Empirically, low surface brightness galaxies (mostly, but with notable exceptions discussed in prior posts at this blog) have lots of "dark matter" effects which are apparent in their dynamics. The X-Ray emissions from low surface brightness galaxies should be high in low surface brightness galaxies with large halos and otherwise small. But, the low surface brightness galaxies that are observed are a poor fit to the CDM predictions to which they are compared in a new paper. They are, however, consistent with what a MOND-like theory would predict, where the apparently dark matter is due to dispersed matter distributions rather than halos creating "failed" spiral galaxies.
Constraining the dark matter halo mass of isolated low-surface-brightness galaxies
(Submitted on 13 Jun 2019)
Recent advancements in the imaging of low-surface-brightness objects revealed numerous ultra-diffuse galaxies in the local Universe. These peculiar objects are unusually extended and faint: their effective radii are comparable to the Milky Way, but their surface brightnesses are lower than that of dwarf galaxies. Their ambiguous properties motivate two potential formation scenarios: the "failed" Milky Way and the dwarf galaxy scenario. In this paper, for the first time, we employ X-ray observations to test these formation scenarios on a sample of isolated, low-surface-brightness galaxies. Since hot gas X-ray luminosities correlate with the dark matter halo mass, "failed" Milky Way-type galaxies, which reside in massive dark matter halos, are expected to have significantly higher X-ray luminosities than dwarf galaxies, which reside in low-mass dark matter halos. We perform X-ray photometry on a subset of low-surface-brightness galaxies identified in the Hyper Suprime-Cam Subaru survey, utilizing the XMM-Newton XXL North survey. We find that none of the individual galaxies show significant X-ray emission. By co-adding the signal of individual galaxies, the stacked galaxies remain undetected and we set an X-ray luminosity upper limit ofL0.3−1.2keV≤6.2×1037(d/65Mpc)2 erg s−1 for an average isolated low-surface-brightness galaxy. This upper limit is about 40 times lower than that expected in a galaxy with a massive dark matter halo, implying that the majority of isolated low-surface-brightness galaxies reside in dwarf-size dark matter halos.
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