We last saw these six little guys in a letter describing how they rotate so slowly that they deviate from the Tully-Fisher relation : their velocity is much slower than expected given their baryonic mass. They're not quite so slow that they lack dark matter, but they are slow enough that we've likely found all their baryons (in most galaxies, a large fraction is missing, usually thought to be very hot, thin gas). Unlike other cases, these are too isolated to be the result of galaxy-galaxy interactions and too far away for distance uncertainties to mess thing up. This latest paper is the follow-up full article to the original letter, wherein they show all their data and not just the sexy money plots.
These particular galaxies are extremely faint, fuzzy little "ultra diffuse galaxies". Since they're extremely faint, and also since the letter didn't show the full maps of each object, it's only natural to wonder if the inclination angle hadn't been computed incorrectly. If they're actually face-on, for instance, we wouldn't be able to measure their rotation speed at all. Similar claims have been made for much brighter galaxies that are spinning more slowly than expected.
Looking at their figures, first I thought, "ooh, they show ordered motions, so they're definitely rotating !", then I thought, "ahh but hang on, the gas is at totally different angle to the stars, so maybe the inclination angle is wrong after all", and then I noticed, "hey, that galaxy's rotation is going the wrong way !". Finally I did what I should have done from the start and read the blasted text. In short, I think this paper reasonably settles any concerns about the inclination angle measurements, though I won't say it's unquestionable.
There's not much doubt that these objects are rotating : one side is clearly at a different velocity to the other. And the velocity axis aligns very well with the morphological axis, except in one case where the resolution is a bit funky. This alignment, they say, means they can dismiss outflows or inflows. Although they don't have much resolution, they have enough to employ some fancy modelling software to extract the true rotation curves of the galaxies. They've tested this on similar simulated galaxies and it works well. And although the stars are all over the place, they point out that the gas in these galaxies is ten times or more massive than the stars, so it's the gas they should use for measuring inclination, not the stars.
Even so, I'm still a bit skeptical about three cases : one looks a bit disordered, one looks too close to face on for reliable measurements, and one is rotating in the wrong direction. The other three cases I'm satisfied with, but no matter how sophisticated the modelling is, I'd still be cautious.
They then compile other samples of galaxies to search for any other possible TFR-deviants. Besides their own sample, there's not much more than a hint of any deviation from the trend. This, as they note, is well-known oddity, since low surface brightness galaxies should deviate from the TFR but generally don't. In fact, so far there haven't been indications of any trend in those few galaxies which do deviate. Here, they find one : the greater the stellar scale length, the greater the deviation (restricting the sample to galaxies with the lowest rotation speeds).
The hell does that mean ? I don't know. They mutter something about the velocity-mass plane not being a well-defined distribution, but that doesn't clear anything up for me.
How do such objects form ? Buggered if I know. They note that the gas discs here are of normal size given their mass, so they're not of exceptionally low gas density. Obviously these particular UDGs can't be "failed" Milky Ways like some other UDGs are reputed to be, since their dark matter content is too low (perhaps the UDG label just doesn't have much physical relevance). Interestingly, if their gas density is normal, their stellar density is low, so stellar feedback may have been weak, preventing outflows from driving their gas away, resulting in these gas-rich, diffuse objects. But of course that doesn't really help explain much : what keeps star formation low in galaxies of normal gas density ?
Just about much dark matter they have remains to be seen. They say the objects are consistent with the average cosmological baryon fraction, meaning we've likely found all their baryons. In that case they'd have about ten times as much dark as visible matter, but this halo would have to be unusually diffuse. But they can't be sure, and given the available resolution, it's possible they actually have no dark matter at all. So higher resolution might yet pin down the little blighters. The research continues.
Robust HI kinematics of gas-rich ultra-diffuse galaxies: hints of a weak-feedback formation scenario
We study the gas kinematics of a sample of six isolated gas-rich low surface brightness galaxies, of the class called ultra-diffuse galaxies (UDGs). These galaxies have recently been shown to be outliers from the baryonic Tully-Fisher relation (BTFR), as they rotate much slower than expected given their baryonic mass, and to have baryon fractions similar to the cosmological mean.
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