Today, two papers on hedgehogs quenched galaxies. It'll make more sense later on, but only slightly.
"Quenched" is just a bit of jargon meaning that galaxies have stopped forming stars, if not completely, then at least well below their usual level. There are a whole bunch of ways this can happen, but they all mostly relate to environment. Basically you need some mechanism to get the gas out of galaxies where it then disperses. In clusters this is especially easy because of ram pressure stripping, where the hot gas of the cluster itself can push out gas within galaxies. In smaller groups the main method would be tidal interactions, though this isn't as effective.
What about in isolation ? There things get tricky. Even the general field is not a totally empty environment : there are other galaxies present (just not very many) and external gas (just of very low density). But you also have to start to consider what might have happened to galaxies there over the whole of time, because conditions were radically different in the distant past.
To cut a long story short, what we find is that giant galaxies seem to have formed the bulk of their stars way back in a more exciting era when things were just getting started. Dwarf galaxies in the field, on the other hand, are still forming stars, and in fact their star formation rate has been more or less permanently constant.
This phenomena is called downsizing, and for a long time had everyone sorely puzzled : naively, giant galaxies ought to assemble more slowly, so were presumed to have taken longer to assemble their stellar population, whereas dwarfs should form more quickly. Simplifying, this was due to host of problems in the details of the physics of the models, and as far as I know it's generally all sorted out now. Small amounts of gas can, in fact, quite happily maintain a lower density for longer, hence dwarfs form stars more slowly but much more persistently.
Dwarfs are, of course, much more susceptible to environmental gas-loss removal processes than giants, and indeed dwarfs in clusters are mostly devoid of gas (except for recent arrivals). And so conversely, any dwarfs which have lost their gas in the field are unexpected, because there's nothing very much going on out there : all galaxies of about the same mass should have about the same level of star formation. There's no reason that some of them should have lost their gas and others held on to it - it should be an all-or-nothing affair.
That's why isolated quenched galaxies are interesting, then. On to the new results !
The first paper concentrates on a single example which they christen "Hedgehog", because "hedgehogs are small and solitary animals" and also presumably because "dw1322m2053" is boring, and cutesy acronyms are old hat. Wise people, I approve.
This particular hedgehog galaxy is quite nearby (2.4 Mpc) and extremely isolated, at least 1.7 Mpc from any nearby massive galaxies. That puts it at least four times further away than expected from the region of influence of any groups, based on their masses. It's a classic quenched galaxy, "red and dead", smooth and structureless, with no detectable star formation at all.
It's also very, very small. They estimate the stellar mass at around 100,000 solar masses, whereas for more typical dwarf galaxies you could add at least two or three zeros on to that. Now that does mean they can't say definitively if its lack of star formation is a really significant outlier, simply because for an object this small, you wouldn't expect much anyway. But in every respect it appears consistent with being a tiny quenched galaxy, so the chance that it has any significant level of star formation is remote.
How could this happen ? There are a few possibilities. While it's much further away from the massive groups than where you'd normally expect to see any effect from them, simulations have shown that it's just possible to get quenched galaxies this far out. But this is extraordinarily unlikely, given that they found this object serendipitously. They also expect these so-called "backsplash" galaxies (objects which have passed through a cluster and out the other side*) to be considerably larger than this one, because they would have formed stars for a prolonged time, right up until the point they fell into the cluster.
* I presume and hope this is a men's urinal reference.
Another option is simply that the star formation in small galaxies might be self-limiting, with stellar winds and supernovae able to eject the gas. This, they say, is only expected to be temporary (since most of the gas should fall back in after a while), so again the chances of finding something like this are pretty slim. But I'd have liked more details about this, since I would expect that for galaxies this small - and it really is tremendously small - the effects of feedback could be stronger than for more typical, more massive galaxies. Maybe stellar winds and explosions could permanently eject much more of the gas, although on the other hand galaxies this small would have fewer massive stars capable of this.
Similarly another possibility, which I don't think they mention, is quenching due to ram pressure in the field. Again, for normal dwarf galaxies, this is hardly a promising option. For ram pressure to work effectively, you need gas of reasonably high density and galaxies moving at significant speeds, neither of which happens in the field. But, studies have shown that galaxies in the field do experience (very) modest amounts of gas loss which correlates with the distance from the large-scale filaments. Ordinarily this is not really anything substantial, but for galaxies this small, it might be. Since a galaxy this small just won't have much gas to begin with, and removing it will be easy because it's such a lightweight, what would normally count as negligible gas loss might be fatal for a tiddler like this.
The most interesting option is reionisation. When the very first stars were formed, theory says, there were hardly any elements around except hydrogen and helium and a smattering of others. Heavier elements allow the gas to cool and therefore condense more efficiently, so today's stars are comparative minnows. But with none of this cooling possible, the earliest stars were monsters, perhaps thousands of times more massive than the Sun. They were so powerful that they reionised the gas throughout the Universe, heating it so that cooling was strongly suppressed, at least for a while. In more massive galaxies gravity eventually overcame this, but in the smallest galaxies it could be halted forever.
Hedgehog, the authors say, is right on the limit where quenching by reionisation is expected to be effective. If so then it's a probe of conditions in the very early universe, one which is extremely important as it's been used a lot to explain why we don't detect nearly as many dwarf galaxies as theory otherwise predicts*. The appealing thing about this explanation is the small size and mass of the object, which isn't predicted by other mechanisms.
* They do mention that the quenched fraction of galaxies in simulations rises considerably at lower masses, but how much of this is due to reionisation is unclear.
This galaxy isn't quite a singular example, but objects like this one are extremely rare. Of course ideally we'd need a larger sample, which is where the second paper comes in.
This one is a much more deliberate attempt to study quenched galaxies, though not necessarily isolated. What they're interested in is our old friends, Ultra Diffuse Galaxies, those surprisingly large but faint fluffy objects that often lack dark matter. In this paper the authors used optical spectroscopy to target a sample of 44 UDGs, not to measure their dynamics (the spectroscopic measurements are too imprecise for that) but to get their chemical composition. With this they can identify galaxies in a post-starburst phase, essentially just after star formation has stopped. That kind of sample should be ideal for identifying where and when galaxies get quenched.
I'm going to gloss over a lot of careful work they do to ensure their sample is useful and their measurements accurate. The sample size is necessarily small because UDGs are faint, and their own data finds that some of the distance estimates were wrong so a few candidates weren't actually UDGs after all. Their final result of 6 post-starburst UDGs doesn't sound much, and indeed it isn't, but these kinds of studies are still in their very early days and you have to start somewhere.
Even with the small size, they find two interesting results. First, the fraction of quenched UDGs is around 20%, much higher than the general field population. The stellar masses are a lot higher than the Hedgehog but still small compared to most dwarfs though, so this one needs to be treated with a bit of caution but it's definitely interesting. Second, while most quenched UDGs do appear to result from environmental effects, a few are indeed isolated. Which is a bit weird and unexpected. UDGs in clusters might form by gas loss of more "typical" galaxies, but this clearly can't work in the field, so why only a select few should lose gas isn't clear at all.
What all this points to isn't all that surprising, though in a somewhat perverse sense : it underscores that we don't fully understand the physics of star formation. The authors of the second study favour stellar feedback as being responsible for a temporary suppression of star formation. If this is common and repeated, with galaxies experiencing many periods of star formation interspersed with lulls, that could also make Hedgehog a bit less weird - if, say, it's forming/not forming stars for roughly the same total amount of time, then it wouldn't be so strange to detect it during a quenched phase. And of course the lower dark matter content of UDGs surely also has some role to play in this, although what that might be is anyone's guess.
As usual, more research is needed. At this point we just need more data, both observational and simulations. That we're still finding strange objects that're hard to explain isn't something to get pessimistic about though. We've learned a lot, but we're still figuring out just much further we have to go before we really understand these objects.
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