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

Abstract

The luminosity of the Tip of the Red Giant Branch (TRGB) provides an\nexcellent measure of galaxy distances and is easily determined in the resolved\nimages of nearby galaxies observed with Hubble Space Telescope. There is now a\nlarge amount of archival data relevant to the TRGB methodology and which offers\ncomparisons with other distance estimators. Zero-point issues related to the\nTRGB distance scale are reviewed in this paper. Consideration is given to the\nmetallicity dependence of the TRGB, the transformations between HST flight\nsystems and Johnson-Cousins photometry, the absolute magnitude scale based on\nHorizontal Branch measurements, and the effects of reddening. The zero-point of\nthe TRGB is established with a statistical accuracy of 1%, modulo the\nuncertainty in the magnitude of the Horizontal Branch, with a typical rms\nuncertainty of 3% in individual galaxy distances at high Galactic latitude. The\nzero-point is consistent with the Cepheids period-luminosity relation scale but\ninvites reconsideration of the claimed metallicity dependence with that method.\nThe maser distance to NGC 4258 is consistent with TRGB but presently has lower\naccuracy.\n