COMBO-17 measurements of the effect of environment on the type-dependent galaxy luminosity function

We have developed a method to calculate overdensities in multicolour surveys, facilitating a direct comparison of the local density contrast measured using galaxy samples that have different redshift error distributions, i.e. for red and blue, or bright and faint galaxies, respectively. We calculate overdensities in small redshift slices (Delta z =0.02, which at z=0.3 corresponds roughly to Delta r_comoving=53 h^-1 Mpc) for 9176 galaxies with R<=23.65, M_B(Vega)-5log h<=-18, and z<= 0.7, in three COMBO-17 fields (measuring 31'X 31' each). The mean redshift errors of this sample are approximately sigma_z/(1+z)~0.015. In the Chandra Deep Field South we identify a region that is underdense by almost a factor 2 compared to the other two fields in the same redshift range (0.25<~ z <~0.4). This can be used for an investigation of the variation of the colour-dependent luminosity function with environment: We calculate the luminosity function in this redshift range for red sequence and blue cloud galaxies (as defined by Bell et al. 04) in each of the fields separately. While the luminosity function of the blue galaxies remains unaffected by different density contrasts, the luminosity function of the red galaxies clearly has a more positive faint-end slope in the Chandra Deep Field South as compared to the other two COMBO-17 fields. The underdensity there is thus mainly due to a deficiency of faint red galaxies. This result is in qualitative agreement with the trends seen at z=0.1, e.g. in the 2dFGRS (Croton et al. 05), or in the SDSS (Zandivarez et al. 06).