Ultra-diffuse galaxies (UDGs) are among the faintest and lowest-surface-brightness (mu_0 > 24 mag/arcsec^2, R_e>1.5 kpc) galaxies known in the Universe. Being so diffuse and faint, in order to survive cluster tides UDGs should host a large amount of dark matter.
This idea has brought ever-increasing attention to the detection and study of UDGs and has given these galaxies a special role in the realm of the LSB Universe. Given the extremely low baryonic mass density, UDGs are indeed considered particularly suitable laboratories for testing the formation of galaxies in the LambdaCDM framework. Recent results from the LEWIS project, which map the resolved stellar kinematics with the integral-field spectrograph MUSE@ESO-VLT, revealed that UDGs do not rotate and have a very low velocity dispersion. These properties were not known before, when most of the studies were concentrated on imaging data. Hence, some questions raised: are UDGs gravitationally stable? Which is the lifetime of these systems? To address these issues, we should map the number density of the UDGs (N_UDGs) with redshift. To date, most of the data are collected in the nearby universe (z<0.2) and only one measured point is available at higher redshift, z>0.4. We propose to search for deep imaging data from the HST archive to search for UDGs in clusters at z>2 and look for the relation between N_UDGs and z. If UDGs are not stable systems, because they are not supported by rotation, we would expect and increasing of the number of UDGs with z.
Riferimento: Enrichetta Iodice