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

project: early-type galaxies at z=0.2..1.1

Figure:  The colour-magnitude relation of galaxies out to z~1.

The evolution of early-type galaxies directly reflects of the importance and number of major galaxy mergers, and therefore strongly constrains hierarchical models of galaxy formation and evolution. We have explored the rest-frame colors and luminosities of ~25,000 galaxies with R<24 from 0.78 square degrees from the COMBO-17 survey. We find that the distribution of galaxy colors is bimodal at all redshifts out to z~1. The blue star-forming peak has colors that do not evolve with redshift; however the number of luminous blue galaxies evolves strongly with redshift. In contrast, the red non-star-forming galaxies have colors that evolve with redshift in a way which is consistent with passive aging of ancient stellar populations. The rest-frame B-band luminosity density in red galaxies evolves only mildly with redshift in a Lambda-dominated cold dark matter Universe. We marginally detect the signature of an increase in stellar mass in red galaxies by a factor of two since redshift one, in excellent agreement with hierarchical models of galaxy formation and evolution. The largest source of error is large-scale structure, implying that considerably larger surveys are necessary to further refine this result.

Contact person:  Eric Bell, MPIA

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Last update Jan 9, 2003, CW