Giants in the deep

Here's a fun little paper about hunting the gassiest galaxies in the Universe.

I have to admit that FAST is delivering some very impressive results. Not only is it finding thousands upon thousands of galaxies – not so long ago, the 30,000 HI detections of ALFALFA was leagues ahead of everything else, this has already been surpassed – but in terms of data quality too it looks like it's delivering. This paper exploits that to the extreme with the FAST Ultra Deep Survey, FUDS.

Statistically, big, all-sky surveys are undeniably the most useful. With a data set like that, anyone can search the catalogue and see if anything was detected at any point of interest, and at the very least they can get an upper limit of the gas content of whatever they're interested in. Homogeneity has value too. But of course, with any new telescope you can always go deeper, as long as you're prepared to put in the observing time. That can let you find ever-fainter nearby sources, or potentially sources at greater distances. Or indeed both.

It's the distance option being explored in this first FUDS paper. Like previous ultra-deep surveys from other telescopes, FUDS tales a pencil-beam approach : incredibly sensitive but only over very small areas. Specifically it's about 12 times more sensitive than AGES but in an area almost 50 times smaller (or, if you prefer, 44 times more sensitive than ALFALFA but in an area 1,620 times smaller). This paper looks at their first of six 0.72 square degree fields, concentrating on the HI detections at redshifts at around 0.4, or a lookback time of about 4 Gyr. Presumably they have redshift coverage right down to z=0, but they don't say anything about that here.

They certainly knock off a few superlatives though. As well as being arguably the most distant direct detection of HI (excluding lensing) they also have, by a whisker, the most massive HI detection ever recorded – just shy of a hundred billion solar masses. For comparison, anything above ten billion is considered a real whopper.

All this comes at a cost. It took 95 hours of observations in this one tiny field and they only have six detections at this redshift. On the other hand, there's really just no other way to get this data at all (with the VLA it would take a few hundred hours per galaxy). Theoretically one could model how much HI would be expected in galaxies based on their optical properties and do much shorter, targeted observations which would be much more efficient. But this redshift is already high enough that optically the galaxies look pretty pathetic, not because they're especially dim but simply because they're so darn far away. So there just isn't all that much optical data to go on.

As you might expect, these six detections tend to be of extraordinarily gas-rich galaxies, with correspondingly high star formation rates. While they're consistent with scaling relations from local galaxies, their number density is higher than the local distribution of gas-rich galaxies would predict. That's probably they're most interesting finding, that we might be seeing the effects of gas evolution (albeit at a broad statistical level) over time. And it makes sense. We expect more distant galaxies to be more gas-rich, but exactly how much has hitherto been rather mysterious : other observations suggest that galaxies have been continuously accreting gas to replenish at least some of what they've consumed. For the first time we have some actual honest-to-goodness data* about how this works.

* Excluding previous results from stacking. These have found galaxies at even higher redshifts, but since they only give you the result in aggregate and not for individual galaxies, they're of limited use.

That said, it's probably worth being a bit cautious as to how well they can identify the optical counterparts of the HI detections. At this distance their beam size is huge, a ginormous 1.3 Mpc across ! That's about the same size as the Local Group and not much smaller than the Virgo Cluster. And they do say that in some cases there may well be multiple galaxies contributing to the detection. 

A particular problem is here is the phenomenon of surface brightness dimming. The surface brightness of a galaxies scales as (1+z)4. For low redshift surveys like AGES, z is at most 0.06, so galaxies appear only about 25% dimmer than they really are. But at z=0.4 this reaches a much more worrying factor of four. And the most HI-rich galaxy known (apart from those in this sample), Malin 1, is itself a notoriously low surface brightness object, so very possibly there's more galaxies contributing to the detections than they've identified here. It would be interesting to know if Malin 1 would be optically detectable at this distance...

On the other hand, one of their sources has the classic double-horn profile typical of ordinary individual galaxies. This is possible but not likely to arise by chance alignment of multiple objects : it would require quite a precise coincidence both in space and velocity. So at least some of their detections are very probably really of individual galaxies, though I think it's going to take a bit more work to figure out exactly which ones.

It's all quite preliminary so far, then. Even so, it's impressive stuff, and promises more to come in the hopefully near future.

Going through a phase

Still dealing with the fallout from EAS 2024, this paper is one I looked up because someone referenced in in a talk. It caught my attention because the speaker mentioned how some galaxies have molecular gas but not atomic, and vice-versa. But we'll get to that.

I've no idea who the author is but the paper strongly reminds me of my first paper. There I was describing an HI survey of part of the Virgo cluster. This being my first work, I described everything in careful, meticulous detail, being sure to consider all exceptions to the general trends and include absolutely everything of any possible interest to anyone, insofar as I could manage it. Today's paper is an HI survey of part of the Fornax cluster and it is similarly careful and painstaking. If this paper isn't a student's first or early work, if it's actually a senior professor... I'm going to be rather embarrassed.

Anyway, Fornax is another nearby galaxy cluster, a smidgen further than Virgo at 20 compared to 17 Mpc. It's nowhere near as massive, probably a factor ten or so difference, but considerably more compact and dense. It also has less substructure (though not none) : the parlance being "more dynamically evolved" meaning that it's had more time to settle itself out, though it's not quite finished assembling itself yet. Its velocity dispersion of ~400 km/s is quite a bit smaller than than the >700 km/s of Virgo, but like Virgo, it too has hot X-ray gas in its intracluster space.

This makes it a natural target for comparison. It should be similar enough that the same basic processes are at work in both clusters : both should have galaxies experiencing significant tidal forces from each other, and galaxies in both clusters should be losing gas through ram pressure stripping. But the strengths of these effects should be quite different, so we should be able to see what difference this makes for galaxy evolution.

The short answer is : not all that much. Gas loss is gas loss, and just as I found in Virgo, the correlations are (by and large) the same regardless of how the gas is removed. I compared the colours of galaxies as a function of their gas fraction; here they use the more accurate parameter of true star formation rate, but the finding is the same. 

The major overall difference appears to be a survival bias. In Virgo there are lots of HI-detected and non-detected galaxies all intermingled. While there is a significant difference in their preferred locations, in Fornax this is much stronger : there are hardly any HI-detected galaxies inside the cluster proper at all. Most of the detections appear to be on the outskirts or even beyond the approximate radius of the cluster entirely. Exact numbers are a bit hard to come by though : the author's give a very thorough review of the state of affairs but don't summarise the final numbers. Which doesn't really matter, because the difference is clear.

What detections they do have, though, are quite similar to those in Virgo. They cover a range of deficiencies, meaning they've lost different amounts of gas. And that correlates with a similar change in star formation rate as seen in Virgo and elsewhere. They also tend to have HI extensions and asymmetrical spectra, showing signs that they're actively in the process of losing gas. Just like streams in Virgo, the total masses in the tails aren't very large, so they still follow the general scaling relations. 

So far, so standard. All well and good, nothing wrong with standard at all. They also quantify that the galaxies with the lowest masses tend to be the most deficient, which is not something I saw in Virgo, and is a bit counter-intuitive : if a galaxy is small, it should more easily lose so much gas as to become completely undetectable, so high deficiencies can only be detected in the most massive galaxies. But in Fornax, where the HI-detections may be more recent arrivals and ram pressure is weaker, this makes sense. They also quantify that the detections are likely infalling into the cluster for the first time in a now-standard phase diagram* which demonstrates this extremely neatly.

* Why they call them this I don't know. They plot velocity relative to the systemic velocity of the cluster as a function of clustercentric distance, and have nothing at all to do with "phases" in the chemical sense of the word.

The one thing I'd have liked them to try and this point would be stacking the undetected galaxies to increase the sensitivity. In Virgo this emphatically didn't work : it seems that there, galaxies which have lost so much gas as to be below the detection threshold have really lost all their gas entirely. But since Fornax dwarfs are still detectable even at higher deficiencies, then the situation might be different here. Maybe some of them are indeed just below the threshold for detectability, in which case stacking might well find that some still have gas.

Time to move on to the feature presentation : galaxies with different gas phases. Atomic neutral hydrogen, HI, is thought to be the main reservoir of fuel for star formation in a galaxy. The fuel tank analogy is a good one : the petrol in the tank isn't powering the engine itself. For that, you need to allow the gas to cool to form molecular gas, and it's this which is probably the main component for actual star formation.

There are plenty of subtleties to this. First, there's some evidence that HI is also involved directly in star formation : scaling relations which include both components have a smaller scatter and better correlation than ones which only use each phase separately. Second, galaxies also have a hot, low density component extending out to much greater distances. If the molecular gas is the fuel actually in the engine and the HI is what's in the tank, then this corona is what's in the petrol station, or, possibly, the oil still in the ground. And thirdly, cooling rates can be strongly non-linear : left to itself, HI gas will pretty much mind its own business and take absolutely yonks to cool into a molecular state.

Nevertheless this basic model works well enough. And what they find here is that while most galaxies have nice correlations between the two phases – more atomic gas, more molecular gas – some don't. Some have lots of molecular gas but no detectable HI. Some have lots of HI but no detectable molecular gas. What's going on ? Why are there neat relations most of the time but not always ?

Naively, I would think that CO without HI is the harder to explain. The prevailing wisdom is that gas starts off very hot indeed and slowly cools into warm HI (10,000 K or so) before eventually cooling to H₂ (perhaps 1,000 K but this can vary considerably, and it can also be much colder). Missing this warm phase would be weird.

And if we were dealing with a pure gas system then the situation would indeed be quite bewildering. But these are galaxies, and galaxies in clusters no less. What's probably going on is something quite mundane : these systems are, suggest the authors, ones which have been in the cluster for a bit longer. There's been time for the ram pressure to strip the HI, which tends to be more extended and less tightly bound, leaving behind the H₂ – which hasn't yet had the time to fully transform into stars. So all the usual gas physics is still in play, it's just there's been this extra complication of the environment to deal with.

What of the opposite case – galaxies with HI but no H₂ ? How can you consume the H₂ without similarly affecting the HI ? There things might be more interesting. They suggest several options, none of which are mutually exclusive. It could be that the HI is only recently acquired, perhaps from a tidal encounter or a merger. The former sounds more promising to me : gas will be preferentially ripped off from the outskirts of a galaxy where it's less tightly bound, and here there's little or no molecular gas. Such atomic gas captured by another galaxy may simply not have had time to cool into the molecular phase, whereas in a merger I would expect there to be some molecular gas throughout the process. 

Tidal encounters could have a couple of other roles, one direct, one indirect. The direct influence is that they might be so disruptive that they keep the gas density low, meaning its cooling rate and hence molecular content remains low (the physics of this would be complicated to explore quantitatively but it works well as a hand-waving explanation). The indirect effect is that gas at a galaxy's edge should be of lower metallicity : that is, purer and less polluted by the products of star formation. The thing is, we don't detect H₂ directly but use CO as a tracer molecule. Which means that if the gas has arrived from the outskirts of a galaxy, it may be CO-dark. There could be some molecular gas present, it's just that we can't see it. Of course, to understand which if any of these mechanisms are responsible is a classic (and well justified) case of "more research is needed".