Exploring the Sagittarius stream with RR Lyrae stars from Gaia Data Release 3

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, Bologna 40129, Italy
² Dipartimento di Fisica e Astronomia, Alma Mater Studiorum – University of Bologna, Via Piero Gobetti 93/2, Bologna 40129, Italy

^★ Corresponding author: tatiana.muraveva@inaf.it

Received: 26 May 2025
Accepted: 16 July 2025

Abstract

Context. The Sagittarius (Sgr) dwarf spheroidal galaxy is one of the most prominent satellites of the Milky Way (MW). It is currently undergoing tidal disruption, forming an extensive stellar stream that provides key insights into the assembly history of the MW halo.

Aims. Our goal is to investigate the structure and metallicity distribution of the Sgr stream using RR Lyrae stars (RRLs).

Methods. We analyzed RRLs provided in Gaia Data Release 3 (DR3), for which new estimates of photometric metallicities are available in the literature, and accurate distances were calculated using the reddening-free period–Wesenheit–metallicity relation in the Gaia G, G_BP, and G_RP bands.

Results. We determine the mean metallicity of RRLs in the Sgr stream to be [Fe/H] = −1.62 ± 0.01 dex. We measure a metallicity gradient as a function of stripping time from the Sgr progenitor of 0.05 ± 0.02 dex/Gyr, indicating that the metal-poor RRLs were stripped earlier during the accretion process. The far arm is found to be the most metal-poor structure of the Sgr stream, with a mean metallicity of [Fe/H] = −1.98 ± 0.37 dex, which is remarkably lower than, but still consistent within the errors with, that of the leading (−1.69 ± 0.31 dex) and trailing (−1.64 ± 0.28 dex) arms. Our findings show that the RRLs in the far arm of the Sgr stream exhibit a bimodal metallicity distribution with peaks at [Fe/H]=−2.4 dex and −1.7 dex. The main body of the stream is the most metal-rich structure, with a mean metallicity of [Fe/H] = −1.58 ± 0.31 dex and a radial gradient of −0.008 ± 0.005 dex/kpc. We find almost negligible metallicity gradients of (−0.2 ± 0.3) × 10⁻³ dex/deg in the trailing arm and (−1.0 ± 0.5) × 10⁻³ dex/deg in the leading arm, in agreement with previous studies. Finally, we investigate the bifurcation of the Sgr stream and conclude that the metallicity difference between the faint and bright branches is not confirmed based on the RRLs in our sample.