halo occupation and assembly bias

My research highlight today was a conversation with MJ Vakili about the paper he wrote this summer about halo occupation and what's known as “assembly bias”. Perhaps the most remarkable thing about contemporary cosmology is that the dark-matter-only simulations do a great job of explaining the large-scale structure in the galaxy distribution, despite the fact that we don't understand galaxy formation! The connection is a “halo occupation function” that puts galaxies into halos. It turns out that incredibly simple prescriptions work.

I have always been suspicious about halo occupation, because galaxy halos are not fundamental objects in gravity or cosmology; they are defined by a prescription, running on the output of a simulation. That is, they are just effective crutches, used for convenience. There was no reason to put any reality onto a halo (or a sub-halo or anything of the sort). Really there is just a density field! However, empirically, the halo description of the Universe has been both easy and useful.

Now that cosmology is seeking ever higher precision, work has started along the lines of asking what halo properties (mass, velocity amplitude, concentration, and so on) are relevant to the galaxies that form within them. The answer from the data seems to be that mass is the main driving factor. The community has expected a bias or occupation that depends on the time of formation of the halo (which itself relates to he halo concentration parameter). Vakili has been testing this, and the main punchline is that if the effect is there, it is a small one! It is a great result and he is nearly ready to submit.

My question is: Can we step out of the halo box and consider all the ways we might put galaxies into the dark-matter field? Could the data tell us what is most relevant?

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