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.
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