Interpreting the Observed Clustering of Red Galaxies atz∼ 3

Daddi et al. have recently reported strong clustering of a population of red galaxies at z~3 in the Hubble Deep Field-South. Fitting the observed angular clustering with a power law of index -0. 8, they infer a comoving correlation length r₀~8 Mpc/h; for a standard cosmology, this r₀ would imply that the red galaxies reside in rare, M>10^13 Msun/h halos, with each halo hosting ~100 galaxies to match the number density of the population. Using the framework of the halo occupation distribution (HOD) in a ΛCDM universe, we show that the Daddi et al. data can be adequately reproduced by less surprising models, e. g. , models with galaxies residing in halos of mass M>Mₘin=6. 3x10^11 Msun/h and a mean occupation Navg (M) =1. 4 (M/Mₘin) ^0. 45 above this cutoff. The resultant correlation functions do not follow a strict power law, showing instead a clear transition from the 1-halo dominated regime, where the two galaxies of each pair reside in the same dark matter halo, to the 2-halo dominated regime, where the two galaxies of each pair are from different halos. The observed high-amplitude data points lie in the one-halo-dominated regime, so these HOD models are able to explain the observations despite having smaller correlation lengths, r₀~5 Mpc/h. HOD parameters are only loosely constrained by the current data because of large sample variance and the lack of clustering information on scales that probe the 2-halo regime. If our explanation of the data is correct, then future observations covering a larger area should show that the large scale correlations lie below a θ^-1. 8 extrapolation of the small scale points. Our models of the current data suggest that the red galaxies are somewhat more strongly clustered than UV-selected Lyman-break galaxies and have a greater tendency to reside in small groups.