Today's paper is a long one, so I'm going to heavily condense it down to the main result. I should say in advance that, although I think there are a couple of points (literally, just two) which are just gut-wrenchingly wrong, it's an excellent, detailed yet very readable work which is interesting from start to finish.
First, you might remember a couple of oddballs found in the Virgo cluster. The first is SECCO 1, a collection of very young, blue stars without any older component that's apparently just wandering around the cluster nowhere near any plausible parent galaxy. It's got gas, and simulations show it can survive moving slowly over such long distances, but why it only has a young stellar population is a mystery. What started this recent episode of star formation, given that there's bugger all around it ?
The second object is more controversial. It appears to be a case of gas stripping from a ultra diffuse (faint, spread out) galaxy with star formation occurring in the stripped gas. That would be the first time we've seen such a galaxy caught in the act of gas loss, and relatively few of them appear to even have any gas, so this would be a pretty neat result. And it would make sense : lots of UDGs known in clusters but virtually all are gasless, whereas those with gas are a bit more common in other, less dense environments.
But then, another team (actually the same as today's) came along and said that the UDG was actually not in the cluster at all, and that the star forming stuff was embedded in a stream of hydrogen coming from another galaxy.
I'm actually a bit skeptical about this second result. We've seen galaxy distance estimates be revised and revised again, and revised some more... so I wouldn't automatically trust any one claim. And the "stream" of hydrogen looks... well, maybe. It's not definitely wrong, but it's not definitely not wrong either. It looks quite a lot like an artifact of smoothing the data, to me.
Still, there are weird bits of star formation happening in the Virgo cluster. Today's paper is an attempt to tackle such objects a bit more systematically. The authors decided to do a search of the entire Next Generation Virgo Survey data and get a more complete catalogue of such objects, rather than stumbling on them at random as in all previous cases.
They found... a total of six candidates, including those two previous cases. And one of those they rule out, as it's most likely just a bunch of distant background galaxies. So three genuine new objects, bring the total to a staggering five.
Yes, that was slightly sarcastic, but never mind.
As usual with such small number studies, it's unclear if these objects really are all that similar to each other. They're not found in any particular location in the cluster. It's not really known if they have a distinct gas content - two are detected and have similar amounts of neutral hydrogen (a few tens of millions of solar masses), but of the other three, their redshifts are so low that any gas would be hard to distinguish from the much brighter gas in our own Galaxy. Only one is at all a significant non-detection, but if its gas was even just slightly less than the others, it would still be extremely gas rich compared to typical galaxies.
But in other ways they are actually remarkably similar. They're all very blue patches of starlight with no obvious underlying old stellar component - everything seems to have happened in the last hundred million years or even less. Their morphologies also look similar, and their total stellar masses are all a few tens of thousands of solar masses. And they have similar chemical compositions, indicating they weren't formed from primordial gas, but likely from material that was pre-enriched inside another galaxy. Star formation rates are also similar, and if they're not found in the same particular region of the cluster, they do seem to have similar levels of isolation - which is pretty unusual by itself for objects in a cluster.
So, how do you form such an object ? There are various possibilities. None are especially compelling, although one does seem more promising than the rest.
The mechanism I would be naturally drawn towards is that the objects might be very faint galaxies, with their stars bound in a dark matter halo just like much brighter ones. This, however, is the first point where I think the authors commit and aaaaargh noo this is wrong level of mistake. It doesn't help that they cite me in order to point out that there are "few convincing" results to date : this is technically correct, but very much missing the point. Worse is the claim that because the gas is not primordial, the objects themselves cannot be primordial and therefore they cannot be galaxies.
I don't think this is fair. If you want to decide if something is a galaxy, you need to look at its dynamics to infer its dark matter content. Whether or not it's composed of primordial material is secondary, especially given that nobody know much about how chemical enrichment works in objects like these. Two points in particular are that they can't rule out an old stellar population, and they say the stars detected are so blue they're hard to fit with a model even assuming very young ages. So there might be something unknown at work here. As it stands, this is confusing two quite separate issues.
Much more convincing is the data on dynamics which does exist, little though it is, doesn't suggest any hint of the high rotation speeds that would indicate the presence of dark matter. If anything it seems that their velocities are extremely low. Ironically, given the size of the objects, this implies they're not gravitationally self-bound and so are likely transient : they exist for just a short time and then disperse.
But how can this be ? How they reach a high distance from their parent galaxy, suddenly form stars, and then bugger off into void ?
There are two possibilities. One is that they could be tidal dwarf galaxies, formed from material stripped off in gravitational encounters between two or more galaxies. But this has a lot of problems. Tidal dwarfs need to be quite massive to survive the gravitational field from their parent galaxies (otherwise they just disintegrate immediately), and these guys are just too little. They could be from smaller parents, but then it's hard to eject material with the necessary velocity to get them to such isolation. And there's a relationship between mass and metallicity, which these objects would violate : their chemistry suggests bigger parents, not smaller.
A more promising alternative is that these are a new class of stellar system : ram pressure dwarfs. Note the lack of the word "galaxy" here, commendably emphatic in its absence*. We know ram pressure in clusters can push out gas at basically any velocity – this is dependent on the mass of the cluster itself, not that of the galaxy. We also know that star formation can happen in this stripped gas, with star formation initially boosted by the compression, though declining afterwards in the parent as its gas supply is rapidly exhausted.
*Claiming a new type of galaxy is tantamount to claiming you've discovered a new planet in the Solar System, so kudos for them for not doing this.
So this sounds great. This mechanism would allow the big, metal–rich parents to eject gas at high enough velocities to produce these slightly star-forming blobs that appear to be very isolated. Hurrah !
But even this is not without issue. One is the second oh god no why did you say that moment. They note that the objects are only found in clusters as evidence for their ram pressure origin. Well, yes, ram pressure is a distinctly cluster–based process... but they only looked for these objects in clusters ! Who knows if they're found in groups or not - they're not at all easy to spot, and no-one's been looking.
Certainly it's fair to say that they must be produced by some mechanism that does operate in clusters, however. The other issues here might be more difficult : the objects don't seem to be found near to objects with known gas tails where we know stripping is still ongoing, and they estimate that such objects could remain detectable for about half a billion years – a not insignificant amount of time.
There's a couple of possible future options for really nailing down what's going on. First, their search was "visual", meaning they took the data and looked at it. That's fine (although I would have like a lot more details), but doesn't tell us anything about their expected completeness, which could surely be quantified, and search techniques refined. Second, getting molecular gas measurements with ALMA or other instruments would help understand if star formation really is occurring the way they think it is.
So in the end this feels like an incremental result. And it is. But it's important to realise the amount of work that goes into some of these incremental results, which is... not small. A better way to think about it would be that this one interesting result has spawned the potential for yet more interesting results, in a never–ending cataclysmic chain reaction of interest that will eventually consume us all. Or something.
