In the last post I looked at a probably-nonsense claim about massive red but gas-rich galaxies that for some reason aren't forming stars. It's not totally rubbish by any means : some of the sample almost certainly are really interesting objects. It's just that many of them clearly are forming stars, you can see it very clearly just by looking, and if their numbers say otherwise, then I bet against their numbers.
Today's paper makes for a nice counterpoint. Here the authors make essentially the exact opposite claim, that all early-type galaxies (popularly supposed to be "red and dead") are forming stars. Not very many stars necessarily, but some.
One of the problems in establishing whether such galaxies really have star formation activity is that there's a degeneracy in the methods used for estimating star formation rates. Typically, H-alpha observations tell you about star formation occurring right now, that is, on timescales of at most a few tens of millions of years. UV, on the other hand, tells you about both ongoing and recent (few hundred millions of years) activity. You can use estimates from other wavelengths, but they tend to be less accurate, so H-alpha and UV tend to be the go-to preferred choices. And H-alpha is relatively difficult to do, so UV can be a bit easier.
Here's where the degeneracy comes in. UV emission can be from hot young stars but it can also be form much older objects : asymptotic giant branch stars and even stellar remnants like white dwarves. So while UV emission is known in early-type galaxies (ellipticals and lenticulars, both are which are smooth and structureless, with the latter being more disc-like), that doesn't automatically mean it's indicating star formation.
The best way to break the degeneracy, say the authors, is by high resolution observations. If the UV is just from the normal old stellar population that dominates early-type galaxies, then it should appear to have the same basic morphology, smooth and symmetrical, as the bulk of the red stars. Star formation, by contrast, is by definition something that happens in distinct structures, so in this case the UV should appear clumpy, maybe disc-like and otherwise distinct from the rest of the stars in the galaxy.
What they do here is get a sample of 32 galaxies with high-resolution UV observations from the UVIT instrument on AstroSat. I wasn't even aware of this before so this alone is quite interesting. They are really quite careful in defining their sample to make sure it's representative and that the observations are sufficiently deep. They seem to cull this in half a dozen different ways, making it sometimes quite tedious to follow, and they actually miss out on what to me seem like the obvious and most interesting selection criteria (which I'll get back to). And the sample is, of course, very small indeed for claiming that all such galaxies are forming stars – this is one hell of an extrapolation.
Even so, it's careful and meticulous work. They measure the structure in the UV using three different parameters : concentration (essentially how much of the total emission is within some radius), asymmetry (how different the emission is comparing two arbitrary opposite sides), and clumpiness (how much of the emission is in small-scale structures compared to larger-scale emission). This too I found interesting, as these appear to be much more readily-quantifiable morphological parameters than the standard Hubble type, which tells you whether a galaxy is a spiral, irregular, or other sort of object.
Exactly how well these work in practise I'm not sure, so I'm going to take their word for it. I would imagine that a clumpy galaxy would also be asymmetrical since the clumps on one side won't be at the same location as on the opposite – whether the asymmetry involves some level of smoothing to account for this, I don't know.
Anyway, the nifty thing about their observations is that they have much higher resolution observations than typical UV data (i.e. from the GALEX satellite that did an all-sky survey), in fact about six times sharper. So while their overall findings in terms of the other parameters are quite similar to previous studies, they show quite definitively that indeed the UV emission is clumpy. Previous studies also showed the UV was structured, so already indicating it was from star formation, except that it didn't appear to be clumpy at all. This paper, then, has an an incremental but still important new finding, with that higher resolution being quite definitive. They also show images of the galaxies and it's quite obvious that yes, GALEX could not possibly have seen this because it's too blurry, whereas UVIT is definitely up to the mark.
Okay so this is quite nice. It's more incremental than I was expecting because I'm not familiar with star formation in ellipticals, but it's still definitely progress. As I said, they're hardly forming stars at the same rate as spirals, but still it's ticking over, which is in marked contrast to the popular "red and dead" moniker. And over cosmic time, this looks to have been significant : they did not form all their stars early on, as it commonly supposed.
What about those selection criteria I mentioned ? Well, that's a bit disappointing. Star formation needs gas, so the obvious question is : how much gas do they have and where did it come from ? This they don't mention. This is intimately connected with the next question : where are these particular galaxies ? Ellipticals in the general field are known to have significant amounts of gas, so it would be interesting but unsurprising to discover that they're forming stars. By contrast, those in clusters such as Virgo appear to have virtually no gas whatsoever. Not merely very little gas, but orders of magnitude less. So if those are forming stars, that would be very much more interesting. Hence the two unmentioned selection criteria of gas content and environment could have made the paper all the more exciting.
But no matter. Despite the small sample size and exaggerated extrapolation to "all" early-type galaxies, it seems like a much surer finding than the previous paper. And it would certainly be interesting to see what UVIT reveals about those supposedly-quenched galaxies. I'd bet heavily that some of them at least are really-UV bright and heavily structured... time will tell, perhaps.
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