The problem of dwarves that aren't hot or gassy enough

Another attempt at figuring out whether ultra-diffuse galaxies as as massive as giant spirals or are just very extended puny little dwarfs. This is a problem which refuses to die, though practically everyone seems to think they're probably dwarves.

The issue with these ghostly beasties is that if they're dwarves, we have to explain why they're so much more extended than the ones we've found previously. Of course all existing studies have been biased because we couldn't easily see things this faint before, so what we once thought of as "typical" may be anything but. I suspect a lot of people still haven't realised the potential implications of this yet.

Making dwarves more or less extended could be a challenge, but probably a solvable one. More difficult would be the case that these objects are actually really massive but contain hardly any stars. Galaxy formation models seemed to be working well at the high-mass end, so to find a large number of unexpected giant galaxies would be a real headache. And it's not at all easy to see how a massive dark matter halo could ever avoid accumulating enough gas to form large amounts of stars : the large-scale cosmic web of dark matter implies that where there's dark matter, there you will also find gas and stars.

The authors of this paper attempt a novel method to constrain the mass of the UDGs by looking at their hot X-ray gas content. For more "normal" galaxies there's a nice correlation between X-ray luminosity and total dynamical mass. Unfortunately none of the ~50 UDGs in this sample are detected in X-rays, but in this case that's a good thing. By stacking the observations, the authors can put an upper limit on the average amount of hot gas present. And it's way lower, even accounting for possible uncertainties like distance, than for known massive galaxies. According to the normal relation, these things are much more likely to be dwarves than giants.

But I for one and far from convinced by this interpretation. It's a clever approach and an interesting study, but I don't buy their main conclusion. The problem is that this relies heavily on the assumption that the galaxies did, at one point, accumulate the usual amount of baryonic matter but some process quenched the star formation. It's not at all obvious to me why this should be the case. A much more natural interpretation would be (in my opinion) to say that the galaxies simply never accumulated much gas in the first place, so they've never ever had a chance at forming many stars. If so, there'd be absolutely no reason to assume they'd have as much hot gas as other giant galaxies. And anyway, any process capable of quenching the star formation in a giant galaxy presumably involves driving away the cold star-forming gas, and since hot gas is even easier to remove, I don't see a reason to expect the hot gas to still be hanging around. In short, the logic here is somewhat circular : conventional relations say these objects don't challenge theories of galaxy formation, therefore these objects don't pose a challenge to conventional theories.

So it's an interesting take, but to my mind this is still assuming the conventional scaling relations apply when it might instead provide evidence that they do not. Without direct dynamical mass measurements, we simply can't say if the conventional luminosity-mass relation still holds true.

Constraining the dark matter halo mass of isolated low-surface-brightness galaxies

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.