Tidal tales of galaxies without dark matter

A year ago almost to the day, a team of astronomers led by Pieter van Dokkum reported the discovery of a galaxy apparently lacking dark matter, known as NGC 1052-DF2. Maybe not quite entirely lacking, but having so little compared to other galaxies that it's clearly very strange.

The implications of this object have yet to be fully realised. There was, predictably, criticism - some of it very silly, but others much more valid. The dark matter mass of a galaxy can't be measured directly but must be inferred by how fast its stars and gas are moving. In this case there are only stars, which appear to be swirling about randomly rather than rotating in a disc, but this is quite normal and doesn't make much difference to the measurement procedure. The point is that there are two main ways the mass could have been overestimated : either the measured velocity dispersion was too low, or the estimated distance to the galaxy was too high.

Since the galaxy is extremely faint, van Dokkum et al. originally used the galaxy's globular clusters to measure the velocity dispersion as these are brighter and easier to measure. This was only possible for a handful of objects, and one of them hinted at a much higher velocity dispersion. This led to a lot of criticism about their statistical inference of the true dispersion, though van Dokkum refuted this (personally I thought their low dispersion measurement seemed much more plausible from the data than the claims of a higher one). Subsequently they improved their observations, which eliminated their errant data point : as with most weird outliers, it was a spurious measurement. And later they improved their measurements still further through deeper observations that allowed them to get the dispersion directly from the stars, showing pretty definitively that they are indeed moving very, very slowly.

If the velocity dispersion is correct, what about the distance ? That came in for stronger criticism, which showed that the galaxy became a pretty typical object if it was rather closer than reported. The original team hit back, saying that their original measurement was correct. Then a completely independent team did their own measurements, which agreed with van Dokkum's original claim.

So it looks as if this is a genuinely weird object. Does it represent a distinct class of objects or just an example of exotica ? It's too early to tell, but van Dokkum found a second, very similar galaxy close to the first one, while another team found a much smaller, much closer galaxy that also appears to be deficient in dark matter. The latter group also investigated how such an object could form, suggesting it might be in part due to tidal encounters with more massive objects that could rip out much of the dark matter. That echoes an earlier study claiming a similar origin was possible for NGC 1052-DF2.

The paper below reports deeper observations of the NGC 1052 group and finds several tidal structures : streams and arcs of stars pulled out of galaxies by their gravitational interactions. Here's their main figure :

The second galaxy claimed without dark matter is NGC 1052-DF4, upper right.

It's abundantly clear from this that tidal interactions have played a role in the evolution of this group. NGC 1052-DF2 is, intriguingly, very close indeed to a narrow stream originating from NGC 1052 itself. So could this enigmatic object just be some form of transient debris ? Almost certainly not. The structure and density of DF2 are very different from the stream, and it's not obvious why a thin, faint stream should be broken at the end but related to a smooth, much higher density object just a bit further away (if it was embedded within the stream, though, that would be a different matter). The authors conclude the alignment is most likely just pure chance.

Could it be something more stable but produced in a similar way - a tidal dwarf galaxy ? If enough material is pulled from a galaxy, it can sometimes become self-gravitating and collapse to form a stable object without any significant dark matter needed to bind it together. These are controversial objects : while there are some clear examples known which are still embedded in their parental streams, it's much harder to decide if an isolated object was formed by this mechanism. While the authors here favour an old tidal origin for these objects, tidal dwarfs should be much more vulnerable to disruption than normal galaxies because they're much less massive (and it should be noted that the authors are known fans of tidal dwarf galaxies).

Personally I'd say that the presence of tidal streams makes the case for a tidal origin more likely, but it's nowhere near definitive. One of the main points of controversy is whether all the galaxies here are at the same distance, so the presence of streams is pretty strong confirmation of that. That we know the galaxies are interacting with each other does make it more plausible that there's tidal debris floating around, but we suspected that anyway. So we're no closer to knowing if the interactions within this group are capable of producing objects similar to the weird galaxies that have been detected there, or if they could survive in such an environment for several billion years. Crucially, a point which remains addressed is that the velocity dispersions are so low that it's difficult to see how the objects could have become this smooth and spherical given the available timescale - the age of the Universe means there's only been time for their stars to complete a couple of orbits !

Finally, what do these galaxies mean for physics ? That too remains highly controversial. If they're formed by tidal encounters, how many other galaxies might have been produced in the same way ? If they're not, then does that mean we have to revise our theory of galaxy formation and/or gravity ? Alas it seems that this object cannot be used to rule out theories of modified gravity - as van Dokkum admitted, earlier claims about this were simply wrong; the objects appear to be equally consistent with both the standard model and its alternatives. A year on, these objects remain as controversial and enigmatic as ever.

A tidal tale: detection of multiple stellar streams in the environment of NGC1052

The possible existence of two dark matter free galaxies (NGC1052-DF2 and NGC1052-DF4) in the field of the early-type galaxy NGC1052 presents a challenge to theories of dwarf galaxy formation according to the current cosmological paradigm.