Letter to the Editor

Calibrating the tip of the red giant branch and measuring Magellanic Cloud distances to ∼2% exclusively with Gaia

¹ Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290, Versoix, Switzerland
² Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, 37077, Göttingen, Germany

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 14 February 2026
Accepted: 15 March 2026

Abstract

We have calibrated the tip of the red giant branch (TRGB) using our recent catalogue of homogeneous globular cluster (GC) distances. The GC distances were determined via a global joint fit to optical period–Wesenheit relations of their member RR Lyrae stars and type-II Cepheids, anchored by trigonometric parallaxes; all data were taken from the ESA Gaia mission’s (early) third data release (GDR3). Using I-band measurements of 48 GCs from P. Stetson’s database, we determined M_I,0 = −3.948^+0.037_−0.034 mag (1.6% in distance). Calibrating the TRGB using Gaia’s homogeneous, space-based RP photometry of 53 GCs, we found M_RP,0 = −3.807^+0.041_−0.035 mag (1.8%). The stated uncertainties include statistical and systematic effects, including the correlated nature of the GC distances. The robustness of our calibrations was evaluated via tests against small-number statistics and analysis choices. Within the (small) uncertainties, no significant metallicity effect is detected in our sample of old, low-metallicity GCs. We measured ∼2% distances to the Large and Small Magellanic Clouds, 18.447^+0.036_−0.042 mag (48.9 ± 0.9 kpc) and 18.898^+0.049_−0.054 mag (60.2 ± 1.4 kpc), respectively, using a single photometric system: RP (spectro-)photometry from GDR3. Our new TRGB distances, whose absolute scale derives from Gaia parallaxes, are fully independent of the well-known detached eclipsing binary (DEB) distances and agree with them to within the uncertainties. Combining our new TRGB and existing DEB distances, we illustrate how additional constraints can be incorporated into the Local Distance Network and obtain H₀ = 73.52 ± 0.80 km s⁻¹ Mpc⁻¹. Expected improvements thanks to the upcoming fourth Gaia data release are discussed.