Simultaneous analysis of SDSS spectra and GALEX photometry with starlight : method and early results.

We combine data from the Sloan Digital Sky Survey (SDSS) and the Galaxy Evolution Explorer (GALEX) to simultaneously analyse optical spectra and ultraviolet photometry of 231 643 galaxies with the starlight spectral synthesis code using state-of-the-art stellar population models. We present a new method to estimate GALEX photometry in the SDSS spectroscopic aperture, which proves quite reliable if applied to large samples. In agreement with previous experiments with CALIFA, we find that adding UV constraints leads to a moderate increase on the fraction of ∼107–108 yr populations and a concomitant decrease of younger and older components, yielding slightly older luminosity-weighted mean stellar ages. These changes are most relevant in the low-mass end of the blue cloud. An increase in dust attenuation is observed for galaxies dominated by young stars. We investigate the contribution of different stellar populations to the fraction of light in GALEX and SDSS bands across the UV optical colour–magnitude diagram. As an example application, we use this λ dependence to highlight differences between retired galaxies with and without emission lines. In agreement with an independent study by Herpich et al. we find that the former shows an excess of intermediate-age populations when compared to the later. Finally, we test the suitability of two different prescription for dust, finding that our data set is best-fitting using the attenuation law of starburst galaxies. However, results for the Milky Way extinction curve improve with decreasing τV, especially for edge-on galaxies. [ABSTRACT FROM AUTHOR]