Classifying Objects by Medium-Band Observations
- a spectrophotometric 17-filter survey -

project: galaxy luminosity function at z=0.2..1.2


Figure:  The evolution of the galaxy luminosity function is clearly differential by SED type.


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 (z=0.2..1.2) for galaxies of different spectral energy distributions (SED). The results are based on ~25000 galaxies (R<24) with redshift measurements (error ~0.03) and SEDs across 350..930 nm. The redshifts and SEDs were derived from medium-band photometry in 17 filters, observed as part of the COMBO-17 survey 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, j(r).
  2. Galaxies resembling present-day Sa- to Sbc-colours show a co-moving number density and contribution to j(r) 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(r) 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 factor of 1.7, 2.0 and 5.0 in restframe r, b and 280 nm, respectively. At z=1.1, galaxies redder than Sbc's, contribute 40% to the total j(r), which increases to 75% by z=0. For j(280), the increase is from 12% to 25% over this 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.

Contact person:  Christian Wolf, Oxford


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Last update Sep 19, 2002, CW