Tip of the Red Giant Branch Distances. II. Zero‐Point Calibration

The luminosity of the Tip of the Red Giant Branch (TRGB) provides anexcellent measure of galaxy distances and is easily determined in the resolvedimages of nearby galaxies observed with Hubble Space Telescope. There is now alarge amount of archival data relevant to the TRGB methodology and which offerscomparisons with other distance estimators. Zero-point issues related to theTRGB distance scale are reviewed in this paper. Consideration is given to themetallicity dependence of the TRGB, the transformations between HST flightsystems and Johnson-Cousins photometry, the absolute magnitude scale based onHorizontal Branch measurements, and the effects of reddening. The zero-point ofthe TRGB is established with a statistical accuracy of 1%, modulo theuncertainty in the magnitude of the Horizontal Branch, with a typical rmsuncertainty of 3% in individual galaxy distances at high Galactic latitude. Thezero-point is consistent with the Cepheids period-luminosity relation scale butinvites reconsideration of the claimed metallicity dependence with that method.The maser distance to NGC 4258 is consistent with TRGB but presently has loweraccuracy.Comment: accepted for publication into Ap