Halo-model signatures from 380 000 Sloan Digital Sky Survey luminous red galaxies with photometric redshifts

We analyse the small-scale clustering in 'MegaZ-LRG', a large photometric redshift catalogue of luminous red galaxies extracted from the imaging data set of the Sloan Digital Sky Survey. MegaZ-LRG, presented in a companion paper, spans the redshift range 0.4 < z < 0.7 with an rms redshift error sigma(z) approximate to 0.03(1 + z), covering 5914 deg(2) to map out a total cosmic volume 2.5 h(-3) Gpc(3). In this study we use 380 000 photometric redshifts to measure significant deviations from the canonical power-law fit to the angular correlation function in a series of narrow redshift slices, in which we construct volume-limited samples. These deviations are direct signatures of the manner in which these galaxies populate the underlying network of dark matter haloes. We cleanly delineate the separate contributions of the 'one-halo' and 'two-halo' clustering terms and fit our measurements by parametrizing the halo occupation distribution N(M) of the galaxies. Our results are successfully fitted by a 'central' galaxy contribution with a 'soft' transition from zero to one galaxy, combined with a power-law 'satellite' galaxy component, the slope of which is a strong function of galaxy luminosity. The large majority of galaxies are classified as central objects of their host dark matter haloes rather than satellites in more massive systems. The effective halo mass of MegaZ-LRG galaxies lies in the range log(10)(M-eff/h(-1) M-circle dot) = 13.61-13.80 (increasing with redshift assuming large-scale normalization sigma(8) = 0.8) for corresponding number densities in the range n(g) = 5.03 - 0.56 x 10(4) h(-3) Mpc(-3). Our results confirm the usefulness of the halo model for gaining physical insight into the patterns of galaxy clustering.