Spectral classification of emission-line galaxies from the Sloan Digital Sky Survey I. An improved classification for high redshift galaxies

Aims. We study the spectral classification of emission-line galaxies as star-forming galaxies or Active Galactic Nuclei (AGNs). From the Sloan Digital Sky Survey (SDSS) high quality data, we define an improved classification to be used for high redshift galaxies Methods. We classify emission-line galaxies of the SDSS according to the latest standard recipe using [OIII]λ5007, [NII]λ6584, [SII]λ6717+6731, Hα , and Hβ emission lines. We obtain four classes: star-forming galaxies, Seyfert 2, LINERs, and composites. We then examine where these galaxies fall in the blue diagram used at high redshift (i.e. log([OIII]λ5007/Hβ ) vs. log([OII]λλ3726+3729/Hβ ). Results. We define new improved boundaries in the blue diagram for star-forming galaxies, Seyfert 2, LINERs, SF/Sy2, and SFLIN/comp classes. We maximize the success rate to 99.7% for the detection of star-forming galaxies, to 86% for the Seyfert 2 (including the SF/Sy2 region), and to 91% for the LINERs. We also minimize the contamination to 16% in the region of star-forming galaxies. We cannot reliably separate composites from star-forming galaxies and LINERs, but we define a SF/LIN/comp region where most of them fall (64%).