2003A&A...401...73W

We present a detailed empirical assessment of how the galaxy luminosity function and stellar luminosity density evolves over the last half of the universe's age (0.2<z<1.2) for galaxies of different spectral energy distributions (SED). The results are based on ∼25000 galaxies (R≲24) with redshift measurements (σ_z≃0.03) and SEDs across λ_obs≃350... 930nm. The redshifts and SEDs were derived from medium-band photometry in 17 filters, observed as part of the COMBO-17 survey (``Classifying Objects by Medium-Band Observations in 17 Filters'') over three disjoint fields with a total area of 0.78 square degrees. Luminosity functions (LF), binned in redshift and SED-type, are presented in the restframe passbands of the SDSS r-band, the Johnson B-band and a synthetic UV continuum band at 280nm. We find that the luminosity function depends strongly on SED-type at all redshifts covered. The shape of the LF, i.e. the faint-end power-law slope, does depend on SED type, but not on redshift. However, the redshift evolution of the characteristic luminosity M<sup>*</sup> and density φ<sup>*</sup> depends strongly on SED-type: (1) Early-type galaxies, defined as redder than a present-day reference Sa spectrum, become drastically more abundant towards low redshift, by a factor of 10 in the number density φ<sup>*</sup> from z=1.1 to now, and by a factor of 4 in their contribution to the co-moving r-band luminosity density, j<sub>r</sub>. (2) Galaxies resembling present-day Sa- to Sbc-colours show a co-moving number density and contribution to j<sub>r</sub> that does not vary much with redshift. (3) Galaxies with blue spectra reflecting strong star formation decrease towards low redshift both in luminosity and density, and by a factor of 4 in their j<sub>r</sub> contribution. Summed over all SED types and galaxy luminosities, the comoving luminosity density decreases towards low redshift, between z=1.1 and now, by a small amount in restframe r and B, but by a factor of ∼6 in restframe 280nm. At z=1.1, galaxies redder than Sbc's, contribute 40% to the total j<sub>r</sub>, which increases to 75% by z=0. For λ<sub>rest</sub>=280nm, this increase is from 12% to 25% over the same redshift interval. Comparison of the three independent sight-lines shows that our results are not significantly affected by large-scale structure. Our lowest redshift bin at z=[0.2,0.4] largely agrees with the recent assessment of the present-day galaxy population by SDSS and 2dFGRS and deviates only by an excess of ``faint blue galaxies'' at z∼0.3 compared to very local samples. Overall our findings provide a set of new and much more precise constraints to model the waning of overall star formation activity, the demise of star-bursts and the strong emergence of ``old'' galaxies, with hardly any young population, over the last 6-8 Gigayears.