Today's paper is another one that combines my favourite topics of dark galaxies, ultra diffuse galaxies (UDGs), and ram pressure stripping. There have been a few of these lately, for example, this one from May where the authors postulated that "fake" dark galaxies could form as a result of ram pressure stripping, and in particular this one from April claiming that they'd found an UDG doing something similar.
I would strongly recommend reading that second link before reading this post, as today's paper is a direct response to that, with this being a saga that's been running since 2015. In brief, there's this large, very faint galaxy (a UDG candidate) in the Virgo cluster that's close to this other faint, disc-like galaxy-thing. Both of these are embedded in a common envelope of atomic hydrogen gas (HI). So the previous authors said that maybe we were witnessing RPS occurring from the UDG - that is, the gas in the UDG being pushed out as it moves through the hot, thin gas that fills the cluster.
In that case, the edge-on discy thing would probably be stars that formed out of the stripped gas, and not really a galaxy in the conventional sense. This would all be pretty neat, especially as seeing RPS from a UDG would give important clues about how such things form and evolve.
The connection to truly dark, starless galaxies is a bit tangential. With earlier optical data, it wasn't at all obvious if the HI had any optical counterpart at all, whereas now it's clear that it does - it's just very faint.
This paper is partly a re-analysis of the HI data and partly new optical data that gives independent constraints on the distances to the objects. One of the nice things about working with cluster galaxies is that they're all at about the same distance, or at least similar enough that the minor variations don't make a jot of difference to any calculations. And Virgo is especially nice because there don't seem to be any significant numbers of foreground galaxies to mess things up. So once you've got a galaxy in this general area and you know it's velocity, that's enough to give you a pretty solid distance estimate, and then of course if you spot alignments like this one, you know it's physically meaningful and not just a projection effect (i.e. a coincidence).
... Or so the legend goes. These authors disagree. They note that not only are the disc and UDG embedded in a common envelope of HI, but that both are connected by a much longer stream to a pair of other Virgo members. One of those already (VCC 2037) has independent distance measurements saying it's actually much closer. Whereas Virgo is at about 17 Mpc distance, new measurements say it's not more than 10 Mpc away and possibly as close as 5 Mpc. But a note of caution needs to be sounded here : there are two different papers, each using the same data but different methods, giving these two quite different results. That's a bit worrying in itself - and not at all uncommon either.
The authors of the current work say that as well as this, the UDG candidate is also likely to be much closer, maybe 7 Mpc away. So it's not forming from RPS at all, since it isn't even in the Virgo cluster.
What's actually going on here then becomes much more complicated. According to the new measurements, the faint disc does seem to be a Virgo member, but it doesn't have the normal scaling relations expected of a galaxy of similar stellar mass. So it might still be the result of RPS after all, just from a different galaxy (VCC 2034) : there's no reason that clouds of stars formed in this way should in any way resemble conventional galaxies.
In this case, the very good alignment between the HI stream and the other galaxies (the UDG candidate and VCC 2037) is just an unfortunate coincidence. Hence their title "two deceptive dwarfs"; the farting dwarf is indeed a lie.
But I am not at all sure about this. Given the big discrepancies in the distance estimates, particularly given the ping-pong distance estimates we've seen recently with galaxies claimed to lack dark matter, I think it makes sense to be very cautious about this. It seems a heck of a coincidence for an HI stream - of which only a handful are known in the cluster - to be superimposed so neatly on not one but two faint, unusual and interesting objects, again with very few foreground objects known. Both of these would also have to have quite extreme peculiar velocities, easy to explain if they're in the cluster, but not at all obvious if they're in the foreground where there's nothing much to disturb them. So this scenario seems to me to be very anti-Occam : possible, yes, but I would want a lot more data to be fully convinced of it.
To make matters worse, they also note that RPS is not the only possible gas removal mechanism, though it is the most likely. The alternative is a tidal encounter, but in this case narrowing down the possible perturbing galaxy is very difficult : there are many candidates, but none of them are much good. Saying anything very firm about this is even more difficult than usual.
There are two bits of good news to end on though. First, they already have more data themselves for at least three other objects, so additional papers are in progress. Second, the objects studied here should all fall within our very own WAVES survey, which is much more sensitive than the HI data used here. Having done a similar analysis of another object, these ones are natural things to take a look at. Watch this space.
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