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

## TL;DR

This paper measures the distance to NGC 1365 using the Tip of the Red Giant Branch method with Hubble Space Telescope data, providing a precise distance estimate that supports existing Cepheid-based measurements and informs the Hubble Constant determination.

## Contribution

It presents a high-precision TRGB distance measurement to NGC 1365, demonstrating consistency with Cepheid distances and refining the error budget for Hubble Constant calculations.

## Key findings

- TRGB magnitude for NGC 1365 is F814W = 27.34 mag
- Distance modulus to NGC 1365 is 31.29 mag (18.1 Mpc)
- Measurement supports no significant difference between Population I and II distance estimates

## Abstract

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 $D = 18.1 \pm 0.3_{stat} \pm0.4_{sys}$ 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.