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∼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 ∼ 8h Mpc; for a standard cosmology, this r0 would imply that the red galaxies reside in rare, M ≥ 10hM⊙ 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 = 6.3 × 10 hM⊙ 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 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, r0 ∼ 5h 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 θ 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. Subject headings: galaxies: halos – galaxies: high-redshift – large-scale structure of universe