The rest-frame optical colours of 99 000 Sloan Digital Sky Survey galaxies

We discuss the colors of 99,088 galaxies selected from the Sloan Digital Sky Survey Data Release 1 "main" spectroscopic sample (a flux limited sample, rPet < 17.77, for 1360 deg 2 ) in the rest-frame Stromgren system (uz, vz, bz, yz). This narrow-band (�200 u photometric system, first designed for the determination of effective tem- perature, metallicity and gravity of stars, measures the continuum spectral slope of galaxies in the rest-frame 3200-5800 u wavelength range. We synthesize rest-frame Stromgren magnitudes from SDSS spectra, and find that galaxies form a remarkably narrow locus (�0.03 mag) in the resulting color-color diagram. Using the Bruzual & Charlot population synthesis models, we demonstrate that the position of a galaxy along this locus is controlled by metallicity and age of the dominant stellar popula- tion. The distribution of galaxies along the locus is bimodal, with the local minimum corresponding to an �1 Gyr old single stellar population. The position of a galaxy per- pendicular to the locus is independent of metallicity and age, and reflects the galaxy's dust content, as implied by both the models and the statistics of IRAS detections. Comparison of the galaxy locus in the rest-frame Stromgren color-color diagram with the galaxy locus in the H�-Dn(4000) diagram, utilized by Kauffmann et al. (2003) to estimate stellar masses, reveals a tight correlation, although the two analyzed spectral ranges barely overlap. Furthermore, the rest-frame r i color (5500-8500 u wavelength range) can be "predicted" with an rms of 0.05 mag using the rest-frame Stromgren colors. Overall, the galaxy spectral energy distribution in the entire UV to near-IR range can be described as a single-parameter family with an accuracy of 0.1 mag, or better. This nearly one-dimensional distribution of galaxies in the multi- dimensional space of measured parameters strongly supports the conclusion of Yip et al. (2004), based on a principal component analysis, that SDSS galaxy spectra can be described by a small number of eigenspectra. Furthermore, the rest-frame Stromgren colors correlate tightly with the classification scheme proposed by Yip et al. (2004) based on the first three eigenspectra. Apparently, the contributions of stellar popula- tions that dominate the optical emission from galaxies are combined in a simple and well-defined way. We also find a remarkably tight correlation between the rest-frame Stromgren colors of emission-line galaxies and their position in the Baldwin-Phillips- Terlevich diagram. These correlations between colors and various spectroscopic diag- nostic parameters support earlier suggestions that rest-frame Stromgren photometry offers an efficient tool to study faint cluster galaxies and lowsurface brightness objects without performing time-consuming spectral observations.