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

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The Carnegie-Chicago Hubble Program (CCHP) seeks to anchor the distance scale of Type Ia supernovae via the Tip of the Red Giant Branch (TRGB) method. 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 was host to the Type Ia supernova SN 2012fr. We have measured the extinction-corrected TRGB magnitude of NGC 1365 to be F814W =27.34 ±0.03 stat ±0.04 sys mag. In advance of future direct calibration by Gaia, we adopt a provisional I-band TRGB luminosity set at the Large Magellanic Cloud and find a true distance modulus μ 0 =31.29 ±0.04 stat ±0.06 sys mag or D =18.1 ±0.3 stat ±0.5 sys Mpc. This measurement is in excellent agreement with recent Cepheid-based distances to NGC 1365 and reveals no significant difference in the distances derived from stars of Populations I and II for this galaxy. We revisit the error budget for the CCHP path to the Hubble constant based on the analysis presented here, i.e., that for one of the most distant Type Ia supernova hosts within our Program, and find that a 2.5% measurement is feasible with the current sample of galaxies and TRGB absolute calibration.

Original languageEnglish
Article number60
Number of pages15
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 5 Jan 2018


  • galaxies: distances and redshifts
  • galaxies: individual (NGC 1365)
  • stars: Population II

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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