Ultra-diffuse galaxies in clusters: The peculiar gas loss of VCC 1964

Context. Ultra-diffuse galaxies are low surface brightness systems that have been detected in H I in the field, where their line widths sometimes indicate significant dark matter deficits. They are rarely detected in H I in clusters, which makes their dynamical properties difficult to assess. The relation between field and cluster populations is unclear. Aims. Detecting UDGs entering a cluster could give important clues about their evolution in terms of their dynamics, but also as to whether they are structurally similar, that is, if cluster UDGs are generally the same as field UDGs, but with less gas and an older stellar population. Methods. We used data from two deep Arecibo surveys, the Arecibo Galaxy Environment Survey and the Widefield Arecibo Virgo Environment Survey, to measure the gas content of the UDG-candidate VCC 1964. The optical properties were quantified from the Sloan Digital Sky Survey and DESI Legacy Surveys. Results. We found a significant 9 kpc offset between the H I and optical components of VCC 1964, no evidence of asymmetry in the H I , and only a modest deficiency level. This suggests a wholesale displacement of the gas content. The line width deviates by 4–5 σ from the baryonic form and deviates by over 6 σ from the optical form of the Tully-Fisher relation. The optical component is blue and smooth. Conclusions. VCC 1964 is consistent with a UDG in which gas is displaced by ram pressure as it enters the cluster for the first time. Intriguingly, its dynamics imply a significant dark matter deficit, but we cannot rule out that the reason might be that its gas is displaced out of equilibrium.