The Population II Extragalactic Distance Scale: A TRGB Distance to the Fornax Cluster with JWST

Gagandeep S. Anand, R. Brent Tully, Yotam Cohen, Dmitry I. Makarov, Lidia N. Makarova, Joseph B. Jensen, John P. Blakeslee, Michele Cantiello, Ehsan Kourkchi, Gabriella Raimondo

Published 2024-05-06Version 1

Differences between the local value of the Hubble constant measured via the distance ladder versus the value inferred from the cosmic microwave background with the assumption of the standard $\Lambda$CDM model have reached over 5$\sigma$ significance. To determine if this discrepancy is due to new physics or more mundane systematic errors, it is essential to remove as many sources of systematic uncertainty as possible by developing high-precision distance ladders that are independent of the traditional Cepheid and Type Ia supernovae route. Here we present JWST observations of three early-type Fornax Cluster galaxies, the first of fourteen observations from a Cycle 2 JWST program. Our modest integration times allow us to measure highly precise tip of the red giant branch (TRGB) distances, and will also be used to perform measurements of Surface Brightness Fluctuations (SBF). From these three galaxies, we determine an average TRGB distance modulus to the Fornax Cluster of $\mu$ = 31.424 $\pm$ 0.077 mag, or D = 19.3 $\pm$ 0.7 Mpc. With eleven more scheduled observations in nearby elliptical galaxies, our program will allow us set the zero point of the SBF scale to better than 2$\%$ for more distant measurements, charting a path towards a high-precision measurement of $H_{0}$ that is independent of the traditional Cepheid-SN Ia distance ladder.