The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch

TL;DR

The paper addresses the ongoing discrepancy between Planck CMB inferences and local determinations of the Hubble constant $H_{0}$. It implements an independent Cepheid-free distance ladder using the Tip of the Red Giant Branch (TRGB) measured in galaxy halos to calibrate Type Ia supernovae, anchored by the Large Magellanic Cloud distance from detached eclipsing binaries and a Spitzer-calibrated Cepheid Leavitt law. Applying this to the CSP-I SN Ia sample, the authors obtain $H_{0} = 69.8 \,\pm\ 0.8 \,(\mathrm{stat}) \,\pm\ 1.7 \,(\mathrm{sys})$ km s$^{-1}$ Mpc$^{-1}$, placing the result between Planck and SHoES within roughly 1–2 sigma. The TRGB-based, halo-focused method provides a competitive, Cepheid-independent cross-check of the local expansion rate and reinforces the value of diversified distance ladders in diagnosing the Hubble tension.

Abstract

We present a new and independent determination of the local value of the Hubble constant based on a calibration of the Tip of the Red Giant Branch (TRGB) applied to Type Ia supernovae (SNeIa). We find a value of Ho = 69.8 +/- 0.8 (+/-1.1\% stat) +/- 1.7 (+/-2.4\% sys) km/sec/Mpc. The TRGB method is both precise and accurate, and is parallel to, but independent of the Cepheid distance scale. Our value sits midway in the range defined by the current Hubble tension. It agrees at the 1.2-sigma level with that of the Planck 2018 estimate, and at the 1.7-sigma level with the SHoES measurement of Ho based on the Cepheid distance scale. The TRGB distances have been measured using deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of galaxy halos. The zero point of the TRGB calibration is set with a distance modulus to the Large Magellanic Cloud of 18.477 +/- 0.004 (stat) +/-0.020 (sys) mag, based on measurement of 20 late-type detached eclipsing binary (DEB) stars, combined with an HST parallax calibration of a 3.6 micron Cepheid Leavitt law based on Spitzer observations. We anchor the TRGB distances to galaxies that extend our measurement into the Hubble flow using the recently completed Carnegie Supernova Project I sample containing about 100 well-observed SNeIa. There are several advantages of halo TRGB distance measurements relative to Cepheid variables: these include low halo reddening, minimal effects of crowding or blending of the photometry, only a shallow (calibrated) sensitivity to metallicity in the I-band, and no need for multiple epochs of observations or concerns of different slopes with period. In addition, the host masses of our TRGB host-galaxy sample are higher on average than the Cepheid sample, better matching the range of host-galaxy masses in the CSP distant sample, and reducing potential systematic effects in the SNeIa measurements.

Figures (2)

• Figure 1: Images of the HST/ACS images for the nine TRGB galaxy fields observed as part of our program. The halo target fields in each case are outlined by boxes. The hatched regions indicate those analyzed in this study. North is up, east is to the left.
• Figure 2: Images of the HST images for the eight TRGB galaxy fields analyzed by jang_lee_2017b. NGC 5584 was observed with $WFC3$; the remaining 7 galaxies were observed with $ACS$. The Jang & Lee halo target fields in each case are outlined by boxes.The upper three images for M101, M66 and M96 are shown for comparison only; distances to the galaxies have been independently measured as part of the CCHP for the fields illustrated in Figure 1. The hatched regions indicate those analyzed by jang_lee_2017b. North is up, east is to the left.