Curiously cloudy

Today's paper is one that fell through the cracks of time from March 2021 all the way to the present day. Meaning, I should have read it a year ago, but didn't.

Returning to my favourite topic of optically dark gas clouds, this is an early science project of one of the SKA pathfinder telescopes. Naturally they chose a nearby galaxy group as a target. It's not an especially interesting group as far as I can tell, but looking at any galaxy group with the higher sensitivity and resolution of a new instrument is always a good idea.

What they found is... not much. None of the galaxies seem to be doing much of anything. Except, there are these two quite hefty HI clouds with no optical counterparts floating around and no obvious reason how they got there. Hooray !

Unfortunately though, God appears to have been very cruel by aligning both of these with optically bright sources. One is right behind a bright foreground star, which must be within our own galaxy and cannot possibly have anything to do with the cloud. The other is in front of a more distant galaxy, likely too distant to have any relation either. But of course this doesn't mean that the clouds don't have their own optical emission and it isn't simply being wiped out by the brighter intervening sources. They try to model and remove these, and don't find anything obvious, but I'd be very cautious about drawing any strong conclusions about this. There's just too much other optical crap in the way to be sure they've removed it all.

There's still a lot the data can say though. One of the clouds looks quite a lot like a standard rotating disc. The other is an elongated blob, but here the velocity gradient is across the shortest axis which is frankly just confusing - a spinning cylinder ? Doesn't make a lot of sense to me.

Combined with their size of a few kpc, the velocity gradients aren't very high. In fact they're low enough that the objects could be gravitationally self-bound with their HI alone. And their HI mass is pretty substantial, comparable to dwarf galaxies. So are these the long-sought "dark galaxies" of popular radio astronomy myth and legend ?

Maybe ! But no explanation is very satisfying, which is why these are really neat objects.

First, the standard go-to explanation for all HI clouds is that they must be tidal debris. This is a perfectly sensible default option. The problem in these cases is that there aren't many galaxies around, and the only others with HI are many hundreds of kpc away and don't show any signs of disturbances. Where are the long HI tails ? You'd expect to see these even if much smaller amounts of gas were removed, but that we see substantial amounts of gas in just these two clouds implies a factor of several more must have been removed - tidal encounters invariably produce streams, not just isolated clouds.

But given that the clouds appear to be self-bound, this may not be such a problem. Perhaps the encounter just took place a long time ago, so that most of the rest of the gas has dispersed. Without some dedicated numerical simulations, and even more sensitive observations to look for streams, we can't really rule out this scenario yet.

On the other hand, if they were dark galaxies, their self-bound nature is very surprising. The clouds we found in Virgo had such high line widths that they directly imply the presence of a dark matter halo to keep them stable, just like for normal galaxies. These objects don't fit that. The authors search existing numerical simulations for analogues, and they do find a few, but those tend to have higher line widths.

I would also note (though the authors don't plot this themselves) that these clouds have such narrow line widths and such high amounts of gas that they deviate from the standard Tully-Fisher relation for normal galaxies. Actually this looks very similar to those controversial galaxies without dark matter, which have been quite a challenge to explain. So it does seem a bit strange to invoke a dark galaxy for objects where there doesn't seem to be much indication of a dark halo. On the other hand, they do fit the standard mass-size relation for normal dark matter-dominated galaxies. 

What's going on ? I don't know, but this opens up quite a bit of parameter space for HI clouds. The Virgo clouds I've spent so much time on really can't be self-bound by their HI because of their high line widths : they'd have to be so compact that they'd have densities so high we'd expect explosive levels of star formation. These clouds, though they have ten times as much gas, are in a regime of size and velocity where they could indeed sit around quietly doing nothing. That they're so massive is really interesting : why haven't we found more objects like this already ? They were in fact already detected with much less sensitive instruments. That kindof implies a special explanation, rather than them being part of a larger population.

My preference would be old tidal debris. But I can't see any good reason why such clouds are found only in this group and not elsewhere, so this is more instinctive than rational. Those bloody intervening stars and galaxies make things especially tricky... yet even if they do have optical counterparts, that wouldn't necessarily make the situation any clearer : objects which are self-bound by ordinary visible matter are just strange, regardless of whether they're purely gas or purely stars. This is definitely a case where the rallying cry of "we need more data !" is absolutely legitimate.