age and metal content. These represent the minimum uncertainties;
observational uncertainties will be higher. Such calibration efforts may also
provide independent constraints of the age and metallicity of stellar halos
where TRGB distances are best measured. At 3.6 and 4.5 micron, the TRGB
magnitude is predicted to vary up to ~0.15 mag even after corrections for
stellar age and metallicity, making these wavelengths less suitable for
precision distances.
