Environmental effects on real-space and redshift-space galaxy clustering

Galaxy formation inside dark matter halos, as well as the formation of the halo itself, can be affected by the large-scale environment. Evaluating the imprint of environmental effects on galaxy clustering is crucial for construing precise cosmological constraints from galaxy redshift survey data. We investigate the environmental impact on both real-space and redshift-space galaxy clustering statistics using a semianalytic model (SAM) derived from the Millenium Simulation. We compare clustering statistics from original SAM galaxy samples to shuffled samples with the environmental influences on galaxy properties eliminated. Among the luminosity-threshold samples examined, the one with the lowest threshold luminosity (similar to 0.2L(*)) is the most affected by environmental effects, having a similar to 10% decrease in the real-space two-point correlation function (2PCF) after shuffling. By decomposing the 2PCF into five different components based on the source of pairs, we show that the change in the 2PCF can be explained by the age and richness ( galaxy occupation number) dependence of halo clustering. The 2PCFs in redshift space are found to change in a similar manner after shuffling. If the environmental effects are neglected, halo occupation distribution modeling of the real- space and redshift-space clustering may have a less than 6.5% systematic uncertainty in constraining sigma(8)Omega(0.6)(m) m from the most affected SAM sample, and substantially smaller uncertainties for the other, more luminous samples. We argue that the effect could be even smaller in reality. In the Appendix, we present a method to decompose the 2PCF, which can be applied to measure the two-point autocorrelation functions of galaxy subsamples in a volume-limited galaxy sample and their two-point cross- correlation functions in a single run using only one random catalog.