Analyzing Projection Contamination in Cluster Cosmology Anbo Chen & August E. Evrard

1. Analyzing Projection Contamination in Cluster CosmologyAnbo Chen & August E. Evrard Abstract We have constructed a halo model to assess the amount of projection contaminations in galaxy clusters among optical, X-ray and sub-mm surveys. The model includes foreground and background, random and correlated projections via the halo mass function and correlation function. Properly dressed with optical HOD, X-ray profile or SZ emission, it can be used to address projection contamination in multiple wavelengths. We have also developed mock skies using Monte Carlo methods to allow more studies on a more diverse range of aspects. N-body Simulation Projection effect predicted by the model is verified by numerical simulation results. It is also suggested that projections due to correlated structure will dominate at higher redshifts. Furthermore, such effect is even more significant for massive clusters, as they incur a much stronger correlation. Mock Sky Patches Using the halo projection model, we are capable of creating large volume of mock sky patches rendering a full sight line alone a target halo including correlations. Such mock sky patches can be used as test environments for testing cluster finders, analyzing projection effect, etc. We have currently developed an optical HOD model with multi-band color evolution to generate optical galaxies with multi-band photometry. In the above figures we take a snapshot at a halo with mass 2×1014h-1Msol at z = 0.2 and generate the full sight line with halos and then optical galaxies and SZ fluxes. The color on the left plot indicates the G-R color of those galaxies and the logarithm of SZ flux on the right one. These multi-wavelength sight lines enable one to examine covariance in multi-wavelength surveys. In addition, these mock sky patches can take different cosmologies as input and one may look at the impact on cluster properties, especially projection effects, in a different cosmology. Velocity Dispersion Plotted underneath is a comparison to the velocity dispersion analysis in Becker et al. 2007 on SDSS spectroscopic data. The observed redshift histogram of galaxies surrounding a BCG of a cluster with 16~30 galaxies is plotted in green crosses. The model prediction is broken down into true target cluster, correlated and random projection contributions. As illustrated, while the random contribution can be removed with proper background subtraction, the correlated one will remain and increase the scatter observed by approximately 15%~20% depending on the richness.