The Archetypal Ultra-Diffuse Galaxy, Dragonfly 44, is not a Dark Milky Way

Due to the peculiar properties of ultra-diffuse galaxies (UDGs), understanding their origin presents a major challenge. Previous X-ray studies demonstrated that the bulk of UDGs lack substantial X-ray emission, implying that they reside in low-mass dark matter halos. This result, in concert with other observational and theoretical studies, pointed out that most UDGs belong to the class of dwarf galaxies. However, a subset of UDGs is believed to host a large population of globular clusters (GCs), which is indicative of massive dark matter halos. This, in turn, hints that some UDGs may be failed $L_{\star}$ galaxies. In this work, I present Chandra and XMM-Newton observations of two archetypal UDGs, Dragonfly 44 and DF X1, and I constrain their dark matter halo mass based on the X-ray emission originating from hot gaseous emission and from the population of low-mass X-ray binaries residing in GCs. Both Dragonfly 44 and DF X1 remain undetected in X-rays. The upper limits on the X-ray emission exclude the possibility that these galaxies reside in massive ($M_{\rm vir} \gtrsim 5\times10^{11} \ \rm{M_{\odot}}$) dark matter halos, suggesting that they are not failed $L_{\star}$ galaxies. These results demonstrate that even these iconic UDGs resemble to dwarf galaxies with $M_{\rm vir} \lesssim 10^{11} \ \rm{M_{\odot}}$, implying that UDGs represent a single galaxy population.