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Small galaxies, big problems

Today we explore the Satellite Planes Problem with Dr. Marcel S. Pawlowski, PhD at the Argelander Institute for Astronomy in Bonn, and Junior Research Group leader at the Leibniz-Institute for Astrophysics in Potsdam.

David: What is the Satellite Planes Problem?

Marcel: The problem has to do with the distribution and motion of dwarf satellite galaxies around more massive host galaxies. For the Milky Way, it had already been noted in the 1970’s that its satellites are distributed in a flattened spatial arrangement, along a plane perpendicular to the Milky Way disk itself. Proper motion measurements have since shown that many, especially of the brighter satellites, orbit in the same sense along this preferred direction. Indications for similar, spatially flattened distributions of satellite galaxies in other systems have since been found, most notably around Andromeda and Centaurus A. In both cases, line-of-sight velocities of the on-plane satellite galaxies indicate a high degree of kinematic correlation, similar to the Milky Way satellites – though full 3D velocities are still mostly unknown for these systems.

These observations by themselves wouldn’t be a problem, of course. Yet, they are at odds with how satellite galaxy systems should behave within our standard model of cosmology. Cosmological simulations, which are necessary for making predictions in the non-linear regime of structure formation that’s relevant for these dwarf satellite galaxies, show that satellites should be mostly randomly distributed, and display only very mild kinematic correlation. Finding simulated satellite systems arranged in as narrow and as strongly correlated structures as for these three best-studied observed cases is a one in a thousand exception. This mismatch is what’s meant with the planes of satellite galaxies problem. By the way, it is mostly driven by the kinematic correlation. Flattened alignments alone are not quite as rare in simulations, but they are most often spatial coincidences that vanish after a short time. David: Is lots of stretching necessary to resolve it within the standard cosmological model?

Marcel: What makes satellite planes a serious problem for the standard model is that it is concerned with rather fundamental properties: the locations and motions of dwarf galaxies on scales of their host galaxy halo, so several hundred kpc. It’s not some small, detailed feature that is easily affected by peculiarities in the numerical codes, like how or what subgrid physics is implemented. Where a dwarf galaxy is and how it moves is largely defined by the larger-scale structure of the cosmic web, by the overall potential and gravitational interactions of dark matter halos, and its time evolution. There’s simply not much you can tweak to resolve this. Also, there doesn’t seem to be any special properties that a host halo needs to have to enhance the chance of being surrounded by a pronounced satellite plane.

One avenue of research that I consider increasingly promising is to look at the effects more major galaxy interactions have on their satellite galaxy systems. For the Milky Way, we have a rather massive Large Magellanic Cloud falling in right along the satellite plane. For Centaurus A, it appears likely that a major merger within the last two billion years has shaped the host, and its orbit appears to be aligned with the observed satellite planes. Even for Andromeda, some colleagues propose a major merger. That’s even relevant for alternative approaches. For example, say, you’d want to abandon the dark matter paradigm and move to something like Modified Newtonian Dynamics, or MOND. Then you can do a timing argument calculation with the known baryonic masses, distances, and velocities of the Milky Way and Andromeda, and find that – due to the enhanced gravitational acceleration in that model – that they should have had a past encounter in MOND. Which in turn could have disturbed their satellite galaxy systems or even produced new (tidal) dwarfs from the tidal debris of a past encounter. David: Could feedback from star formation and/or black holes contribute to solving the problem?

Marcel: Like I just said, the main driver of the relevant properties, the positions and motions of dwarf galaxies, is the overall potential and not the internal details of how the gas behaves, how stars are formed, and how their feedback acts. Consequently, hydrodynamical cosmological simulations result in very similar degrees of tension with the observed satellite planes as the much more simple ones modeling only dark matter. So star formation doesn’t offer an obvious solution.

I often get asked, from professional colleagues as well as the audience of public talks, whether AGN feedback from a central massive black hole might play a role. After all, the Milky Way’s satellite plane is polar, and also the one around Centaurus A is somewhat aligned with its jets. But again, there’s not obvious mechanism. Even if AGN feedback would, say, quench star formation for dwarf galaxies in certain directions and could introduce some anisotropy, this wouldn’t explain the strong kinematic correlations. David: Given the limited number of galaxies, can’t we dismiss these small-scale issues by way of statistics? Can we relegate them as mere ‘curiosity’?

Marcel: Just dismissing the issue is dangerous, scientifically. Mismatches such as these are what might indicate important clues to the limitations of current models, which is what we all should be looking for if we want to make progress. That’s why I strongly believe that we need to understand the origin of these structures for these well-studied cases. If that understanding then explains why they can be considered as curiosities, that’d be great. But we can’t just assume this to be the case and move on, ignoring a potentially important piece of information.

Yet, I do agree that these are only a few cases. That’s why we’re also working on expanding the sample. In the last year alone, there were several other host galaxies reported that might be surrounded by similar structures, such as NGC 253. Ultimately, we want to study this in a statistical sample, though that’s not easy because for more distant hosts we lack sufficient accuracy in distance measurements to resolve 3D distributions, so only projected distributions can be studied which prohibits identifying face-on structures. Furthermore, at most line-of-sight velocities are accessible observationally, and those are observationally quite expensive. We have made good experiences using IFUs like ESO’s MUSE instrument which is very productive to get the systemic velocities of such dwarfs, but still it is difficult to get time for sufficiently large samples. David: How has the community reacted to your work?

Marcel: Initially it had been tough, sometimes outright hostile. Like when one senior astrophysicist, upon being asked about their highlights of a conference during the closing panel discussion, felt the need to emphasis that he considered my work as clearly the opposite of a highlight. By now, the issue of satellite planes is much more widely acknowledged, and lots of research is being done on it by others as well, on both the observational and the theoretical side. I’m really happy that there’s now a real debate, not only among colleagues at conferences, but also in the published literature different arguments are exchanged. Ultimately, that’s how we progress in science. It’s been interesting to see this change. In part that’s certainly due to improving observational data that confirmed earlier results. Maybe it is a good example for how being persistent helps, too, though I think it was also important that I learned how to present these issues better, and that I became independent of certain more dogmatic debates and perceptions. Funny enough, the person who mentioned my research as an “anti-highlight” later tried to get me to work with him on related issues when I got my own funding as a Hubble Fellow, apparently entirely oblivious to that past interaction. David: Would you be shocked if dark matter and dark energy exited the collective scientific consciousness by the end of the 21st century?

Marcel: That’s an interesting question. I don’t think I’d be shocked, provided that there are solid reasons for this. Thus far, these are good hypotheses that have some issues, but the present alternatives aren’t perfect either. If we were to come up with a well working model that does not require dark matter and dark energy, and that does explain all the current indications we have in their favor, of course we as a research field would move on. Whether that’s likely is another question, and we definitely should not abandon the dark matter and dark energy possibilities prematurely. Yet, I’d also argue that the same should be the case for alternative approaches. And I’m afraid we presently largely fail at that, because such alternatives don’t quite get the attention and respect they deserve.

David: Thank you Professor!

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