Ultra-diffuse galaxies in the Coma cluster: Probing their origin and AGN occupation fraction

Abstract: Ultra-diffuse galaxies (UDGs) exhibit low surface brightness, but their optical extent is comparable to Milky Way-type galaxies. Due to their peculiar properties, it remains ambiguous whether UDGs are the descendants of massive galaxies or they are puffed-up dwarf galaxies. In this work, we explore a population of 404 UDGs in the Coma cluster to study their origin and AGN occupation fraction. To constrain the formation scenario of UDGs, we probe the X-ray emission originating from diffuse gas and from the population of unresolved low-mass X-ray binaries (LMXBs) residing in globular clusters (GCs). It is expected that both the luminosity of the hot gas and the number of globular clusters and hence the luminosity from GC-LMXBs are proportional to the total dark matter halo mass. We do not detect statistically significant emission from the hot gas or from GC-LMXBs. The upper limits on the X-ray luminosities suggest that the bulk of the UDGs reside in low-mass dark matter halos, implying that they are genuine dwarf galaxies. This conclusion agrees with our previous results obtained for isolated UDGs, arguing that UDGs are a homogenous population of galaxies. To probe the AGN occupation fraction of UDGs, we cross-correlate the position of detected X-ray sources in the Coma cluster with the position of UDGs. We identify two UDGs that have a luminous X-ray source at 3.0" and 3.2" from the center of the galaxies, which could be off-center AGN. However, Monte Carlo simulations suggest that one of these sources could be the result of spatial coincidence with a background AGN. Therefore, we place an upper limit of $\lesssim0.5\%$ on the AGN occupation fraction of UDGs.