The MUSE-Faint survey

V. The binary fraction of Leo T

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, PT4169-007 Porto, Portugal
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁴ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
⁶ Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Straße 24/25, D-14476 Potsdam, Germany

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

Received: 30 May 2025
Accepted: 13 February 2026

Abstract

Context. The Leo T dwarf galaxy, the faintest and least massive galaxy known to have recent star formation (≤1 Gyr), exhibits a high dynamical mass-to-light ratio (100 M_⊙/L_⊙), based on its stellar velocity dispersion (7.07^+1.29_−1.12 km s⁻¹), indicating extreme dark matter dominance.

Aims. We present the first measurement of the binary fraction of Leo T using MUSE-Faint multi-epoch spectroscopy. We also determine the binary fraction for both young and old stellar populations separately and gain insights into binary properties in more metal-poor environments than the Milky Way or Magellanic Clouds. Finally, we investigate the potential impact of binaries on the inferred stellar velocity dispersion.

Methods. We employed a forward model methodology combining empirical scaling relations to predict stellar velocity variations and a constrained binary distribution from the literature. To estimate the close binary fraction, we limited the maximum semi-major axis (a < 10 au) and repeated the analysis with a semi-amplitude threshold (≥10 km s⁻¹) to check the impact on the inferred stellar velocity dispersion.

Results. The overall binary fraction of Leo T is estimated to be 55⁺⁴⁰₋₉%, consistent with similar systems. The close binary fraction (a < 10 au) is 30⁺³⁴₋₉%, which is aligned with low-metallicity environments. We found a lower binary fraction for the older stellar population (15⁺⁴³₋₁₅%) when compared to the younger population (35⁺⁴⁰₋₆%). Finally, we found no significant inflation of the velocity dispersion estimate due to binary motions when compared to the dispersion inferred from the co-added spectra. This suggests that the co-added spectra effectively provide period-averaged velocities of the stars, thus mitigating the impact of binaries on the overall velocity dispersion measurement.