Wednesday, 12 March 2014

Dark Matters

Our standard model for dark matter, Cold Dark Matter,
predicts that dark matter haloes of all masses - clusters,
galaxies and dwarfs - should be swarming with
substructures, remnants of the merging hierarchy.
I spent yesterday at the Australian Astronomical Observatory in Sydney, at what is quaintly called a Town Hall meeting, to build community support for a very cool new galaxy survey known as WAVES, which might be carried out on the planned 4MOST instrument on the ESO VISTA telescope. In essence, this was a get-together of various members of the great and good of Australian astronomy research to discuss how great 4MOST will be for their research programmes - ranging from AGN to cold gas in galaxies to surveys to constrain the dark sector of dark matter and dark energy. I'm part of the design team at ICRAR/UWA - alongside my colleagues Simon, who has made the WAVES dream a real possibility, Aaron and Martin. I attended the meeting to talk about my particular interest in testing the nature of dark matter - which is one of the central objectives of the survey.

Observational evidence for some form of non-baryonic (i.e. exotic) dark matter is now well established; what is less well established (i.e. it's not) is its physical nature, by which I mean its identification with one of a myriad of fundamental particles. The model for dark matter that is most widely favoured by the community is cold dark matter (CDM); cold because the CDM particle wasn't in thermal equilibrium when it decoupled in the first few minutes after the Big Bang and it moved at non-relativistic speeds. The CDM model has been kicking around for close to three decades and it's had its ups and downs. Pretty much everything we have learned about galaxy formation (from a theoretical perspective) has been within the framework provided by the CDM model. CDM makes a couple of key predictions -- first, that the massive virtualised structures that are the end-point of gravitational collapse, so-called dark matter haloes, have divergent central dark matter densities; and second, the CDM haloes contain a wealth of small-scale structure, remnants of their hierarchical assembly histories. 

How we properly test observationally whether or not the CDM is an accurate model of the Universe has bugged me for some time, and in a paper I published last year I looked into how we  might go about doing this. I argued that, although CDM haloes have divergent central densities, this is not a good test because astrophysical processes can erase these dark matter cusps. Rather it's better to go after dark matter substructure, the prediction that is the signature of CDM. I ran a series of supercomputer simulations of model universes, comparing a fiducial CDM universe with warmer versions of dark matter -- WDM models. WDM models suppress the formation of low-mass dark matter haloes because, in the jargon, they have reduced power on small-scales. This means that WDM free-streams in the early Universe because the WDM particle was in thermal equilibrium when it decoupled, erasing small-scale matter density perturbations. The bottom lines is that reduced small-scale power in the early Universe broadly translates into fewer low-mass dark matter haloes (and substructures) in the present-day Universe. 

How can this be tested? I argued in the paper - and during my talk - that looking at clustering of low-mass galaxies, minor merger rates and evidence for disturbances might be a good way to test the veracity of the CDM model. These kinds of statistics may not be so sensitive to our understanding of how low-mass galaxies form (another challenging problem) and they can be robustly measured in observational data. I also suggested that the signal probably requires a new approach to analysing and comparing simulations with observations - rather than running a simulation and a best-bet galaxy formation model, making mock catalogues and comparing, we will need to apply advanced statistical techniques to isolate regions of parameter space in the models that are allowed by the data. Fortunately, we have just hired a very clever postdoc to work on precisely this problem… watch this space!