Abstract

We use photometric redshifts to analyse the effect of local environment on galaxy colours at redshifts z ≲ 0.63 in the Sloan Digital Sky Survey (SDSS) data release 6 (DR6). We construct mock SDSS-DR6 catalogues using semi-analytic galaxies to study possible systematic effects on the characterization of environment and colour statistics due to the uncertainty in the determination of redshifts. We use the projected galaxy density derived from the distance to the nearest neighbours with a suitable radial velocity threshold to take into account the uncertainties in the photometric redshift estimates. Our findings indicate that the use of photometric redshifts greatly improves estimates of projected local galaxy density when galaxy spectra are not available. We find a tight relationship between spectroscopic and photometric derived densities, both in the SDSS-DR6 data (up to z = 0.3) and in mock catalogues (up to z = 0.63). At z = 0, faint galaxies show a clear increase of the red galaxy fraction as the local density increases. Bright galaxies, on the other hand, show a constant red galaxy fraction. We are able to track the evolution of this fraction to z = 0.55 for galaxies brighter than Mr = -21.5 and find that the fraction of blue galaxies with respect to the total population progressively becomes higher as the redshift increases, at a rate of 15 per cent/Gyr. Also, at any given redshift, bright galaxies show a larger red population, indicating that the star formation activity shifts towards smaller objects as the redshift decreases. © 2008 RAS.