My research has focused on the optical properties of galaxies in clusters. I'm especially interested in:

• the use of clusters as cosmological indicators
• why clusters have the particular galaxy population that they do
• how clusters are built up over time
• how galaxies evolve differently in clusters than the field as a result of environmental effects
• the formation of Brightest Cluster Galaxies (BCGs)
• fossil groups

I investigate these problems by using imaging data from the Sloan Digital Sky Survey (ugriz data for > 7,000 square degrees), the Blanco Cosmology Survey (griz data for 100 square degrees overlapping with the SZ survey regions of SPT, ACT and APEX), and the Sunyaev-Zeldovich Array (6 square degrees with BVRz and 30 GHz interferometric data). I am an observer for both BCS and the SZA Optical surveys.

My thesis work focuses on the combination of optical and SZ data for the purpose of investigating possible systematic effects in using a survey of SZ-selected clusters for cosmology for will allow the SZ survey to push cluster characterization to a lower mass threshhold. My thesis advisor is Stephan Meyer; I also collaborate frequently with Risa Wechsler (Stanford), Erin Sheldon (NYU), and Mike Gladders (Chicago).

Currently...
I am focusing on thesis work: correlating the optical and SZ signal for the SZA survey.

Previously...
Hansen et al. 2007 is about the galaxy population in clusters as a function of cluster mass, using the maxBCG catalog of clusters from the SDSS. The paper focuses on BCG properties and on the color and luminosity distributions of cluster members as a function of cluster mass, redshift, and distance from the cluster center.

Sheldon et al. 2007 measured the mass-to-light profiles of stacked samples of galaxy clusters as a function of cluster richness and to radii many times the virial radius. We find that the M/L value within r_200 scales with cluster mass as a power law with index 0.33+/-0.02, and approaches an asymptotic value, independent of scale or richness, of (M/L) / b^2_{ml} = 362+/-54 h. Multiplying by the mean luminosity density in the same bandpass we find Omega_m / b^2_{ml} = 0.20+/-0.03, independent of the Hubble parameter.

In Hansen et al. 2005 we used a subset of photometric SDSS data to estimate the characteristic cluster size, r_200, as a function of cluster richness, and investigated the typical radial distribution and luminosity function of galaxies in clusters.

For a complete listing of my work, take your pick of:
Preprints (astro-ph) ... Publications (ADS) ... Publications (SPIRES)