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