Interpreting the observed clustering of red galaxies at z ∼ 3

Daddi et al. have recently reported strong clustering of a population of red galaxies at z similar to 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 r(0) similar to8 h(-1) Mpc; for a standard cosmology, this r(0) would imply that the red galaxies reside in rare, M greater than or equal to 10(13) h(-1) M-circle dot halos, with each halo hosting similar to100 galaxies to match the number density of the population. Using the framework of the halo occupation distribution (HOD) in a LambdaCDM 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 > M-min 6.3 x 10(11) h(-1) M-circle dot and a mean occupation N-avg(M) 1/4(M/M-min)(0.45) above this cutoff. The resultant correlation functions do not follow a strict power law, showing instead a clear transition from the one-halo-dominated regime, where the two galaxies of each pair reside in the same dark matter halo, to the two-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, r(0) similar to 5 h(-1) Mpc. 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 two-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 theta(-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.