Is it a bird, is it a plane ? No, it's a super spiral !

"Super spiral" is not a term I've ever heard used before but apparently this is what the cool kids are calling the largest and most massive of all spiral galaxies. Whereas the Milky Way rotates at about 220 km/s, the fastest rotation of all reaches a whopping 570 km/s.

Now normally in astrophysics all the key breakthroughs are made at the margins of what's technically possible. We build a new telescope to see a wavelength we couldn't see before, or we increase sensitivity to different sorts of features, and right at the limit of what we're able to accomplish we find something unexpected. And physics is usually better behaved on the largest of scales, where nice simple gravity tends to dominate. So to find giant galaxies doing something unexpected is extra-specially unusual, but that's just what this paper has found.

There's one small caveat. These beasties are so massive that they're incredibly rare, meaning you need a large search volume to find any. So measuring their rotation in the best possible way - using a radio telescope to measure their atomic gas - is difficult, because they're just a bit too far away for that (though I suspect it will be possible given a bit more time). But they're close enough to use other reliable measures, in this case via the ionised gas.

What the author's find is that the galaxies are rotating more quickly than the standard Tully-Fisher relation predicts. There's normally a tight correlation between the rotation speed of a galaxy and its combined mass of stars and gas, but these galaxies break that relation. And they deviate systematically, as though they were a different population obeying their own TFR with blackjack and hookers. In essence, it looks as though the galaxies are either rotating too quickly or have less gas and stars than expected.

Neither of these interpretations is easy to explain. The rotation looks very solid : the rotation curve shown is clear and with minimal scatter. It's even still rising at the edges, meaning that the figure of 570 km/s is a lower limit (though I would stress that since this is only a letter, not a full paper, only one rotation curve is shown). So not much chance the rotation has been measured incorrectly. As for the masses, stellar masses are relatively easy to compute, and although the ionised gas might not be the best tracer of the total gas mass, estimates would be have to be wrong by a factor 5-50 to explain the deviation - and that's just not credible.

But actually, it might not be too difficult to explain these whoppers in standard galaxy formation models. The authors say this points to an upper mass limit for galaxy formation, beyond which it's difficult for infalling gas to cool and form stars. To exceed this mass requires mergers instead, which build up galaxy mass and rotation in a different way, neatly explaining the change of slope of the TFR.

Where it gets really interesting is for modified gravity theories like MOND. The rotation signal is very clear and it doesn't seem at all sensible - as MONDian advocates usually do - to argue that the galaxies are out of equilibrium and so not representative of stable conditions. The rotation curve is frankly lovely and shows no signs of distriburbance, and getting anything to rotate at 570 km/s is frickin' hard anyway. And the parameters (size and rotation) of the galaxies are such that they should be showing strong signs of MOND's effects, which predict that they should rotate more slowly than expected, not more quickly ! Even MOND's beloved external field effect - the influence of nearby galaxies - doesn't seem to work, as that too would decrease the rotation speed, not increase it.

Can MOND survive this latest challenge ? It's seen off many a similar bold claim before, albeit often in ways that feel like it's rendered as a barely scientific theory at all, much less a decent one. But with these... I suspect there are too few data points here to be fully convincing, but the noose is tightening.

A Break in Spiral Galaxy Scaling Relations at the Upper Limit of Galaxy Mass

Super spirals are the most massive star-forming disk galaxies in the universe (Ogle et al. 2016, 2019). We measured rotation curves for 23 massive spirals and find a wide range of fast rotation speeds (240-570 km/s), indicating enclosed dynamical masses of 0.6 - 4E12 Msun.