Calibration of the Tip of the Red Giant Branch (TRGB)

Abstract: The Tip of the Red Giant (TRGB) method provides one of the most accurate and precise means of measuring the distances to nearby galaxies. Here we present a VIJHK absolute calibration of the TRGB based on observations of TRGB stars in the Large Magellanic Cloud (LMC),grounded on detached eclipsing binaries (DEBs). This paper presents a more detailed description of the method first presented in Freedman et al. (2019) for measuring corrections for the total line-of-sight extinction and reddening to the LMC. In this method, we use a differential comparison of the red giant population in the LMC, first with red giants in the Local Group galaxy, IC 1613, and then with those in the Small Magellanic Cloud. As a consistency check, we derive an independent calibration of the TRGB sequence using the SMC alone, invoking its geometric distance also calibrated by DEBs. An additional consistency check comes from near-infrared observations of Galactic globular clusters covering a wide range of metallicities. In all cases we find excellent agreement in the zero-point calibration. We then examine the recent claims by Yuan et al. (2019), demonstrating that, in the case of the SMC, they corrected for extinction alone while neglecting the essential correction for reddening as well. In the case of IC 1613, we show that their analysis contains an incorrect treatment of (over-correction for) metallicity. Using our revised (and direct) measurement of the LMC TRGB extinction, we find a value of Ho = 69.6 +/-0.8 (+/-1.1% stat) +/- 1.7 (+/-2.4% sys) km/s/Mpc.

• The paper introduces a multi-wavelength calibration of the TRGB method, using differential analysis of LMC, SMC, and IC 1613 data to simultaneously correct for extinction and reddening.
• The paper validates the TRGB calibration by cross-referencing DEB-based geometric distances and Galactic globular cluster observations, confirming its reliability for measuring extragalactic distances.
• The paper computes a Hubble constant of 69.6 ± 0.8 (stat) ± 1.7 (sys) km s⁻¹ Mpc⁻¹, providing key insights for resolving discrepancies in cosmic expansion rate estimates.

Calibration of the Tip of the Red Giant Branch (TRGB)

The paper "Calibration of the Tip of the Red Giant Branch (TRGB)" by Freedman, Madore, et al. provides an analytical exploration into the distance measurement methodology using the TRGB as a standard candle. This method is central to determining accurate extragalactic distances, which directly impacts the precision of cosmological measurements such as the Hubble constant (H$_0$).

Overview of the TRGB Method

The TRGB method relies on the well-understood astrophysics of low-mass Red Giant Branch (RGB) stars, which undergo a crucial transition at a specific luminosity, referred to as the TRGB. This absolute luminosity is relatively constant across different stellar populations, making it a reliable distance indicator when combined with precise photometry. The present study meticulously calibrates the TRGB using observations from the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and IC 1613, anchored on distances derived from detached eclipsing binaries (DEBs).

Methodology and Results

The authors provide a multi-wavelength calibration across $VIJHK$ bands, ensuring robustness against individual band inaccuracies by utilizing a simultaneous extinction and reddening correction approach. A remarkable feature of this study is its use of both geometric and empirical distance measures to solidify TRGB zero points, yielding highly consistent results across independent evaluations.

1. Multi-Wavelength Approach: The authors utilized observed TRGB stars' properties to iteratively correct for both extinction and reddening simultaneously. This novel approach is supported by the differential analysis of the photometric data from the LMC compared with the SMC and IC 1613, which have low internal reddenings, providing a reliable standard.
2. Distance Calibration Consistency: The calibration achieved through the LMC was verified against DEB-based geometric distances for the SMC, highlighting the TRGB's reliability. The results were also cross-referenced with Galactic globular clusters' observations, further corroborating the calibration's validity through separate methodologies.
3. Hubble Constant Implications: Applying the TRGB calibration, the study deduces H$_0$ = 69.6 ± 0.8 (stat) ± 1.7 (sys) km s$^{-1}$ Mpc$^{-1}$. This figure, resting between earlier H$_0$ estimates, offers a potential resolution in the ongoing debate regarding discrepancies in the measured and theoretically predicted values of the Hubble constant.

Implications and Future Work

The study presents important implications for both theoretical and observational astronomy. By refining the TRGB distance method with precise calibrations, the research impacts how distances in cosmology are determined, influencing constraints on the universe's expansion rate and consequently, dark energy models. The method proposed promises to improve as Gaia parallax data becomes available for nearby TRGB stars, enhancing zero-point definitions.

Further, the TRGB method, with its advantage of operating in the less extincted halos of galaxies, provides a strong case for its application in measuring extragalactic distances across a diverse range of environments. As future observatories like the James Webb Space Telescope become operational, the calibration work laid out in this study sets a robust framework for high-precision astronomical measurements. Through these endeavors, this research plays a crucial role in addressing the present uncertainties and contributes significantly to refining the cosmic distance scale.