2019ApJ...885..141B

The Carnegie-Chicago Hubble Program (CCHP) is building a direct path to the Hubble constant (H₀) using Population II stars as the calibrator of the Type Ia supernova (SN Ia)-based distance scale. This path to calibrate the SNe Ia is independent of the systematics in the traditional Cepheid-based technique. In this paper, we present the distance to M101, the host to SN 2011fe, using the I-band tip of the red giant branch (TRGB) based on observations from the ACS/WFC instrument on the Hubble Space Telescope. The CCHP targets the halo of M101, where there is little to no host galaxy dust, the red giant branch is isolated from nearly all other stellar populations, and there is virtually no source confusion or crowding at the magnitude of the tip. Applying the standard procedure for the TRGB method from the other works in the CCHP series, we find a foreground-extinction-corrected M101 distance modulus of µ₀ = 29.07 ± 0.04_stat ± 0.05_sys mag, which corresponds to a distance of D = 6.52 ± 0.12_stat ± 0.15_sys Mpc. This result is consistent with several recent Cepheid-based determinations, suggesting agreement between Population I and II distance scales for this nearby SN Ia host galaxy. We further analyze four archival data sets for M101 that have targeted its outer disk to argue that targeting in the stellar halo provides much more reliable distance measurements from the TRGB method owing to the combination of multiple structural components and heavy population contamination. Application of the TRGB in complex regions will have sources of uncertainty not accounted for in commonly used uncertainty measurement techniques.