The Galaxy Correlation Function in the CNOC2 Redshift Survey: Dependence on Color, Luminosity, and Redshift

We examine how the spatial correlation function of galaxies from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) depends on galaxy color, luminosity, and redshift. The projected correlation function wp is determined for volume-limited samples of objects with 0.12 ≤ z < 0.51 and evolution-compensated RC-band absolute magnitudes M < -20, over the comoving projected separation range 0.04 h-1 Mpc < rp < 10 h-1 Mpc. Our sample consists of 2937 galaxies that are classified as being either early- or late-type objects according to their spectral energy distribution (SED), as determined from UBVRCIC photometry. For the sake of simplicity, galaxy SEDs are classified independently of redshift: Our classification scheme therefore does not take into account the color evolution of galaxies. Objects with SEDs corresponding to early-type galaxies are found to be more strongly clustered by a factor of ~3 and to have a steeper correlation function than those with late-type SEDs. Modeling the spatial correlation function, as a function of comoving separation r, as ξ(r) = -γ, we find r0 = 5.45 ± 0.28 h-1 Mpc and γ = 1.91 ± 0.06 for early-type objects, and r0 = 3.95 ± 0.12 h-1 Mpc and γ = 1.59 ± 0.08 for late-type objects (for ΩM = 0.2, ΩΛ = 0). While changing the cutoff between early- and late-type SEDs does affect the correlation amplitudes of the two samples, the ratio of the amplitudes remains constant to within 10%. The redshift dependence of the correlation function also depends on SED type. Modeling the redshift dependence of the comoving correlation amplitude r as r(z) ∝ (1 + z)γ-3-ϵ, we find that early-type objects have ϵ = -3.9 ± 1.0, and late-type objects have ϵ = -7.7 ± 1.3. Both classes of objects therefore have clustering amplitudes, measured in comoving coordinates, which appear to decrease rapidly with cosmic time. The excess clustering of galaxies with early-type SEDs, relative to late-type objects, is present at all redshifts in our sample. In contrast to the early- and late-type SED samples, the combined sample undergoes little apparent evolution, with ϵ = -2.1 ± 1.3, which is consistent with earlier results. The apparent increase with redshift of the clustering amplitude in the early- and late-type samples is almost certainly caused by evolution of the galaxies themselves rather than by evolution of the correlation function. If galaxy SEDs have evolved significantly since z ~ 0.5, then our method of classifying SEDs may cause us to overestimate the true evolution of the clustering amplitude for the unevolved counterparts to our early- and late-type samples. However, if color evolution is to explain the apparent clustering evolution, the color evolution experienced by a galaxy must be correlated with the galaxy correlation function. We also investigate the luminosity dependence of the correlation function for volume-limited samples with 0.12 ≤ z < 0.40 and M < -19.25. We detect a weak luminosity dependence of the correlation amplitude for galaxies with early-type SEDs, d log ξ/dM = -0.35 ± 0.17, but no significant dependence for late-type objects, d log ξ/dM = 0.02 ± 0.16.