Sister blog of Physicists of the Caribbean. Shorter, more focused posts specialising in astronomy and data visualisation.

Friday, 13 January 2023

This is exactly what I expected

Today's post is not a paper but a poster (doesn't format well on mobiles so please use an actual PC). I wouldn't normally do anything this preliminary, but we've been waiting long enough for this that I'll make an exception. Keep in mind that content that appears in a poster can change drastically by the time of formal publication, so everything I'm about to say is subject to revision.

To give some necessary context : Robert Minchin and I had this running joke in Arecibo that when set he'd automated a task, he really meant he'd delegated it to the postdoc, i.e., me. Well, it took several years and Robert moved institutions twice, but I finally have my revenge. Mwhahahah !

What's this all about then ? Well, way back in 2012/2013, we published a couple of papers which, among other things, noted the discovery of eight optically dark hydrogen (atomic HI) clouds in the Virgo cluster. These were particularly strange in that they have velocity widths normally associated with rotation-dominated galaxies, but no optical counterparts. They were also pretty isolated, with no obvious nearby parent galaxies to explain them.

What followed for the next decade has been a series of papers looking at the various possible explanations. In the "Flying Snakes" paper, we catalogued other, similar objects, did some simple analytical calculations, and also ran a bunch of simulations to see if we they were more likely to be optically dark galaxies or tidal debris. This was followed up with more, similar but more realistic simulations a little later. And then we tried even more advanced simulations looking at more novel suggestion that the objects were held together by the pressure of the intracluster medium.

Let's cut to the chase. Tidal debris doesn't work for these objects - they are much too far from their parent galaxies and should have dispersed already (clouds with smaller line widths, however, are no problem at all for the tidal debris scenario). And the confining pressure of the ICM just isn't significant.

This leaves the most dramatic explanation : that these objects are indeed rotation dominated, and thus require a substantial amount of dark matter to prevent them from exploding. They are in effect dark galaxies.

... except, this explanation has problems too. We don't really find objects like these except in clusters, and the ram pressure they experience there ought to destroy them almost instantly. So the dark galaxy hypothesis remains at most the best of a bad bunch.

What we've yearned for is data with better spatial resolution. With Arecibo they're just point sources. If we could see their morphology, we could tell whether they had the all-important ordered motions that would truly indicate rotation, while more haphazard motions would mean some other, probably less interesting explanation.

At last, we have it. We put in the proposal to the VLA and got the data back in 2017 (!), but then muggins here left it lying on a shelf ever since. Why ? Basically, stuff kept coming up. Reducing the data is damned hard if you don't know what you're doing and there's always lower-hanging fruit to pick. I mean, sure, I could spend months doing this incredibly difficult thing to get this probably very interesting result, or I could do this easier thing with a guaranteed moderately interesting result instead.

Anyway, Robert has reduced the data and made a shiny poster for the AAS meeting. What did we find ?

Of our eight targets, six were observed. Not sure what happened to the other two but never mind. Of those, three were detected. The non-detection of three is not at all surprising, as the VLA can be hundreds of times less sensitive to low-density gas than Arecibo. So it goes.

Of those three which were detected... (drumroll please)...

Two are galaxies. One is optically dark.

Ho-hum. Exactly as I expected, we found results which have nothing much to do with what were expecting but were interesting all the same - which is exactly why detailed observing proposals are a waste of everyone's time.

One of these, AGESVC1 231, appears to be associated with a very small, compact blue blob. Actually we spotted this blob from the word go, but we can only now say that the HI is associated with it thanks to the higher resolution of the VLA data. Without this, the HI map doesn't really coincide well with the object at all, it's so compact it could be anything. Also the high velocity width would be completely atypical for an object this small, so originally, there was no good reason to think the HI and the blue blob were related. But the match with the VLA data is so good it can't possibly be coincidence.

The other thing the VLA data shows is that the blob appears to be associated with the brightest, narrowest peak of the HI emission, with the high velocity width component being considerably displaced - i.e. a tail. Because of its lower sensitivity, the VLA only detects the narrow, densest component, with the wider, more diffuse component only seen with Arecibo. 

Tentatively, this looks like a dwarf galaxy experiencing ram pressure stripping. The blob is extremely blue (the bluest object in our original sample), which might be because ram pressure can initially compress the gas and enhance star formation before it removes and disperses it. So the object is indeed a galaxy, but whether it has any bearing on the notion of "dark galaxies" is harder to say. It's gas rich but not enormously so. More interesting from this one would be to ascertain the age of the stellar population : given that it's so blue, perhaps it only started forming stars very recently. 

What we're probably seeing here is the narrow velocity width component giving a decent representation of the object's true dynamics. The high width component, which is what marked it out as interesting and made us think the blue bob was unrelated, is likely deceptive : this might exist only because ram pressure is disturbing the gas, not because of rotation. So most likely the dark matter content is nowhere near what we initially guessed, though that needs to be more properly quantified.

The second object, AGESVC1 274, also has an optical counterpart. If you look at Robert's poster I think you'll see why we missed this one : it's truly pathetically faint, and you'd never claim it was associated with the HI based on the lower-resolution Arecibo data. However, we also had more sensitive optical data where this is more clearly visible, so I'm a little surprised I missed this. I'll have to go back and check to see why this one didn't get mentioned.

This object is one of two of our sample which have very low velocity widths. In this case, unlike 231, the VLA and Arecibo spectra are in excellent agreement. In some ways it appears to be a typical though extremely gas rich dwarf galaxy, which puts me in mind of other strange stellar systems. I also wonder if it might classify as an Ultra Diffuse Galaxy. But it really is very gas rich indeed, and certainly has a very low surface brightness. It's a galaxy for sure, but whether this is just an extreme example of the general population or is actually an outlier from the general trends is not yet clear.

The third object, AGESVC1 258, is the most interesting. It's only marginally detected in the VLA, but what we see is a clearly elongated structure. It has no apparent optical emission at all (though maybe I do see a pathetic little blob in the densest part of the gas ?). Now this elongated structure could indicate a disc, but it doesn't show any obvious velocity gradient that would be the smoking-gun signature of rotation. On the other hand, as with 231 unfortunately the VLA is missing a lot of flux compared to the Arecibo data, so we might just not be able to detect the rotation because the thing's so faint. In addition, the density of the detected gas is well below the typical threshold for star formation.

Again tentatively, this object might still be consistent with the dark galaxy hypothesis, but it could also be some really weird debris. If it's debris then it's still a mystery how it survived.

So as usual, this data answers some questions but raises whole new ones. Why are some optically bright but some are dark ? Do they have a common formation mechanism or we looking at completely different objects ? If they are debris then how do they survive and reach such great distances from their parent without dispersing despite their high velocity widths ? How much - if any - dark matter do they have ?

For me this is pretty exciting stuff. It's not a huge amount of new data, but it's more than enough to provoke a whole new inquiry. Watch this space.

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