Un-missing matter

When you find that the missing matter of a galaxy is itself missing, and then you realise that actually it isn't missing missing matter, it's just the regular missing matter, you're in a right proper linguistic pickle.

Readers are advised to consult this previous post for a full summary. In brief, there's this galaxy which doesn't seem to have the usual dark matter (unseen and hence missing by direct observations) needed to hold it together. Based on the mass of its visible stars and gas, its rotation speed is so slow that it's apparently gravitationally bound without needing any of this additional matter at all. And this just doesn't fit the general picture of galaxy evolution : yes, some systems like this can be produced in interactions, but this object is nowhere near anything else. And a generally overlooked detail is that the slow rotation speed means the galaxy wouldn't have had much time to settle into its apparent stability.

My major worry last time was that the rotation speed of the galaxy might have been underestimated. This is dependent on estimating how inclined the object is along our line of sight : if it's directly edge on, we'd be able to measure its full rotation directly, whereas if it's face on, we wouldn't be able to measure rotation at all; at intermediate inclinations we can apply a correction factor. The observations indicate a low inclination angle, which makes this correction already uncertain, but the very ragged edge of the gas disc used for estimating the angle makes this process extra difficult.

To be fair, the authors of the observational studies have had many good arguments in favour of the higher inclination angle they say is preferred by the data. Indeed, it wasn't until the last paper that I would even have questioned this. And they do admit that the inclination angle is the biggest uncertainty.

Today's paper is by a different group coming at the problem with numerical simulations. They cite a lot of private communication with the original team, so it's nice to see that this dispute is being addressed cooperatively.

You might remember my own extensive efforts to model a galaxy without dark matter that met with assorted dismal failures. Only when I added a nice big dark matter halo, to ensure stable circular orbits, did everything magically click into place. This is nothing very new, as the author's of the latest paper explain - in fact, disc stability was even one of the motivations for dark matter in the first place.

What they do is much as you might expect. They model the galaxy as according to the original observations and see what happens. Which, as I found myself, is that the object tears itself apart in short order. Even if they use a very peculiar dark matter halo the original team say the observations do permit, it just doesn't work - it's a bit better, but not much. And then, lo and behold, if they allow a lower inclination angle and hence higher velocities and a big dark halo, everything works out nicely.

This is an awfully tempting explanation. It's just possible that there is some combination of parameters that would be compatible with the observations where a dark halo is not required, but this needs a lot of fine tuning and feels contrived. In contrast, accepting the inclination angle is wrong and there actually is a dark halo after all is a hell of a lot simpler and more satisfying. There is one other option that the object is not in fact stable, and is in the process of disintegrating, but this would seem to be incredibly unlikely in the case of an isolated object.

Not that we should discount these alternatives altogether, mind you. Remember one of those other claimed galaxies without dark matter, which had a prolonged controversy over its distance. A lower value would have made it a very satisfactorily normal galaxy after all, but in fact it was the more puzzling higher value that won out. So galaxies without dark matter are still, for the time being, very much a thing. But in this particular case, my money would now be quite firmly on the "inclination angle measurement" problem. And given that those other cases may well be tidal objects, this particular controversy just might be wrapping itself up.

Not the One Ring but the wrong ring

I have to confess that I read today's paper by mistake. The title is about "HI debris in the NGC 7232 group", which I misread as NGC 7332. And we have our own observations of that group, so I wanted to see if the new data found anything that we didn't. Well, I suppose technically it does, but only because it's of another group entirely...

This meant I was at first disappointed, and then intrigued, then annoyed, then intrigued, and then mildly annoyed again. But overall I'm interested. In fact I think the authors potentially under-sell their main result, which is always better than the more common opposite case.

Their target is a relatively nearby little group of galaxies, using radio data to study gas loss. Why look at groups ? Massive clusters tend to get all the glory because there are so many galaxies crammed in that there's always tonnes of stuff going on, plus you can observe lots of targets in a single observation - but it's small groups where by far most galaxies tend to hang out. Clusters may be sexy, but that's like saying Instagram influencers are sexy. Perhaps they are, but if you wanted to study a cross-section of humanity, looking on Instagram would be about as poorly representative as selecting people randomly from the public library. Galaxy groups, on the other hand, they are much more like the great thriving mass of Joe Public - it's them you want to study if you want to know about more typical behaviours.

The authors use the fancy new MeerKAT array of 64 antennas to get higher resolution and sensitivity HI observations than has been possibly in this group before. And this does help, substantially, though I should add that the column density sensitivity is still about two orders of magnitude worse than Arecibo was capable of. But such is life.

Anyway, at the heart of the group is a triplet of galaxies - two spirals and one lenticular. They find... a great big HI ring (see their figure 3). This is already really strange, but for some reason they don't comment on the morphology at all. I find that to be a very strange omission, especially considering that the ring isn't small - it's about 100 kpc across. Polar rings, which orbit directly around the plane of a galaxy, are at least sort-of understood, but this isn't one of those : the three galaxies are found all on one side of the ring, embedded within its HI gas - there's no galaxy in the centre of the ring. Not mentioning that the gas is a giant ring is a bit like finding a giraffe and neglecting to mention that it was purple.

Actually, they point out that only two of those galaxies may be associated with the ring material. At the velocity of the third spiral, which is quite a bit different from the other galaxies, they don't detect any obvious HI extensions. So probably there are just two galaxies involved here. But the ring isn't anything remotely like the tail and counter-tail structure seen elsewhere and in umpteen numerical simulations of interacting galaxies.

Connected to the ring is a long HI tail terminating in a big blob. A bit further away in the same direction is another, smaller tail, orthogonal to the first and pointing directly towards (almost intersecting) a lenticular galaxy. So this also, they say, might have been involved in the interaction. Okaaay... yes, it might be, you're not wrong, but this is glossing over a lot of interesting stuff ! How do you get a ring-shaped structure with a neat linear tail and then another tail that's orthogonal to it ? That's just weird ! And lenticular galaxies don't often have gas, so that's of note all by itself.

And the mass of the gas present doesn't really help. They estimate the deficiency of the galaxies and find that overall this system has more gas than expected. While the galaxies themselves have lost significant amounts of gas (though I would dispute the use of the words "vast majority" here), overall, the total mass of HI present is actually somewhat more than the whole system would be expected to contain if it was a bunch of normal, isolated galaxies.

What's going on ? To my mind this is one case where accretion ought to be considered a serious possibility. We know that galaxies in general have star formation rates that require external replenishment of gas, but claims for detecting the accretion material are always fraught with the difficult question as to why we don't see such structures everywhere. Most such circumgalactic material (as it's called) can indeed probably be explained as gas removal through tidal interactions. This is the interpretation the authors exclusively employ here, but I don't think it fits well at all. The morphology is all wrong, the kinematics don't fit (though their velocity resolution is low), and the total mass of gas doesn't fit the picture for gas loss. Surely, this deserves some more exotic considerations.

So yeah, this is a really interesting little system. I'm slightly annoyed that they don't cite any of my papers, more annoyed that they restrict themselves to a single interpretation, but it's a very nice result indeed and clearly we need more MeerKAT.

Not interesting but important

Well... it's probably unfair to say it isn't interesting. But when I read this paper I was expecting some cool scientific findings, of which there are none. Instead, this paper presents a new method for analysing HI spectra in a robust, reliable, objective way, providing quantitative ways to describe things like the shape of the profile and whether it's asymmetrical or not. They apply this to the enormous ALFALFA data set, so describing this method and giving the full catalogue on such a large sample is very important. It's just not interesting yet because they don't do any scientific analysis of the results.

Anyway, I've long thought that there's potentially a lot of unexploited information in an HI spectra. For example, the classic double-horn (a.k.a. Batman) profile arises because of flat rotation curves : most of the gas in a galaxy is moving at a single, constant velocity, irrespective of its distance from the galactic centre - the very discovery that led to the idea of dark matter. So could there be other such important discoveries to be made by further exploiting the data ?

Perhaps not. But it's been difficult to examine the spectral shapes in a systematic, objective way, because the measurements are surprisingly hard. Typically, we measure the width of the line at 20% and/or 50% of the peak flux (known as the W20 and W50 parameters) , and that's about it. Nobody does much with such nuances as the shape of the profile because objectively describing the shape is rather hard. 

There's one exception. People do try and measure the asymmetry, by comparing how much flux is found either side of the central velocity. Unfortunately this is the exception that proves the rule, as here the results prove controversial. You'd expect to see wonky profiles for galaxies in dense regions, where there are lots of gravitational interactions to disturb the gas. Some people find this, but some don't, and some find there are anyway strong asymmetries even in isolated galaxies, meaning there could be internal process causing disturbances as well. It's all very confusing, the numbers vary considerably, and not at all satisfying. Nobody can agree on what constitutes a significant level of asymmetry, which is a problem.

What the authors of the present study do here is actually better explained in their previous paper (see section 3.2 and especially figure 1). They use the curve of growth method to measure the HI spectra, something which is actually quite common in optical astronomy but not much used in HI analysis.

Let me explain the problem of the existing measurements a bit more. Usually, the W50 and W20 values are in good agreement. W50 is generally better because it's measured at a higher flux level, but in a strongly asymmetric profile, this can give an estimate of the line width that's much smaller than the true value - you can end up measuring only one of the horns, for example. W20, measured at a lower flux level, is often confused with noise, so can give width estimates which are too high. Although most of the time the two measurements do agree quite well, it'd sure be nice to have something more robust.

The curve of growth method is a pleasingly simple alternative. Starting from the central velocity, the flux is integrated incrementally over larger and larger velocity widths. And all you do is then plot the cumulative flux over these increasing widths, and what you get is roughly a linear slope which then quite suddenly flattens when you hit the edge of the galaxy. Et voilà, a robust, objective way to determine the true velocity width of the galaxy, irrespective of the shape of the profile.

Marvellous ! But there's more. You can parameterise the width in different ways, e.g. by looking for the width that encloses, say, 85 or 90% of the total, giving objective criteria for measuring both width and total flux that account for the signal to noise. And you can do the curve of growth for each half of the spectrum separately, giving you robust flux ratios for measuring asymmetry. 

*Importantly, though the details are boring, the authors also correct for how much the signal to noise level affects the measured asymmetry, meaning you can now reliably quantify the significance level of any measured asymmetry and thus settle any arguments about how much of a flux ratio is really needed to quantify as asymmetry.

This also gives additional parameters. For asymmetric profiles, the slopes of the two curves of growth are different, so you can also compare the slopes as a measure of asymmetry as well as the flux ratio. And more complex parameters can quantify the actual shape of the profile. They favour a parameter they call K, which measures how much the integrated flux compares to the case of a linear increase : the linear case (K=0) corresponds to a flat-topped profile, while K < 0 (less flux than linear) corresponds to double-horned profiles and K > 0 happens for single-peaked profiles.

What this means is they can create reliable, quantifiable, continuous parameters for measuring both asymmetry and shape. What they stop short of doing is any sort of analysis on the results. Are double-horn profiles more common in certain environments, or dependent only on galaxy morphology ? Are there any galaxies with strong HI asymmetries but without optical disturbances ? Does K vary linearly, or does it fall into nice neat groupings ? Do these new parameters work well at low S/N levels ? Can they help with source extraction or this is strictly only for analysis ?

This paper very literally raises a lot more questions than it answers, which is a good thing in this case. Though I'd like to really have a good thorough look at how this technique works in anger, it's got definite potential for being a classic case of, "why hasn't anyone done this already ?". Even if it isn't likely to lead to another discovery as profound as dark matter, it's still likely to give rise to a veritable plethora of spin-off papers and a horde of citations so large the Mongols would be jealous. Well, maybe.