Ultraviolet and X-ray Properties of Coma's Ultra-Diffuse Galaxies
  Many ultra-diffuse galaxies (UDGs) have been discovered in the Coma cluster,
and there is evidence that some, notably Dragonfly 44, have Milky Way-like
dynamical masses despite dwarf-like stellar masses. We used X-ray, UV, and
optical data to investigate the star formation and nuclear activity in the Coma
UDGs, and we obtained deep UV and X-ray data (Swift and XMM-Newton) for
Dragonfly 44 to search for low-level star formation, hot circumgalactic gas,
and the integrated emission from X-ray binaries. Among the Coma UDGs, we find
UV luminosities consistent with quiescence but NUV$-r$ colors indicating star
formation in the past Gyr. This indicates that the UDGs were recently quenched.
The $r$-band luminosity declines with projected distance from the Coma core.
The Dragonfly 44 UV luminosity is also consistent with quiescence, with
SFR$<6\times 10^{-4} M_{\odot}$ yr$^{-1}$, and no X-rays are detected down to a
sensitivity of $10^{38}$ erg s$^{-1}$. This rules out a hot corona with a $M >
10^8 M_{\odot}$ within the virial radius, which would normally be expected for
a dynamically massive galaxy. The absence of bright, low mass X-ray binaries is
consistent with the expectation from the galaxy total stellar mass, but it is
unlikely if most low-mass X-ray binaries form in globular clusters, as
Dragonfly 44 has a very large population. Based on the UV and X-ray analysis,
the Coma UDGs are consistent with quenched dwarf galaxies, although we cannot
rule out a dynamically massive population.
