The MUSE-Faint survey

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 $ \left(100\,\frac{\mathrm{M}_\odot}{\mathrm{L}_{\mathrm{V},\odot}}\right) $, based on its stellar velocity dispersion $ \left(7.07^{+1.29}_{-1.12}\,\mathrm{km\,s^{-1}}\right) $, 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 $ \left(15^{+43}_{-15}\%\right) $ when compared to the younger population $ \left(35^{+40}_{-6}\%\right) $. 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.