Today, two papers for the price of one ! The one is a sequel to the other and they're both quite technical, so let's knock off two birds with one stone in the bush and other mixed metaphors. Paper I is here and paper II is here.
The papers in essence address two related questions on Ultra Diffuse Galaxies – the big faint fuzzy things that often seem to lack dark matter, which I've covered here ad nauseum. Neither paper much addresses the dynamics (i.e. total mass) of the objects, but rather the other fun aspect of these galaxies : why are they so wretchedly bad at forming stars ? Many of them have tonnes of gas, so why aren't they forming stars like normal galaxies are ? Why are they so large and yet so faint ?
The first paper deals with the basics. It tries to determine if the star formation efficiency of UDGs really is weirdly low, or if this is only a selection or measurement effect. Spoiler alert : it really is low. So the second paper then address why this might be – is there something missing in our basic model of star formation, or is it just due to the peculiars and particulars of of these particularly peculiar systems ?
Paper I begins by collecting a sample of 22 UDGs and 35 more typical dwarfs of comparable mass. The UDGs all have atomic HI gas detections, though low resolution so essentially all we know is the mass of gas, nothing about its structure. But it seems that at first glance, UDGs are indeed of systematically lower star formation efficiencies : their star formation rate is less than that of other galaxies of similar gas masses, and given their stellar masses they have more gas than expected. Both of these effects are modest though. It's quite apparent that the population as a whole is systematically offset from the rest, but they're all still within the general scatter. Interestingly, they also note that the properties of the HUDGs (HI-detected UDGs) aren't much affected by environment.
The first question they tackle is whether these objects are really different in terms of their gas content, or if this is just a selection effect. That is, the HI observations might be limiting what can be detected at all. It could be that those of lower gas fractions do exist, it's just that the data isn't sensitive enough to show them. But they find that's not the case : they should be able to find considerably less gassy-objects, so these don't seem to exist at all. These HUDGs are not the tip of a less-gassy iceberg.
Next, their main topic. While the average star formation efficiency of HUDGs appears low, this could just be due to the statistics from using the total gas mass and stellar mass, which smooths over the whole structure of the galaxy. It could be that the star formation efficiency is actually quite normal, just restricted in area. For example the gas could all be concentrated in the centre and forming stars pretty normally, but when averaged over the whole galaxy, this would be "washed out" and it would look like the galaxy was rubbish at forming stars. Overall, they would be, but locally, they wouldn't be that bad. This would imply a significant population of older stars outside the gas-dominated regions.
To test this they use spectral energy distribution fitting. Basically what this does is use many different data points across the optical, UV and IR spectrum to estimate the stellar ages as accurately as possible – this is the best we can do in terms of estimating a galaxy's star formation history. Ideally we'd also like to have resolved measurements of the HI, which sadly they don't have here. But they do SED-fitting for many different points per galaxy, so they can see if the low SFE is something that varies throughout each object or if it's low everywhere.
I'll skip over the details of the SED fitting because I don't understand any of it; I only note that they stress they aren't constructing detailed star formation histories here, just enough to answer their main questions. Their main result is that the low SFE is true on all scales from big to small. It's not just that the gas could be more extended, it's that UDGs are bad at forming stars full stop, even if the gas density is higher. While UDGs are large given their stellar masses, they're not especially large considering their HI masses.
There are a lot of different scaling relations to juggle here, but the end result is very simple : UDGs aren't good at converting gas to stars even when they've got plenty of it. The obvious next question is, of course, why are they so bad at this ? For this we need the sequel.
Paper II takes quite a different approach and is all about modelling. There's a really popular and widely-used relation between the surface density of gas and its star formation activity, but there have been indications for many years that we ought to be using the true, volumetric (3D) density instead. This is harder to measure directly so some assumptions have to be made, but it can be done.
Here they consider a particular version of the volumetric star formation law that depends on the different components of a galaxy together : the gravity from the atomic and molecular gas, the stars, and the dark matter. All have different gravitational contributions. For instance in the centre of a galaxy everything might be extremely dense, whereas further out their might still be lots of atomic gas and stars but less molecular gas, and on the very outskirts only atomic gas and dark matter. So even if the atomic gas has the same surface density, it may need the extra gravity of the stars to help pull it together and collapse.
Again I shall spare you the technical details of the model. This time my essential note is that they consider UDGs to be rather more dark matter dominated than ordinary galaxies, which flies against the prevailing winds in the last few years. More on that in a moment.
They find that this more complex model... works ! It can explain the star formation rates of both normal galaxies and UDGs under very reasonable assumptions : there's no need for any additional physics, no weird mechanism or alien interference that suppresses the star formation. There are many uncertainties, but their sensible default assumptions are enough to give a good result without any sort of fine-tuning needed. So UDGs are, in a sense, pretty normal.
They also find that the model isn't very sensitive to how much molecular gas the UDGs are assumed to contain. That's bad news for anyone trying to detect their molecular content : essentially it implies they could well have very little of it. They can even estimate just how much molecular gas they expect them to have, given their estimated star formation rates and the (rather surprisingly but repeatedly established) independent finding that molecular gas generally has a constant depletion timescale of about 2 Gyr. In short, their conclusion is that detecting the molecular component will be Bloody Difficult. Dwarf galaxies are already hard to detect, but UDGs will be even worse.
What about that choice of a relatively massive dark matter halo ? They explore this, and rather surprisingly it doesn't matter much. It seems that the density of the dark matter is anyway assumed to be low so that reducing the total mass doesn't make a great deal of difference. In fact this can give slightly better agreement with the measured star formation activity, but unfortunately, they say there are too many other uncertainties to say if their model prefers no dark matter at all to a normal mass halo.
So, there we have it. There's no need for any weird physics here : star formation in UDGs apparently follows the same laws as in every other galaxy. The difference isn't so much the gas content itself as the state it happens to find itself in, just as a puppy can be an energetic ball with the madness of a thousand caffeinated suns or the sleepiest thing since Slothy the Sleepy Sloth swallowed an entire bottle of Nytol after a mug of warm milk in a comfy armchair.
Does this mean UDGs aren't weird at all though ? Nope ! You might remember that there were previous claims that UDGs are actually just normal galaxies if you redefine how to measure their radius. That was true, but doesn't mean that the other radius estimates were wrong : it still points to UDGs being anomalous, just not in the way we'd understood.
Here we still don't know what sets the initial conditions of UDGs. Why do they start out so differently to normal galaxies ? Is it their dark matter halos (or lack thereof) and if so, is this compatible with our simulations of the sort of halos we expect to actually exist ? And how come their globular cluster populations appear to be very different to other galaxies ?
We still don't know. We do know that UDGs are weird, but as to whether they're pointing to a deep flaw in our models or just some incompleteness or other... the jury's out. Or more accurately, the trial hasn't even started yet : we need more evidence before we can even really begin. But at least what this paper adds is what evidence we should go after. If UDGs are found to have chonks of molecular gas, that would falsify their model straight away. If they're not, we keep investigating.
