Chained in darkness

This is a very interesting paper about my favourite topic, optically dark gas clouds.

As far as I can tell, the discovery is purely serendipitous. They pointed the shiny new MeerKAT telescope at a galaxy group as part of a larger survey, and happened to find this linear chain of seven clouds. Hydrogen clouds without optical counterparts are pretty rare things, but these ones are among the most unusual. They're spread over an enormous extent (400 kpc !), are each pretty massive (around a billion solar masses, roughly the same gas mass as a chunky dwarf galaxy), have high line widths (~130 km/s, again dwarf galaxy territory), and have no obvious association with anything optical.

Somewhat oddly, they begin with a quite protracted introduction discussing why HI clouds are in general unlikely to be dark galaxies. While I agree, they erroneously cite my papers as supporting this, which of course they don't - rather, we found that certain objects are more likely to be dark galaxies than others. Even so, the introduction is a nice overview, though it feels a bit strange that they don't much discuss the possibility later on.

The paper is largely a discovery and description affair, avoiding much speculation as to the origin and nature of the objects. This is very sensible. From what we found, objects with high line widths, say > 50 km/s, are very difficult to explain as tidal debris in clusters. But these objects aren't in clusters, they're in a small-ish galaxy group (18 members), where the speeds may well change the kinematics of any HI clouds produced in galaxy-galaxy interactions. It's also impossible to ignore the neatly linear alignment of the clouds and the fact that part of the complex forms a sort of V-shaped feature. So debris of some kind does seem to be at least a plausible explanation.

Unfortunately they don't provide optical imaging over a large area, so it's difficult to see which if any galaxies these might relate to. But within the area they examine, there's no galaxy which looks like an obvious source of the gas. It reminds me quite a lot of the Kent Complex, which sadly they don't present as a comparison object (though they do compare it with HI1225+01, which is also similar). And there are certainly no big nearby galaxies with HI tails that one could point to and say, "there, that's the bugger !".

It also seems to resemble the Leo Ring in some aspects, which has comparable mass, size, lack of optical counterparts, and environment. But the kinematics and morphology of the Ring are very different. Although it lives in a different environment (a rich cluster), and measures only about a quarter of the size of this feature, to my mind the Kent Complex is overall more similar : comparable in mass, linear-ish morphology, and with several distinct "clumps" of similar kinematics. The latter is especially important, since line widths this high are extremely unusual.

No other feature is a perfect match though, and importantly, the origin of the Kent feature also remains a mystery. Dynamically, the major source in the MeerKAT object would require some dark matter for it to be stable, though not nearly as much as a typical galaxy. So neither "dark galaxy" nor "tidal debris" is wholly satisfactory in either case. Surely, it can't be a coincidence that this feature is so close to a galaxy group, but what's especially strange is that the group is HI-poor. How do you remove so much gas - nigh-on ten billion solar masses - without much affecting the optical component of the galaxies ? How do you remove the gas so completely from the parent galaxy ? At least some ought to be left in the galaxy itself, deep in the potential well.

All in all, a very interesting read about a fascinating object. Unusually, further observations here require not better spatial resolution (they already have that) but better kinematics - a resolution of 45 km/s is pretty awful, really. Even so, it's not at all obvious if better velocity resolution would help. My bet is that it will remain mysterious for quite some time to come.

Yo Dawg, Herd You Like Missing Matter

So we took your missing matter away so you can miss your missing missing matter.

Ahem.

Galaxies lacking dark matter are a reoccurring topic these days and they're back in the headlines again. Time to take a look at the paper behind the headlines.

To recap, there have been a few different galaxies reported all showing the same basic trend : their stars and/or gas are moving much more slowly, at any given radius, than normal galaxies. Whereas most galaxies rotate so quickly that they require a huge amount of extra, invisible dark matter to hold them together (or equally, their rotation can be said to imply that there's something wrong with our theory of gravity), these ones don't. Apparently, in these handful or peculiar cases, just ordinary stars and gas and good ol' Newton are more than enough.

Thus far, Six galaxies present especially nice examples. Unlike some other, more prominent cases (we'll get back to those later), these are too far away to attribute this rotational oddity to distance measurement errors. To determine the amount of dark matter, we need to know both the speed at which any part of it is moving and the distance of that point from the centre of the galaxy itself. In essence, it's the speed as a function of radial distance, not speed alone, which is the important number. We don't ever measure total dark matter content directly - instead we get the amount of dark matter enclosed within a certain radius.

Measurements of the speed don't directly depend on knowing the distance of the galaxy, but the radial (galactocentric) distance does. And in these six cases, the uncertainty on the distance is far too low to be a possible explanation for their unusual properties. Which is very helpful.

But, there's an added complication. We don't really get the speed directly either. Well, we do, but not the true rotational speed. We get how fast things are moving along our line of sight. To convert this to rotation, we need to correct for how the galaxy is inclined towards us : for edge-on galaxies the measurement is directly equivalent to rotation; for face-on galaxies we can't measure rotation at all. For galaxies somewhere in between, knowing the inclination angle allows us to make a correction. And that, arguably, is a potentially big difficulty.

In the current paper prompting the latest press release, the authors revisit one of their old favourites, using new high-resolution data of the atomic hydrogen content to estimate the inclination angle. They find good agreement with their old data, essentially supporting their original inclination angle estimate and conclusion that it lacks dark matter. They also go much further in their efforts to establish the inclination, looking at a wide variety of possibilities for the true nature of the galaxy, e.g. considering different thicknesses, orientations on the sky, and find a model which minimises the residuals : that is, when subtracting the model galaxy from the original data, it leaves behind nothing but noise.

All this seems to indicate that the galaxy is well-fitted by a model disc of pure gas and stars at an inclination of about 32 degrees. They go further, and try at some considerable length to find if there's even any dark matter model at all which could account for the observations. In short, there isn't : the dark matter would either have a bafflingly low concentration, or just gets the velocity measurement all wrong. The disagreement is so low that it could even be called non-physical.

Interestingly, this galaxy also poses difficulties for alternative theories of gravity. Now Modified Newtonian Dynamics does allow for galaxies which rotation curves like this, but only if they're near to massive galaxies. This one apparently isn't. It seems to be isolated and in nice stable equilibrium. And so MOND, just like the standard model, predicts this galaxy should be rotating much faster than it actually is.

(It's worth remembering that this is ordinarily an advantage of MOND in that it does just as well as the standard model in predicting rotation speeds. But whereas the standard model doesn't absolutely require the dark and normal matter to always be associated with each other, MOND says that in most cases, the same mass distribution should give rise to the same rotation speed. So galaxies like this one ought to be if anything more problematic for MOND than the standard model of dark matter.)

What's the catch ? Is this just a really, really weird galaxy, or is something more pragmatic at work ?

I've favoured different possibilities on this one in the past, but this paper unwittingly swings me (slightly) in favour of the latter. Here's their money figure. It shows an optical image of the galaxy with the hydrogen gas as a white contour. Their best-fit inclination (32 degrees) is shown by the black line, with the blue dashed line at the much lower 11 degrees needed for this galaxy to have a more normal sort of rotation speed :

And for all their hard work and sophisticated modelling, I'm just not convinced that they can really rule out the lower inclination angle possibility. They use the gas disc to estimate the inclination, but look how ragged that edge is... is it really such a stretch to imagine that deeper observations would reveal a better fit for the lower angle model ? Observational errors have a nasty habit of being larger than we would like (estimates of errors should always be treated with caution), and it seems like the galaxy might need to have only a marginally unusual morphology for it to be well-fitted with this lower angle. After all, the inclination angle of the gas and stellar discs can be different, and the angle measured is already very low.

But, this is not to say the case is settled. Far from it. Recall the similar case of NGC1052-DF2, which was also thought to lack dark matter. A lower distance would have settled all the anomalies of that galaxy, so from that perspective a lower distance seemed far more likely... but the best, most recent available distance measurement does appear to be the larger, weirder value (at least, no-one has challenged the claim yet, as far as I'm aware). So I'm going to be very cautious indeed about reaching any kind of firm conclusion here.

To credit the authors, they admit that the inclination angle is the most crucial measurement. And I remain persuaded that their previous arguments about how the low velocity dispersion variation (the random motions of the gas distinct from the rotation) are much more consistent with a nice stable disc than any large-scale disordered motions. So my instinct to say that this oddball galaxy is probably due to a weird inclination measurement problem is not a very strong one; maybe 60:40 odds. As they say, if they could get rotation estimates at even larger distances, this would be really interesting : their interpretation clearly predicts that here we should see declining rotation velocities. How they might get these, though, is hard to say.

Obviously, then, this isn't mystery solved. Rather, we're still at the stage of determining if there is even a mystery at all*.

* Which is a lot better than the recent approach of the Metropolitan Police in investigating crimes.