When Is Something Massive But Not Massive ?

... when it's a massive galaxy that's not as massive as it should be.

What's that you say ? It's been more than five minutes since you heard about galaxies without any pesky dark matter, and you're desperate for a fix ? Look no further !

To briefly summarise : it looks as though the two candidates which started the ball rolling in this field really do lack dark matter, but a plausible, specific collisional formation mechanism can be invoked to explain them. The stellar masses of these objects are very low, as are those of the other major candidates... though those particular (ultra diffuse) galaxies are harder to explain because they're much more isolated, and additionally only seem to be partially deficient in dark matter rather than lacking it entirely.

The latest candidate, the subject of today's post, is very different to the others. It's much brighter, with a stellar mass three or four orders of magnitude greater. That immediately raises some flags that maybe this is altogether a stranger and even more interesting object : in general, you can do anything you like as long as your galaxy is pathetic enough, but the bright ones tend to fight back. If they're doing something weird, you're on much firmer footing in claiming to have found something seriously strange.

There's a press release here which is quite nice. Of course, I tried to read as much as the original paper as I could, but I have to say I struggled with this one. Large parts are extremely dry and technical and read more of an instruction manual than a scientific analysis. It's absolutely no bad thing to go into this level of detail - in fact, authors, I salute you for your efforts ! - but I do think it could have been structured more effectively. An awful lot of this belongs in an appendix.

I also have to say I found some of the wording... odd. Not linguistically odd at all, just arranged weirdly, jumping from topic to topic, flitting between technical details of the procedures and the scientific analysis, and occasionally coming across as though they meant to say the opposite of what they seemed to be claiming. Narratively, to be honest I just don't like it, it doesn't flow. So after wading though the methodology as far as I could, I skipped ahead to the discussion section, which is it least comprehensible even if it's a bit off.

But let's start with at the beginning. Here they introduce the current prevailing view on the formation of elliptical galaxies, which I'm not all that familiar with. As per much earlier suggestions, it looks like both of the major paradigms of galaxy formation theory (monolithic collapse, suggested in the 1960s, and the much more recent idea of hierarchical merging) might be at work for these objects. Monolithic collapse - the simple collapse of a great big singular cloud of gas - creates an initial, highly compact "nugget", which then grows and expands over time through mergers with other (gas poor*) galaxies. Since those objects lack gas which can dissipate the energy of the collision, the nugget expands spheroidally rather than forming a rotating disc.

* If instead it encounters gas-rich galaxies, presumably this would assemble a spiral/disc galaxy.

There's no guarantee that a nugget must experience this second phase of merger-based growth though. Oh, most will, just because galaxies are belligerent and numerous. But in rare cases it's possible that they don't, and the result of the nuggets evolving passively is a very rare "relic galaxy", of which only a few dozen candidates are known.

The second really interesting point in the introduction kindof makes the rest of the paper feel like a bit of a let-down : that this particular candidate relic galaxy, NGC 1277, is already known (since 2015) to lack dark matter out to a distance of 4.6 kpc from its centre. This study extends that to... 6 kpc. No matter how much the authors try, it's a bit difficult to get excited by this undeniably important yet also undeniably incremental change. Fortunately for them, since I never heard about this before, it's still a very interesting object. (And it's pretty neat that we're still making discoveries like this from NGC objects, a catalogue which is more than a 130 years old ! This is quite the reminder of our sheer ignorance.)

The new observations measure the stellar velocities over a field which roughly corresponds to the following :

NGC 1277 is upper right, while the larger galaxy near the centre is NGC 1278.

Zooming out, this is a crowded part of the Perseus cluster. This causes some complications, but the obvious advantage is that they get two galaxies for the price of one.

What they find is that the motions of the stars of NGC 1277 are consistent with what's expected from the mass of the stars alone (i.e. slow), whereas those in NGC 1278 are what you'd expect if the galaxy also had the usual amount of dark matter (fast). These measurements seem pretty robust, so it's not at all likely that people will dispute the accuracy of the data, unlike in some of the earlier cases.

Straight off, this is bad but not fatal news for dark matter alternatives. Dark and normal matter are in principle separable - there's no reason that the one must accompany the other. We've seen this more famously in the Bullet Cluster and many similar systems), where the gravitational lensing indicates the dark matter has become displaced exactly as theory predicts it should be. Here we see it in in the case of individual galaxies, and we see something similar elsewhere too.

But if instead dark matter isn't a thing, then this is quite the challenge to explain. If it's something else, something related to the visible matter that causes the strange motions, then if you have two otherwise identical systems, then their stars should always have identical motions. There are exceptions. Notably, the external field effect in MOND says that if one system is near another, more massive system, the external gravity can restore Newton-like dynamics in the smaller system. This is why globular clusters, which are well-known in our own galaxy and don't have any hint of dark matter, aren't immediately fatal to the whole idea. 

This option is, however, exceedingly strange and arguably non-physical, but as there's a dedicated recent paper all about the philosophy of MOND, I'll leave that for a forthcoming post. It does seem awfully convenient that we haven't already found more such systems; one would expect this to be a predictable, testable effect, yet the number of systems found which either lack dark matter (or equivalently experience the EFE) is vanishingly small. And it just can't apply to isolated systems, like the UDGs mentioned earlier.

But even if galaxies like this one actually support the dark matter paradigm rather than undermining it, they may still very plausibly challenge the Standard Model in other ways. Nobody expected the existence of large numbers of dark matter-free galaxies, especially not really massive ones. The authors mention that the rotation curve of this galaxy resembles those seen in the earlier universe which have been claimed to be declining at the outer edges, which would also hint at a lack of dark matter. That's not necessarily such a problem for cosmology because galaxy formation is a complex process, though naively one would expect that it's dark matter that drives the accumulation of baryons, not the other way around. That said, I found those claims for the distant galaxies unconvincing. It would be interesting, though, if dark matter was a sort of effect that gradually manifested itself as time progresses, much as dark energy seems to do...

But that's a much more exotic, hand-waving speculation. Best to avoid that. More pragmatically, while dark matter is mass-dominant over normal matter on large scales, locally this is often not true at all.

Unfortunately one the key points of the paper is lost on me. They say that whether the galaxy is compatible or not with the Standard Model depends on where exactly one measures the dark matter content : should it be at the half-light radius (interior to which the stars emit half the light, a standard, objective measure of galaxy size) or at a fixed physical distance value ? And I have no idea what the difference is supposed to be here, since they boil down to doing exactly and quantitatively the same thing, measuring the dark matter enclosed at five times the half light radii or within 6 kpc - which are the same ! And yet they say that if the former measurement is used, this contradicts the standard model expectations, whereas with the latter, the tension is much smaller. How this works, I know not. I'm missing something.

As I said, this paper has some parts which are badly expressed. Though I think it's very wise of them to explicitly refrain from speculations about whether the result really does challenge the Standard Model or not.

Thankfully, their speculations about how such a galaxy could form are on generally safer ground (and easier to follow). Since this galaxy is already very rare as a relic galaxy, it's natural to assume their might be a common cause between these two weird aspects (albeit that this is dealing with VERY small samples). And there's a plausible physical connection here too : dynamical friction. The presence of dark matter tends to act as a gravitational "drag" on anything orbiting within it, making the mergers of satellite galaxies that much easier. As this galaxy doesn't have a dark matter halo, it makes sense that it never experienced the merging phase that would be normally responsible for turning the nugget into a normal galaxy.

So, dark matter-free galaxy => relic nugget instead of a normal galaxy. Good.

But... why doesn't it have a dark halo to begin with ? It certainly seems counter-intuitive, as they say, that such a dense galaxy would form without a correspondingly dense halo - though my own experience suggests otherwise. I saw in my simulations that during a monolithic collapse, the dark matter actually helps to smooth everything out, and without it you get runaway densities.

Anyway, even if the lack of a halo helps explain why this galaxy is a relic, that still doesn't explain why it doesn't have a dark halo in the first place. The authors suggest two possibilities (they say three, but I count two). It could have had a "strong interaction with the environment", with simulations suggesting that dark matter is preferentially stripped because it orbits at generally higher distances from the galactic centre than the stars and gas. Similarly, it could have experienced the same sort of collisional origin that seems to explain some of the other, much smaller dark matter-free galaxies. 

The main caveats to these scenarios is that it's not known if they could explain the frequency of such galaxies, especially since this is the only known massive galaxy without dark matter : if it's due to interactions within the assembly of a galaxy cluster, why don't we see such objects in every cluster ? It also seems doubtful, they say, that tidally stripping the dark matter could account for the very high density of NGC 1277. On the other hand, the cluster environment does seem like a good place for such objects to survive, since the very high velocities of the galaxies means mergers (which would transform the nugget into a normal galaxy) will be rare.

The other option is that it does have a dark halo, it's just much less concentrated than in typical galaxies - so we need observations to greater radial distances to see its effects. Powerful feedback early on, from star formation and/or the supermassive black hole, could in principle expel large amounts of normal matter from the inner regions, with the gravitational field thus being able to disperse the dark matter as well; again, dark matter dominates globally, not necessarily locally. The problem is that feedback this powerful ought to have shut down star formation before it produced such a dense inner core.

So more careful analysis of simulations would help understand if objects like this are expected to be the extreme rarity that a naïve, intuitive view would suggest, or if actually they're not quite such a total anomaly after all (though they will almost certainly turn out to be highly unusual). Better observations, in particular to measure the velocities out to greater distances, would also help constrain the nature of the object, especially to see if it's truly lacking dark matter or only deficient in its innermost regions.

This latter would also help test for the effects of MOND. Unfortunately, this is likely to be extremely difficult. To get to the distance where the accelerations ought to be able to distinguish between MOND and Newton requires distances of 13 kpc, more than twice what they achieve here. And that starts to blend into NGC 1278. Additionally, the very crowded field here surely means the EFE cannot be neglected.

My guess is we'd need to a different approach here : a statistical study of where such galaxies are found and how strong the EFE is. And given how long it's taken to obtain this quality data even for a nice bright NGC object, that's not going to happen any time soon. It's also unfortunate - and I think a wee bit suspicious - that this galaxy looks visually so damn similar to other galaxies, almost as though the dark matter was unimportant...*

* But see that last link. My Master's project was partially motivated by trying to show that dark matter played little apparent role in galaxies apart from making them spin faster; as it turned out, this is just not the case at all. Appearances can be deceiving.

What's the take-home message here ? Well, the good news is that this does appear to be a relic galaxy, which is pretty neat, and it's in a cluster, which is a good place for such objects to survive. Furthermore its lack of dark matter could help explain why it's a relic. The even better news is that we have no idea why it's lacking dark matter to begin with. As with many such anomalies, the fun part is that you can clearly point to them and say, "That thing THERE ! That's WEIRD, that is !". The frustrating part is that going beyond that, following up to confirm exactly how weird it is, whether it presents a challenge to physics or is just one of those things, isn't necessarily any easier just because you know where to look.

More research is needed.