The Tip of the Red Giant Branch Distances to Type Ia Supernova Host
  Galaxies. IV. Color Dependence and Zero-Point Calibration
  We present a revised TRGB calibration, accurate to 2.7% of distance. A
modified TRGB magnitude corrected for the color dependence of the TRGB, 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 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 times
smaller than those of the 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 ((F606W-F814W)_0
> 2.5). We provide the revised TRGB calibration in several magnitude systems
for future studies.
