ar X iv : a st ro - p h / 03 07 03 0 v 1 1 J ul 2 00 3 Interpreting the Observed Clustering of Red Galaxies at z ∼ 3

Daddi et al. (2003) have recently reported strong clustering of a population of red galaxies at z∼3 in the Hubble Deep Field South (HDF-S). 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., 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 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 1-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 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 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