The Archetypal Ultra-Diffuse Galaxy, Dragonfly 44, is not a Dark Milky Way. (arXiv:2009.07846v2 [astro-ph.GA] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Bogdan_A/0/1/0/all/0/1">Akos Bogdan</a>

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 ($M_{rm vir} gtrsim 5times10^{11} rm{M_{odot}}$) 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.

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 ($M_{rm vir} gtrsim 5times10^{11} rm{M_{odot}}$) 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.

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