The velocity function of missing satellites

A very intriguing paper on astro-ph yesterday, submitted to ApJ but not yet accepted.

This is yet another one attempting to solve the missing galaxy problem. The problem is that at low masses there are far fewer galaxies than simulations predict. Simulations generally only use dark matter since it's computationally cheap and normal matter only accounts for about 10% of the total mass anyway. This means it can be tricky to predict what the observable mass (i.e. of normal matter) of the simulated galaxies would really be. So people have started predicting their rotation velocities instead. This should depend much more strongly on the dark matter mass and less on the normal matter, so the prediction should be more robust. Past simulations have found there's still a great big problem, with galaxies of low velocities (i.e. low total mass) being much too numerous in the simulations compared to reality.

This paper compares simulations that use only dark matter with ones that use normal matter too. They find that the physics of the gas can explain the discrepancy very well. In their model, it turns out that the gas discs don't trace out the full size of the dark matter halos, so the rotation that would actually be measured would be lower than the true maximum of the halo. Also, the gas mass in many of the smaller halos (which rotate less quickly) would be so low it would be undetectable. That brings the number of small, slowly-rotating galaxies into good agreement with the observations.

It's a very interesting result and definitely one to watch as it evolves with the the referee reports. One thing they barely mention is the "too big to fail" problem, where there seems to be missing galaxies which are so large there's no way they should have been able to avoid forming stars. A potentially more serious flaw is (if I understand them correctly) that they select galaxies only if they have a gas mass above a certain threshold. If so, this will introduce a weird selection effect because real observations aren't like that. The observed "brightness" of the gas depends on its apparent velocity width as well as its mass. So I'm not convinced their results are really directly comparable to observations, but this should be an easy problem to address.
https://arxiv.org/abs/1701.07835