Galaxy clustering from COMBO-17: The halo occupation distribution at <z>=0.6

We present measurements of galaxy clustering at redshift <z>=0.6 using 10360 galaxies with photometric redshifts over an area of 0.78 square degree from the COMBO-17 survey. To obtain a result that is unaffected by redshift uncertainties, we calculate the projected correlation function w(r_p), giving results for red sequence and blue cloud galaxies separately. The correlation function of the red galaxies displays clear deviations from a power law at comoving separations around 1 to 3 h^-1 Mpc, and similar but weaker trends are suggested by the data for the blue galaxies. To interpret these results, we fit the correlation functions with analytical predictions derived from a simple halo occupation model. This combines linear clustering of the underlying mass with a description of the number of galaxies occupying each dark-matter halo (the halo occupation distribution). If the occupation numbers are taken to be a simple power law with N proportional to M^alpha, then alpha ~ 0.5 and alpha ~ 0.2 for red and blue galaxies respectively. These figures are little different from the values required to fit present-day clustering data. The power-spectrum shape is assumed to be known in this exercise, but we allow the data to determine the preferred value of sigma_8, the linear power-spectrum normalization. The average normalization inferred from red and blue galaxies at <z> =0.6 is sigma_8=1.02+-0.17 at zero redshift, consistent with independent estimates of this local value. This agreement can be regarded as a verification of the hierarchical growth of the halo mass function.