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 10 360 galaxies with photometric redshifts over an area of 0.78 deg(2) 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 N proportional to M-alpha, then alpha similar or equal to 0.5 and alpha similar or equal to 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.