Sunday, September 16, 2012

What if dark matter isn't just plain vanilla CDM?

The cold dark matter (CDM) hypothesis has been enormously successful in describing a wide variety of cosmological observations - from the Cosmic Microwave Background to the large scale distribution of galaxies and the properties of galaxies themselves. But, there are a few areas where things aren't so clear cut. These cases are all somewhat ambiguous - because they all involve the complex physics of galaxies and baryons which we don't understand well enough to make really strong statements. But, it's intriguing enough that there's been a lot of interest in alternative types of dark matter recently.

As a result of this, we've just pushed substantial new functionality to Galacticus v0.9.1 (coinciding with a new paper, which appears on arXiv today) which allows it to follow the formation of dark matter structure in non-CDM universes. It's currently set up to work specifically for warm dark matter (WDM) cases, but the underlying algorithms can handle any type of dark matter (for those in the know, I'll add the caveat that it's any type of dark matter whose physics can be described by modifications to the linear theory power spectrum and the barrier for collapse in excursion set theory), due to a neat numerical algorithm figured out by my former student Arya Farahi.

The results look good. Here's a comparison of the dark matter halo mass function with an N-body simulation of warm dark matter by Schneider et al. (2012):

The yellow/red circles were measured from Schneider et al.'s simulation (at low masses the simulation becomes unreliable - shown by the tiny circles - so ignore it in that region). The solid blue line is the result from Galacticus - in excellent agreement with the N-body result and showing the expected cut-off at low masses (well, not that low, Schneider et al. use a relatively light, 0.25keV, WDM particle which isn't consistent with current constraints - but here we're only concerned with comparing the two techniques).

Schneider et al. only include one effect of WDM - the suppression of small scale power in the linear theory power spectrum. A secondary effect, due to the velocity dispersion of the WDM particles, is to make it more difficult for small overdense regions of the Universe to collapse. To make a fair comparison, I ignored that effect in Galacticus also. But, if I add it back in, the result is the green dot-dashed line - it has a very strong effect on the number of low mass halos around the cut-off. (If you want to know what the pink dotted line is all about, check out the preprint!)

The modular nature of Galacticus makes it relatively easy to add in new algorithms such as this - all of its galaxy formation physics will continue to work happily with merger trees built from WDM.

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