Red Sequence Cluster Finding in the Millennium Simulation

Details Red Sequence Cluster Finding in the Millennium Simulation Red Sequence Cluster Finding in the Millennium Simulation

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...20921508e&link_type=abstract

2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #215.08; Bulletin of the American Astronomical Society,

Computer Science

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We use galaxy populations derived from the Millennium Simulation to investigate the nature of high redshift clusters identified by red sequence-based, optical techniques. We build localized sky survey catalogs using projected locations and adjusted r-z colors of galaxies at fixed-time epochs z=0.4, 0.7 and 1. The catalogs are z-band magnitude limited near L*, and generate samples of one million galaxies in (500 Mpc/h)2 projected regions at each redshift. The simulated galaxies in massive, dark matter halos tend to lie on a well-defined red sequence (RS), and we use the knowledge of the RS location at each redshift to inform a simple circular overdensity (CO) algorithm. Using galaxy membership as a way to match clusters with halos, we examine the fidelity of this algorithm. Due to projection, essentially all identified clusters contain galaxies from multiple halos, but we categorize them into two simple classes depending on whether (`clean' clusters) or not (`blended' clusters) a single halo contributes a majority of cluster member galaxies. The CO algorithm, with appropriately tuned overdensity, is extremely efficient at z=0.4; fewer than 10 percent of clusters with 20 or more members are blends, and the relation betweeen observed optical richness and best-matched halo mass is a nearly unbiased version of the true underlying RS halo occupation distribution. At z=1, the performance deteriorates, with the fraction of blended systems increasing to 30 percent. We show that this degradation is driven by weaker evolution in r-z color at high redshift, which effectively increases the projected lengthscale over which galaxies remain within the RS color cut. We discuss implications for existing and upcoming cluster surveys using multi-band optical imaging. In particular, we expect that a significant fraction ( 30%) of high redshift RCS clusters will be underluminous in their X-ray luminosity.

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