Ultraviolet and X-ray Properties of Coma's Ultra-Diffuse Galaxies

TL;DR

This study investigates the ultraviolet and X-ray properties of ultra-diffuse galaxies in the Coma cluster, revealing their recent quenching of star formation and absence of hot gas, consistent with them being quenched dwarf galaxies.

Contribution

It provides the first deep UV and X-ray analysis of Coma's UDGs, constraining their star formation activity and hot gas content, and discusses their possible nature as quenched dwarfs or massive galaxies.

Findings

UDGs show UV luminosities consistent with quiescence.

No X-ray emission detected, ruling out hot coronae with M > 10^8 M_sun.

Dragonfly 44 has very low star formation rate and lacks bright X-ray binaries.

Abstract

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.