Nothing to see here

The last few years have seen a lot of hoo-hah about galaxies apparently lacking dark matter, but this may at last be drawing to a rather boring conclusion.

First, we had a whole kerfuffle about two dwarf galaxies, NGC 1052-DF2 and DF4. These seemed to have such a low velocity dispersion that they wouldn't require any additional matter apart from their stars to hold them together. There was some dispute about the magnitude of the velocity dispersion (in my opinion not a very credible one) which went away quite quickly because yes, their velocities really were very low. A much more protracted and complex dispute arose about their distance, which was only settled quite recently and found that yes, they are far away (and lacking dark matter) after all.

Those first discoveries were pure starballs. Then, we've also had the saga of galaxies with gas, for which we can much more easily examine the rotation velocity. They were also so far away that there was no prospect of a distance controversy. But just the other day, despite many arguments in favour of these being a population of weird objects, simulations showed that such objects can't be stable. It's far more likely that there's been a slight measurement error in their inclination angle. I wouldn't personally call that one settled just yet, but it's definitely going in that direction.

There was also bad news for those of us hoping the original objects would turn out to be something truly weird : simulations found a plausible way of producing objects like this through unusual but entirely plausible encounters, specifically requiring direct collisions between galaxies, which are relatively rare. Which is great news for the boring old standard model of cosmology.

Today's paper feels like another nail in the coffin for anyone hoping these objects would turn out to be truly strange. Not the final nail by any means, and it's still just about possible they could burst back out like an enraged vampire, but it's definitely making it harder for any would-be children of the night to go on a killing spree.

It's a nice little paper but I do have to wonder why on earth it made it into Nature. Noting the simulations showing that collisions are a viable explanation, they go out and have a look at the context of DF2 and DF4 on a larger scale. They show they're in a neat line of maybe 11 objects in total, stretching over 2 Mpc. This is nicely consistent with the collisional origin hypothesis.

And honestly... that's it. Yes, it's a nice discovery, but they don't go on - as I assumed they would - to do numerical simulations to back this up. They don't have any new observations of these additional galaxies to see if any of them have anomalously low velocity dispersions. They just report the basic discovery along with their (undeniably) very interesting hypothesis. I mean, I'm glad they reported this, but it feels much more worthy of a letter to MNRAS than to Nature.

As I said though, this is not the final nail. More observations of those new galaxies are essential to determine their distances and velocities, otherwise this is just a chance alignment. More simulations to examine exactly what we expect in a collision similar in quantitative detail (major kudos to the authors for proposing this, which largely offsets publishing it in the wrong journal) would also help, a lot. But it's awfully tempting to declare the mystery... well solved is too strong a word, but shall we say, addressed then ? Yeah, that'll do. Mystery addressed !

There is also perhaps some prospect of a weird afterlife for these oddballs. If they do form through good old-fashioned collisions, they're like teeny-weeny versions of the famous Bullet Cluster. This has caused a lot of controversy because Standard Model enthusiasts have pointed and laughed at MOND believers by saying, "Hahah ! Look, we found a case where dark matter can be separated from normal matter, and you can't do that just by modifying gravity !". To which MOND devotees responded by saying, "Yes it can, you don't understand our theory at all, and look the collision velocity you need is just too dang high." But then the Standard Modellers retorted with, "Yeah, but you don't have a unique relativistic solution, and anyway, the collision velocity is way lower than we though, so pfffffp !".

Or something like that anyway. I tend to heavily favour the Standard Model, but if we had more cases to compare, especially on such dramatically smaller scales like these galaxies, this might give us a much more rigorous comparison. The thing about MOND though it's that it gives such similar results to the dark matter paradigm that one wonders what the point of it is. So, we'll see.

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.

Is it a plane ? No, it's a... uhh... orbital anisotropy, or something

We haven't heard from satellite planes from a while, so let's re-open that old wound and fill it with hornets. 

My stance on satellite planes calcified quite some time ago. Claims that satellite galaxies orbit their parents in narrow planes are, in my view, mainly due to an over-reliance on numerical estimates of statistical significance and an apparent disdain for actually looking at the bloody data. Most such structures don't look like anything very much at all to me, let alone the insurmountable challenge to the standard models of cosmology they're supposed to be. See the previous link for a much more in-depth and angry examination of this.

The major exception is the plane around our own Milky Way galaxy. This one is visually unmistakable, and regardless of whether such planes are indeed common or not, this surely deserves an explanation. Of course, it's possible our own galaxy just happens to be unusual, but this isn't very satisfying.

This paper takes another look and says there's nothing remarkable about this at all.

Frankly, I am not sure I either agree with or fully understand their arguments here. They examine the plane only using the 11 brightest "classical" satellites, noting that the anisotropy is dominated by just one or two objects. Move them around a bit and actually the "plane" becomes nothing very remarkable.

Hmm... maybe. It would have been nice to see an example, but more fundamentally, they don't seem to consider that basically all the more recent satellite galaxies also follow the plane. I think it's unfair for people to add other features like stellar streams as independent evidence of the plane (because if the galaxies are in a plane, then all features resulting from their interactions must be there too), but ignoring the other galaxies seems strange. To be fair, this seems motivated by previous studies which also limited themselves to this subset.

Their other claims feel on firmer (or at least clearer) ground. Earlier studies couldn't reproduce the planes, they say, because of numerical limitations in their simulations. Apparently some satellite galaxies are disrupted through purely numerical effects (though I am not sure what exactly), but they are able to account for this and reproduce systems more closely resembling the Milky Way.

What seems to me their strongest argument is new data from Gaia for proper motions of the galaxies. If the plane was indeed long-lived and stable, as claimed, we'd expect there to be little velocity dispersion perpendicular to its axis of rotation : in fact, they say, the dispersion both parallel and perpendicular is about the same. That's not at all consistent with rotation, and indicates the plane is just a chance alignment. So while people have generally concentrated on what fraction of galaxies in simulations show planes at any one moment, it might be interesting to consider what fraction ever show a plane at any point in their history.

All this is nice, but I can't help looking at the structure and still thinking, "really ? that's normal, is it ?" Just as it would take something dramatic to convince me of the counter-claim that "planes are everywhere !", so it's going to take me a bit more than this present work to convince me that "the Milky Way's plane is nothing special."