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

We discuss the colours of 99 088 galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 1 'main' spectroscopic sample (a flux-limited sample, r(Pet) < 17.77, for 1360 deg(2)) in the rest-frame Stromgren system (uz, vz, bz, yz). This narrow-band (similar to 200 angstrom) photometric system, first designed for the determination of effective temperature, metallicity and gravity of stars, measures the continuum spectral slope of galaxies in the rest-frame 3200-5800 angstrom wavelength range. We synthesize rest-frame Stromgren magnitudes from SDSS spectra, and find that galaxies form a remarkably narrow locus (similar to 0.03 mag) in the resulting colour-colour 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 population. The distribution of galaxies along the locus is bimodal, with the local minimum corresponding to an similar to 1-Gyr-old single stellar population. The position of a galaxy perpendicular 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 colour-colour diagram with the galaxy locus in the H delta(A)-D-n(4000) diagram, utilized by Kauffmann et al. to estimate stellar masses, reveals a tight correlation, although the two analysed spectral ranges barely overlap. Furthermore, the rest-frame r-i colour (5500-8500 angstrom wavelength range) can be 'predicted' with an rms of 0.05 mag using the rest-frame Stromgren colours. Overall, the galaxy spectral energy distribution in the entire ultraviolet to near-infrared 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 multidimensional space of measured parameters strongly supports the conclusion of Yip et al., based on a principal component analysis, that SDSS galaxy spectra can be described by a small number of eigenspectra. Furthermore, the rest-frame Stromgren colours correlate tightly with the classification scheme proposed by Yip et al. based on the first three eigenspectra. Apparently, the contributions of stellar populations 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 colours of emission-line galaxies and their position in the Baldwin-Phillips-Terlevich diagram. These correlations between colours and various spectroscopic diagnostic parameters support earlier suggestions that rest-frame Stromgren photometry offers an efficient tool to study faint cluster galaxies and low surface brightness objects without performing time-consuming spectral observations.