Sub-per-cent determination of the brightness at the tip of the red giant branch in the Magellanic Clouds

Abstract

The value of the Hubble constant as constrained by type Ia supernovae is directly tied to the zero point of the extragalactic distance scale, which is in turn set by the calibration of astrophysical distance indicators such as the tip of the red giant branch (TRGB). In this article, a calibration of the TRGB luminosity is determined in the Magellanic Clouds. Composite colour-magnitude diagrams are constructed for the Small and Large Magellanic Clouds using regions in which the TRGB could be unambiguously identified. As a result, a sub-per-cent measurement of the TRGB in the Clouds is determined. The I versus (V − I) relation of the TRGB is found to be consistent with a constant I magnitude over colours 1.45 < (V−I)₀ < 1.95 mag, and a shallow, quadratic curvature is confirmed when including more metal-rich (up to (V−I)₀ = 2.2 mag) tip stars into the fit, and is the preferred solution. This study's TRGB measurements also constrain the three-dimensional tilt of the Large Magellanic Cloud as well as the distance between the Small and Large Clouds. Both findings are in agreement with the independent, geometric constraints derived from the detached eclipsing binaries and establish a better than 0.02 mag cross-consistency (1% in distance) between the latest detached eclipsing binary measurements, red clump reddening maps and the TRGB measurements of this study.