The Tip of the Red Giant Branch Distances to Type Ia Supernova Host Galaxies. IV. Color Dependence and Zero-point Calibration

Abstract

We present a revised Tip of the Red Giant Branch (TRGB) calibration, accurate to 2.7% of distance. A modified TRGB magnitude corrected for its color dependence, the QT magnitude, is introduced for better measurement of the TRGB. We determine the color-magnitude relation of the TRGB from photometry of deep images of HST/ACS fields around eight nearby galaxies. The zero-point of the TRGB at the fiducial metallicity ([Fe/H] = -1.6 ({(V-I)}_0,{TRGB}=1.5)) is obtained from photometry of two distance anchors, NGC 4258 (M106) and the Large Magellanic Cloud (LMC), to which precise geometric distances are known: M_QT,TRGB = -4.023 ± 0.073 mag from NGC 4258 and M_QT,TRGB = -4.004 ± 0.096 mag from the LMC. A weighted mean of the two zero-points is M_QT,TRGB = -4.016 ± 0.058 mag. Quoted uncertainty is ∼2× smaller than those of previous calibrations. We compare the empirical TRGB calibration derived in this study with theoretical stellar models, finding that there are significant discrepancies, especially for red color ({({{F}}606{{W}}-{{F}}814{{W}})}₀≳ 2.5). We provide the revised TRGB calibration in several magnitude systems for future studies.