D. A. D. Vaz, J. Brinchmann, S. Kamann, S. Saracino, P. A. C. Cunha, M. P. Júlio
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⊙) 100 M ⊙ L V , ⊙ , based on its stellar velocity dispersion (7.07+1.29−1.12 km s−1) 7 . 07 − 1.12 + 1.29 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% 55 − 9 + 40 % , consistent with similar systems. The close binary fraction (a < 10 au) is 30+34−9% 30 − 9 + 34 % , which is aligned with low-metallicity environments. We found a lower binary fraction for the older stellar population (15+43−15%) 15 − 15 + 43 % when compared to the younger population (35+40−6%) 35 − 6 + 40 % . 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.
Keywords
binaries: spectroscopic / galaxies: dwarf / galaxies: individual: Leo T / galaxies: kinematics and dynamics / Local Group / galaxies: stellar content
Astronomy & Astrophysics
Volume 708, Article Number A101, Number of pages 15
2026 April
