Missing ? So what ?

This is an interesting approach to the missing satellite galaxy problem, where there are far less dwarf galaxies found around the Milky Way than predicted by standard, purely dark matter simulations. The main approach to this so far has to assume that when the complex physics of normal matter - gas and stars - is eventually included, a bunch of stuff could happen to solve the problem. There might not be enough gas in some of the smallest dark halos to ignite star formation, or there might be so much star formation early on that it blows out the gas and prevents the smallest galaxies from forming many stars. Or the smallest galaxies might be more vulnerable to the effects of tidal encounters, due to the presence of a thin heavy disc that's not found in the smooth, spheroidal, pure dark matter halos. Gas and stars are much clumpier than pure dark matter.

I like this paper because it uses pure dark matter simulations to try and address the problem, rather than relying on assumptions about what might happen when gas is included. First, they note that the the number of satellites correlated with the total mass of the parent galaxy - and that's uncertain by a factor two or so for the Milky Way. But they also show that there's a correlation with other parameters of the dark halo : shape, spin, and especially concentration (how much of its mass is found within a certain radius). Accounting for this, the problem can be reduced by 40-70%. So other factors are still needed, but potentially the problem isn't as severe as has been previously thought. It also has important consequences for anyone modelling those other factors.

This is a purely statistical approach to tackling the problem, and I think that's a legitimate approach here - others are working on the physics anyway. One of the major critiques that's been levelled at anyone trying to solve the missing satellites is that most explanations require the Milky Way to be unusual. What they show here, quite convincingly I think, is that the Milky Way is unusual in several key respects that mean it consequently should be expected to have fewer satellites than more naive predictions. They don't address why it's like this, as that's well beyond the scope of the paper. The point is that once you independently know the Milky Way is unusual, you can't still assume it's a typical average galaxy (as we know, the average galaxy is statistical construct that doesn't actually exist anyway). Knowing that it's got these unusual properties, rather than being evidence that the whole model is just plain wrong as some people would have it, is actually evidence for its particular formation history. That's a more Bayesian, anthropic approach of re-evaluating assumptions given evidence, rather than the blunter method of chucking the whole thing out.

Of course what this implies is that galaxies which do have more typical parameters should not suffer from a missing satellite problem. We can't test this yet, but eventually we'll be able to. The research continues.
http://adsabs.harvard.edu/abs/2018arXiv180705180F