Research Interests: Joanne D Cohn
Physical cosmology:
mergers (galaxy, galaxy cluster), galaxy clusters (finding them, characterizing them, situating them in the cosmic web), galaxy evolution in clusters and elsewhere, and counterparts at high redshift. I use simulations (mostly dark matter only) and analytic methods.
(The figure source is here.)
Current work
I started in physical cosmology with an interest in major mergers (which build up cosmological structure) and in analytic approaches to structure formation more generally. These areas are very rich, and even better, relevant observational data for many of the questions/consequences are becoming available or about to become available. Having theoretical predictions with which to compare is becoming more and more useful as the precision of the data allows more discriminating questions to be asked and answered. On the way, it is often crucial to make "mock catalogues" where one creates a simulation that is similar to the observed sky in order to see what observational strategies and analyses may produce/constrain. This is also useful for people doing the observations, to compare strategies and consequences of assumptions/tradeoffs.
Clusters: Galaxy clusters are the largest (and thus rarest) virialized objects in the universe and correspondingly their counts and clustering are extremely sensitive to cosmological parameters. This has inspired many observational surveys, now underway, to collect large samples of them. For counts and clustering, and other uses of galaxy clusters (as environments for galaxy formation, as the most extreme halos/intersections in the cosmic web, etc.), one wants to start with a very basic property of each cluster, its mass. This is extremely difficult to measure better than 20%, and even methods this accurate are difficult to come by. Simulating the counts, positions, mergers and masses of galaxy clusters in dark matter simulations is relatively straightforward nowadays, but going from this to observables, which tend to involve gas physics on some level or another, is more difficult to do precisely. I have worked on comparisons of analytic methods to simulated cluster merger rates, dark matter cluster assembly histories and the roles of major mergers and more recently on more general cluster properties, including measuring their clustering, and finding them in observational data. My most recent cluster work has been on finding and weighing them in simulations, with an eye to understanding what the simulations have to get right in order to use the same finding methods reliably in (specifically optical and SZ) observational data, and there are many interesting questions there to continue pursuing. I am part of the Dark Energy Survey cluster working group.
Galaxies/subhalos: A second merger related direction is that of galaxies, which Andrew Wetzel, Martin White and I have been investigating using subhalos in dark matter halos. Galaxy mergers are thought to drive quasar activity, starbursts and the evolution of galaxies into the red sequence, among other things. Identifying these subhalos within dark matter halos is not yet done in a standard way, however these objects are expected to correlate most directly with galaxies, and many observed galaxy properties have been reproduced with this identification. Pushing this analysis further to learn about galaxy evolution and merger properties is a very rich topic, especially at high redshift where observational data is now becoming available.
Reionization: Reionization is the end of the "dark ages", when the universe became ionized again due to photons from structure formation (e.g. stars), at redshifts somewhere between 12 and 6. The role of various possible sources, their strengths and how reionization proceeds are becoming more accessible to observational measurements in the next decade. There are huge numbers of uncertainties involved and characteristic properties of many possible scenarios are being calculated for comparison with these observations. I have worked on the role of mergers in reionization (where the sources that reionize the universe are commonly assigned to only have a photon flux depending upon their mass, rather than whether they have recently merged and are starbursting or hosting an active quasar), putting in various models into analytic calculations with Tzu-Ching Chang. Martin White and I then used numerical simulations to measure merger rates and their scatter more accurately, and more directly compute their effects on reionization. This also gave insight about the high redshift halos and mergers and their comparison to various analytic estimates commonly used, and suggests several future extensions related to reionization.
The most up to date list can be found by looking at my publications .
I've also worked on analytic methods for baryon oscillations, strong gravitational lensing analyses of some systems, and issues to do with analyzing cluster correlation functions, among other things. In the last Millennium I worked on early universe field theory, and string theory.