Still Missing

The missing satellite problem, a.k.a. the dwarf galaxy problem, is that standard cosmology predicts far more small galaxies than are actually observed. Specifically the missing satellite problem is that there are far fewer dwarf galaxies found around larger galaxies like the Milky Way.

This paper, which was first submitted two years and has only now been accepted, explores another variation on that. It turns out that there are also huge numbers (about a factor of five) of missing galaxies even in the field - that is, the general population of galaxies that aren't in groups or clusters. So even isolated galaxies appear to be missing. This is significant because in a galaxy group the interactions between galaxies could be very significant, so one could wave one's hands and say, "gas physics is haaaaard !". After all, large-scale simulations use just the dark matter, not the gas because it's tricky to model. They have to use clever prescriptions to work out how much gas and how many stars each dark matter halo would contain.

Galaxy groups offer another complication : satellite galaxies seem to be orbiting in planes, not orbiting at random as the theory predicts. That could potentially offer a way out of the missing satellite problem : maybe the gas flows down filaments into the central galaxy, so satellite dark matter halos outside the filament never accrete much gas and never form stars. Maybe. But this simply doesn't work at all for isolated galaxies. There's no reason to think that so many isolated dark matter halos should avoid accreting gas.

Perhaps one could still get away with saying, "gas physics is haaaaard !" for isolated galaxies. When the first stars form, stellar winds and supernovae might blow the remaining gas out and prevent much star formation from ever happening. Most galaxies would then be very hard to detect. The authors of this paper say no, this can't work. Although this might be OK for very small galaxies, it shouldn't work for larger galaxies - which are also missing (the old, "too big to fail" problem).

It's also worth noting that the authors try and minimize the effects of the still poorly-understood baryonic (that is, gas) physics. They compare theory and observations not through brightness but by looking at how fast the galaxies are rotating. Although they still have to do some corrections for the gas and stars, they say this is far less significant than if you look for galaxies of a certain brightness. Essentially, how fast everything is rotating should always be a good indicator of the mass of the galaxy, whereas brightness can vary for many more complicated reasons.

Unfortunately since the paper was submitted so long ago, the authors don't comment on the recent discoveries of huge numbers of very faint galaxies in clusters (see http://astrorhysy.blogspot.cz/2015/10/moaaaarr-galaxies.html). As far as I know we don't yet have good measurements of the kinematics (rotation) of these new galaxies. So possibly a lot of very faint galaxies still remain to be discovered, even ones which should be too big to avoid forming stars. Of course those galaxies are also problematic.

In short, this exposes another bloody great gap in our ignorance.
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