The Carnegie-Chicago Hubble Program. III. The Distance to NGC 1365 via the Tip of the Red Giant Branch

In Sung Jang, Dylan Hatt, Rachael L. Beaton, Myung Gyoon Lee, Wendy L. Freedman, Barry F. Madore, Taylor J. Hoyt, Andrew J. Monson, Jeffrey A. Rich, Victoria Scowcroft, Mark Seibert

Published 2017-03-30Version 1

The Carnegie-Chicago Hubble Program seeks to anchor the distance scale of Type Ia supernovae via the Tip of the Red Giant Branch (TRGB). Based on deep $Hubble$ $Space$ $Telescope$ ACS/WFC imaging, we present an analysis of the TRGB for the metal-poor halo of NGC 1365, a giant spiral galaxy in the Fornax Cluster that is host to the supernova SN2012fr. We have measured its extinction-corrected TRGB magnitude to be F814W = 27.34 $\pm 0.03_{stat} \pm0.01_{sys}$ mag. In advance of future direct calibration by $Gaia$, we set a provisional TRGB luminosity via the Large Magellanic Cloud and find a true distance modulus $\mu_0 = 31.29 \pm 0.04_{stat}\pm0.05_{sys}$ mag or $D = 18.4 \pm 0.03_{stat} \pm0.04_{sys}$ Mpc. This high-fidelity measurement shows excellent agreement with recent Cepheid-based distances to NGC 1365 and suggests no significant difference in the distances derived from stars of Population I and II. We revisit the error budget for the $CCHP$ path to the Hubble Constant based on this analysis of one of our most distant hosts, finding a 2.5\% measurement is feasible with our current sample.