New diagnostic methods for emission-line galaxies in deep surveys

Abstract

We present new quantitative classification methods for emission-line galaxies, which are specially designed to be used in deep galaxy redshift surveys. A good segregation between starbursts and active galactic nuclei, i.e. Seyfert 2s and LINERs, is obtained from diagnostic diagrams involving the [O II] γ3727 Å, [Ne III] γ 3869 Å, Hβ and [O III]γ5007Å relative intensities or the [O II] 23727 Å and Hβ equivalent widths. Furthermore, the colour index of the continuum underlying [O II] γ3727 Å and Hβ provides an additional separation parameter between the two types of emission-line galaxies. We have applied the equivalent widths method to the 0<z≤0.3 emission-line galaxies of the Canada-France Redshift Survey. Our results are in very good agreement with those obtained using the standard diagnostic diagrams including all the strong optical emission-line intensity ratios.