There are two reasons the issue isn't settled. First, observationally there are large parts of the sky where we just can't detect anything because of the Milky Way, and we also know the satellite galaxies aren't distributed isotropically. So it's not at all straightforward to extrapolate from the galaxies that we have found to the total number really present. Secondly, the simulations have got much more sophisticated but far more complicated, making it very unclear if they really predict anything or have just been fine-tuned to get results in agreement with observations.
This excellent paper takes a thorough look at the problem of whether the missing satellite problem is really a problem or not. They begin with a very nice overview of the different factors (potentially) at work :
- Survey incompleteness. It could be that we simply haven't found all the expected satellite galaxies because they're very faint. We also have to be careful that the discoveries match the predictions, i.e. have approximately the correct mass - sheer numbers aren't enough.
- Host galaxy properties. It could be that the Milky Way is unusual in some way and therefore the predictions based on typical galaxies seen in simulations aren't correct. This is the main focus of the paper.
- Baryonic physics. The satellites could be fainter than expected due to the 'orribly complicated physics governing star formation, which is still not at all well understood. Maybe they didn't form many stars, or perhaps the baryons had other effects that disrupted their formation (e.g. tides).
- Exotic physics. Perhaps dark matter itself doesn't behave quite as we expect. This is not covered in any detail here as it's not the topic of the investigation.
Given all this, it's difficult to believe the missing satellite problem isn't solvable. But whether we can actually find the correct solution - that's a much, much harder question.
When it comes to planes of satellites, I've suggested that while simulations (arguably) show that they're expected to be rare, this might be misleading. They may be rare overall, but in fact be very common given the right conditions. If you selected an animal at random from somewhere on the Earth, you might get a giraffe... but you wouldn't conclude this was a typical animal until you had much better statistics. In fact there may not even be such a thing as typical animal, given their huge diversity. Likewise there may not even be a typical galaxy, so predicting how common planes should be could be giving a very confusing picture.
This is the approach the author's adopt for the missing satellite problem. This has hitherto largely been quantified in terms of the mass of the parent galaxy. But even in simulations which use only dark matter, there host galaxy has other properties. Specifically : (i) concentration, which measures the fraction of mass within some radius; (ii) spin parameter - more complicated than rotation, this measures how much of a role rotation (as opposed to random motions) plays in keeping the halo from collapsing; (iii) shape, since dark matter halos tend to be a bit squashed; (iv) halo merger history, with halos which form earlier tending to have fewer satellites later on.
All of these parameters are relatively easy to get from simulations, and they're correlated with each other to various degrees. So it may very well be the case that there is no such thing as a typical spiral galaxy, even if its morphology and mass are precisely specified. It's harder to estimate these parameters for real galaxies, but it can be done, albeit with large errors. And the Milky Way does look quite extreme in some respects. The naive assumption that it's typical is a good one if there's no supporting data, but it would be silly to maintain this assumption if the evidence goes against it. The authors note that even the total mass is uncertain by as much as a factor of two.
How much of a difference do these parameters make to the number of predicted satellites ? They examine this using 45 high resolution pure dark matter simulations. They look at the correlation with various parameters, including using just concentration (which turns out to be the dominant factor after mass) and using a combination of all parameters, as well as comparing this to a model which doesn't use concentration. It's pretty tough going, to be honest, but the answer is interesting : a bit.
Why is that interesting ? Well, it means the missing satellite problem may have been somewhat exaggerated, but it still appears very much to be a problem. Because of all the errors, it's hard to quantify how much of a difference this makes for the smallest satellites (they say the Milky Way should have anything from 1% to 30% fewer tiny satellites than previous predictions), but it's more secure for larger ones (19-52% fewer). This means that although there's still a problem, it's not as dramatic as first thought, and possible solutions may have been over-correcting and therefore need to be revised. Moreover, at the very largest masses, they find the so-called "too big to fail problem" is now even worse than before : the Milky Way appears to have more of the most massive satellites compared to average galaxies, since their results show a decrease in satellites of all masses.
So this is definitely a very interesting piece of work. The missing satellite is still a problem, albeit a potentially solvable problem, but the TBTF problem is getting worse. They note that the major uncertainty is the mass of the Milky Way and (for the TBTF aspect) the resolution of the simulations. Both of these can be addressed, so in time we'll have a better way to tackle the problem of how much of a problem this problematic problem is causing a problem.
Predictably Missing Satellites: Subhalo Abundance in Milky Way-like Halos
On small scales there have been a number of claims of discrepancies between the standard Cold Dark Matter (CDM) model and observations. The 'missing satellites problem' infamously describes the overabundance of subhalos from CDM simulations compared to the number of satellites observed in the Milky Way.
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