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. Seyferts 2s and LINERs, is obtained from diagnostic diagrams involving the [O II]\lambda 3727 \AA, [Ne III]\lambda3869 \AA, H\beta and [O III]\lambda5007 \AA relative intensities or the [O II]\lambda 3727 \AA and H\beta equivalent widths. Furthermore, the colour index of the continuum underlying [O II]\lambda 3727 \AA and H\beta provides an additional separation parameter between the two types of emission-line galaxies. We have applied the equivalent widths method to the 0 < z \leq 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.