The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch (1907.05922v1)

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.

• The paper presents an independent Hubble constant measurement using the TRGB method to yield H₀ = 69.8 ± 0.8 (stat) ± 1.7 (sys) km/s/Mpc.
• It employs deep HST imaging with LMC calibration and Type Ia supernovae data to attain nearly 1% distance precision.
• The study highlights the TRGB method’s role in addressing the Hubble tension by offering a robust alternative to Cepheid-based measurements.

An Independent Determination of the Hubble Constant Using the Tip of the Red Giant Branch

The Carnegie-Chicago Hubble Program (CCHP) aims to provide an independent estimate of the Hubble constant ($H_0$) using the Tip of the Red Giant Branch (TRGB) as a fundamental distance indicator. In a climate where different methods yield conflicting values of the Hubble constant—a discrepancy known as the "Hubble tension"—this research offers an alternative approach to measuring cosmic expansion rates. The paper reported a value of $$H_0 = 69.8 \pm 0.8 (\text{stat}) \pm 1.7 (\text{sys})$$ km/s/Mpc, positioning it between the values attained by the Planck satellite and other local measurements, specifically those based on Cepheid variable stars.

Methodology and Results

The TRGB method is a robust means of determining distances to galaxies, hinging on the brightness of red giant branch stars at the onset of helium burning. This technique leverages the TRGB's minimal dependence on metallicity and extinction by galactic dust, particularly when stars are observed in galaxy halos. Within the CCHP, distances to galaxies hosting Type Ia supernovae were calibrated using TRGB stars, leading to an $H_0$ value closely aligning with Planck's CMB measurements ($67.4 \pm 0.5$ km/s/Mpc), and less so with measurements relying on Cepheid variables, such as those reported by the SHoES team ($74.03 \pm 1.42$ km/s/Mpc).

A key aspect of this research involves using deep imaging from the Hubble Space Telescope’s Advanced Camera for Surveys to perform TRGB measurements, calibrated using the Large Magellanic Cloud (LMC) as a zero-point anchor. With such calibrations, equating the TRGB distance scale to a precision verging on 1% is projected, bolstered by ongoing Gaia parallax updates. Moreover, the Carnegie Supernova Project I provides Type Ia supernovae measurements which serve as distant anchors for these newly computed distances, applying both old and current supernova datasets ensuring reliability and extended reach.

Implications

The implications of such a meticulously calculated $H_0$ are multi-faceted. The TRGB method offers a complementary check on the accuracy of locally derived $H_0$ values compared to those inferred from cosmological data sets such as the CMB. The paper substantiates that the TRGB method not only circumvents potential issues tied to Cepheid-based distance scales (e.g., variable extinction and metallicity effects) but also brings to light the critical role of understanding and resolving the Hubble tension. Differences noted between the Cepheid and TRGB $H_0$ values underscore the importance of refining calibration techniques and highlight the necessity of continued investigation into systematic errors in distance scaling.

Future Directions

Looking forward, the TRGB analysis exemplified by the CCHP proposes pivotal advancements. With improved parallax data from future Gaia releases and continued observations using space-based telescopes like JWST, an even more precise determination of $H_0$ is attainable. These steps are crucial in setting a benchmark for local universe value concordance with cosmological models, either affirming or challenging the standard $\Lambda$CDM model.

In conclusion, this paper demonstrates that the TRGB distance method is not only viable but essential in refining our understanding of the universe's expansion rate. By leveraging the strengths of this alternative method, and coupling it with current datasets, the authors contribute critical data towards solving one of astronomy's most pressing dilemmas.