Looking into the faintest with MUSE: Integral-field spectroscopy of ultra-diffuse galaxies.

In this talk, I would like to present the LEWIS project (Iodice et al. 2023 A&A, 679, 69). Looking into the faintEst WIth MUSE (LEWIS) is an ESO large program, granted with 133.5hrs with MUSE@ESO-VLT., is the first homogeneous integral-field spectroscopic survey of 30 extremely low-surface-brightness (LSB) galaxies in the Hydra I cluster of galaxies, where the majority of LSB galaxies in the sample (22 in total) are ultra-diffuse galaxies (UDGs). Because of their LSB nature, getting spectroscopic data for UDGs is challenging. To date, as opposed to the availability of deep images, we still lack a statistically significant sample of UDGs with spectroscopy, which strongly limits our constraints and conclusions on their stellar populations and DM content. Doubling the number of spectroscopically studied UDGs, with the LEWIS project we will make a decisive impact in this field. With LEWIS we will map, for the first time, i) the 2D stellar kinematics, ii) the stellar population, and iii) establish the GC population and their specific frequency of a complete sample of UDGs in a galaxy cluster with IF spectroscopic data. The structural properties and the baryonic versus DM fraction are compared with hydrodynamical models of UDGs to establish their formation channels as a function of their location in the cluster. With LEWIS we are probing the MUSE capabilities to map the galaxy structure down to the faintest surface brightness levels.