Dey's Blue Blobs

Today's paper is more exciting than I can fully let on.

In the last few years there have been a handful of seemingly-innocuous discoveries in Virgo that don't quite fit the general trends for normal galaxies. They're very faint, very blue, metal-rich*, and some are incredibly gas-rich. The most convincing explanation thus far is that they're ram pressure dwarfs : not galaxies exactly, but bound systems of stars that formed from condensations of gas stripped by ram pressure. 

* Meaning they have lots of chemicals besides hydrogen, because astronomers have weird conventions like that.

The advantages of this explanation is that ram pressure is a high speed phenomenon, so could easily explain why the objects are so far from any candidate parent galaxies (tidal encounters can do this too, but usually require lower interaction velocities), as well as why they're so metal-rich. Primordial gas is basically nothing but hydrogen and helium, and to get complex chemistry you need multiple cycles of star formation, which makes it virtually certain that the gas here must have originated in galaxies. Why exactly they've only just started forming stars is unclear, though it's possible they do have older stellar populations which are just too faint to identify. And these things really are faint, with just a few thousand solar masses of stars... in comparison to the usual millions or billions expected in normal galaxies.

One of the main problems in understanding these objects has been the understandably crappy statistics. With only a half-dozen or so objects to work with, any conclusions about the objects as a population are necessarily suspect. That's where this paper comes in.

Finding such objects isn't at all easy. They're difficult to parameterise and tricky for algorithms to handle, so they opt for a visual search. And quite right too ! Humans are very, very good at this, as per my own work (which I'll get round to blogging soon). Having just one person run the search would risk biases and incompleteness, so they use a citizen science approach based on Galaxy Zoo. 

The result was a total of nearly 14,000 "blue blob"* candidates. But this is being extremely liberal, and many of these might just be fluff : noise or distant background objects or whatever. A more rigorous restriction in which at least three people had to identify each candidate independently reduces this to just 658. Further inspection by experts trimmed this to 34 objects – a still more than respectable improvement over previous studies. And while I previously berated them for claiming that the objects only exist in clusters without having looked elsewhere, this time they at least looked at Fornax as well as Virgo. Fornax is another cluster, but interestingly no candidates were found there.

* C'mon guys, this is the name we're going with ? Really ? Oh. Well, fine. Suit yourselves.

But they don't stop with the results of the search. They cross-correlate their results with HI gas measurements from ALFALFA and, yes, AGES (thanks for the citations, kindly people !), and also observe eight of them with the 10m-class Hobby-Eberly Telescope for spectroscopy of the ionised gas. This is extremely useful as it provides a robust way of verifying that these objects are indeed in the cluster and not just coincidentally aligned, and also shows the the gas in the objects is being affected by the star formation.

Let me cover the main conclusions before I get to why I'm so excited by this work. First, their findings are fully consistent with and support the idea that these are ram pressure features. Their spectroscopy confirms the high metallicity of the objects, comparable to tidal dwarfs – so they have indeed formed by material which was previously in galaxies. They avoid the very centre of the cluster (where they'd likely be rapidly destroyed) and are preferentially found where ram pressure is expected to be effective. 

There's also an interesting subdivision within these 34 candidates. 13 of these are "rank 1", meaning they are almost certainly Virgo cluster objects, whereas the others are "rank 2" and are likely to have some contamination by background galaxies. Most of the rank 2 objects follow the general trends in colour and magnitude as for normal galaxies, but the rank 1 are noticeably bluer. They're also forming stars at a higher than expected rate (though, interestingly, not if you account for their total stellar mass). So indeed these are galaxy-like but not at all typical of other galaxies : they are galaxian, not galaxies.

Now the fun stuff. They identify two supposedly optically dark clouds I found in Virgo way back when and have since based most of my career on, hence – exciting ! They do have optical counterparts after all, then. Actually, one these is relatively bright, and I suggested it as a possible counterpart back in 2016. But it wasn't convincing, and its dynamics didn't seem to match well at all. These days of course everyone is all about the weird dynamics, but back then this seemed like a pretty good reason to rule it out. Since then, our VLA data has independently confirmed the association of the stars and the gas, and Robert Minchin is writing that one up as a publication.

That object has about twenty times as much gas as stars. The second object is altogether fainter, having a thousand times or more gas than stars ! Even with our VLA data we couldn't spot this*, and I probably wouldn't even believe this claim if they didn't have the optical spectroscopy to support it. It looks likely that in this case we're witnessing the last gasp of star formation, right at the moment the gas dissolves completely into the cluster.

* The VLA data has much better resolution than the original Arecibo data, so it can localise the gas with much greater accuracy and precision. This means that it can show exactly where the HI is really located, so if there's even a really pathetic optical counterpart there, we can be confident of identifying it. But of course, that counterpart must be at least visible in the optical data to begin with.

While they comment directly on two of our objects, they actually implicitly include another three measurements in the table. We never identified these as being especially weird; they just look like faint blue galaxies but nothing terribly strange. And that really underscores the importance of having enough resources to dedicate to analysing areas in detail, which, frankly, we don't. It also shows how important it is to quantify things : visual examination is great for finding stuff, but it can't tell you if an object is a weird outlier from a specific trend. Even more excitingly, almost certainly it means that there are a lot more interesting objects in our data that have already been found but not yet recognised as important.

But the most fun part came from doing a routine check. Whenever anyone publishes anything about weird objects in our survey fields, I have a quick look to see if they're in our data and we missed them, just in case. Every once in a while something turns up. This is very rare, but the checks are easy so it's worth doing. And this time... one of the other blue blobs has an HI detection in our data we previously missed.

Which is very cool. The detection is convincing, but there are very good reasons why we initially missed it. But I don't want to say anything more about it yet, because this could well become a publication for my PhD student. Watch this space.