Spectral classification of emission-line galaxies from the Sloan Digital Sky Survey

Abstract

Aims. In this paper we present a classification of emission-line galaxies at intermediate and high redshifts (0.5 ≲ z ≲ 1.0 for optical spectra, z ≳ 2.5 for near-infrared spectra), using the Dn(4000) index as a supplementary diagnostic. Our goal is to complement the diagnostic based only on emission-line ratios from the blue part of the spectra, which suffer from some limitations for the classification of Seyfert 2 and composite galaxies.