Ultra-diffuse galaxies (UDGs) are extreme low-surface brightness (LSB) galaxies (μg ≥ 24 mag/arcsec²) with a size of several kpc, up to that of the Milky Way, with at least 100 times smaller stellar masses. Despite the increasing number of studies on UDGs, mainly on imaging data, their observed structural properties do not fit in a single formation scenario. Therefore, questions about their formation processes and dark matter (DM) content are still open.

LEWIS is a large program started in 2021, approved during the ESO period 108, to obtain the first homogeneous integral-field spectroscopic survey of UDGs in a cluster environment with MUSE. The target is the Hydra I cluster, located at ~50Mpc, where a complete sample of UDGs and low-surface brightness (LSB) galaxies, composed by 32 objects in total, have been recently discovered inside 0.4Rvir of the cluster. 

With the MUSE data, we will 

• confirm their cluster membership,

• measure their stellar kinematics 

• constrain the stellar populations.

For all confirmed cluster members, from the stellar kinematics, we can estimate the dynamical mass Mdyn and, therefore, the dark matter (DM) content in each galaxy of the sample. The stellar population properties (ages and metallicity) together with the baryonic versus DM fraction will be used in comparison with hydrodynamical models of UDGs to establish their formation channels as a function of their location in the cluster. 
The MUSE data will also establish the GC population and their specific frequency within 1.5Re for each UDG. This might provide an independent handle on the DM content in these systems.