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

We discuss the colors of 99088 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 (vz, bz, yz). This narror-band (~200 A) 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 A 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 population. The distribution of galaxies along the locus is bimodal, with the local minimum corresponding to ~1 Gyr old single stellar population. The position of a galaxy perpendicular to the locus is indipendent of metallicity and age, and reflects the galaxy dust content, as implied by both 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 Hdelta-D4000 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 A) wavelength range) can be predicted with an rms 0f 0.05 mag using the rest-frame Stromgren colors. Overall, the galaxy spectral energy distribution in the entire ultaviolet 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 multi-dimensional space of measured parameters strongly supports the conclusion of Yip et al. (2004), based on a principal copmponent analysis, that SDSS galaxy spectra can be described by a small number of eigenspectra. Furthermore, the rest-frame Stroemgren colors 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 colors of emission-line galaxies and their position in the Baldwin-Phillips-Terlevich diagram. These correlations between colors 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.

Methods : statistical; Surveys; Galaxies : fundamental parameters; Galaxies : seyfert; Galaxies : statistics

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