EDP Sciences
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Volume 401, Number 1, April I 2003
Page(s) 73 - 98
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021513

A&A 401, 73-98 (2003)
DOI: 10.1051/0004-6361:20021513

The COMBO-17 survey: Evolution of the galaxy luminosity function from 25 000 galaxies with 0.2< z <1.2

C. Wolf1, 2, K. Meisenheimer1, H.-W. Rix1, A. Borch1, S. Dye1, 3 and M. Kleinheinrich1, 4

1  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
2  Department of Physics, Denys Wilkinson Bldg., University of Oxford, Keble Road, Oxford, OX1 3RH, UK
3  Astrophysics Group, Blackett Lab, Imperial College, Prince Consort Road, London, UK
4  IAEF, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

(Received 14 August 2002 / Accepted 7 October 2002 )

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 ~ $25\,000$ galaxies ( $R\la24$) with redshift measurements ( $\sigma_z\approx 0.03$) and SEDs across $\lambda_{{\rm obs}}\approx 350\ldots
930$  nm. 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 280 nm. 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* and density $\phi^*$ 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 $\phi^*$ from  z=1.1 to now, and by a factor of 4 in their contribution to the co-moving r-band luminosity density, jr. (2) Galaxies resembling present-day Sa- to Sbc-colours show a co-moving number density and contribution to jr 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 jr 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 280 nm. At z=1.1, galaxies redder than Sbc's, contribute 40% to the total jr, which increases to 75% by z=0. For  $\lambda_{{\rm rest}}=$280 nm, 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 $z\sim 0.3$ 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.

Key words: techniques: photometric -- surveys -- galaxies: evolution -- galaxies: distances and redshifts

Offprint request: C. Wolf, cwolf@astro.ox.ac.uk

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