Stop judging the gassy dwarfs

Last year there was an interesting attempt to constrain the mass of those pesky Ultra Diffuse Galaxies by measuring their X-ray gas content. This was clever : UDGs are normally so faint that determining their rotation to get a direct mass measurement is very difficult. Measuring the X-ray content, or at least placing an upper limit on it, is relatively easy, and since there's a tight relation between X-ray luminosity and total mass, this should give a decent constraint on the mass of the galaxies. Basically the more hot, tenuous gas a galaxy can cling on to, the stronger its gravitational field must be.

Since they didn't detect any hot X-ray gas at all, the authors concluded that they're more consistent with being dwarfs (which don't have much hot gas) than giants (which do). Which is a shame because if they were giants they'd be much more interesting, pointing to some unsuspected problem in star/galaxy formation at the high-mass end, where models were thought to be reasonably okay.

Here one of the authors splits from the band to start a solo career. While still controversial, it seems to me that most people would accept that most UDGs are probably dwarfs. But what about the most famous candidate giant objects ? Here the author takes a good hard look at two of the most prominent examples using very deep data.

Again, he doesn't find anything, so concludes that even these are just weirdly large dwarfs. But the same objections I raised last time still apply. If UDGs formed in a somewhat different way to brighter giant galaxies, why should we assume that'd have the same sort of X-ray content ? The whole reason they're interesting is that they have far fewer stars than other galaxies of their size, so I don't see why we should expect them to have the same amount of hot gas. Likewise, if you assume that conventional scaling relations are always correct - even if they have very low scatter - you'll never find anything that deviates. So this is circular reasoning : if you assume they follow the standard relations, you automatically exclude them from being weird.

The author does make the good point that X-rays originate from sources than just hot diffuse gas, however. They also originate from high-energy binary stellar systems (white dwarfs orbiting black holes and suchlike). But it's not obvious that this makes any difference. If UDGs form far less stars than similar sized galaxies, why expect them to have a similar fraction of such binary systems ? I remain unconvinced. It's still an interesting observation, but I'd change the title and emphasise that these observations are only consistent with dwarf galaxies, not proof that UDGs aren't giants.

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.