The Canada-France redshift survey .7. Nature of emission-line field galaxy population up to z=0.3

We present a spectroscopic study of the 138 field galaxies to a redshift z = 0.3 from the I-selected Canada-France Redshift Survey. 117 (85 per cent) spectra exhibit at least H alpha in emission, and the remaining 21 (15 per cent) are purely absorption-line spectra. We focus our analysis on spectra with H alpha and H beta in emission, accounting for about half of this low-z sample, which we classify using emission-line ratio diagrams. Using photoionization models, we determine the extreme boundaries of H II galaxies in these diagnostic diagrams, and demonstrate that the emission-line ratios of a significant fraction of galaxies require harder photoionization sources than massive O stars. We find that about 17 per cent of the field galaxies have emission-line ratios consistent with active galaxies, e.g., Seyfert 2 or LINERs. After correcting for stellar absorption under the Balmer lines, we conclude that the fraction of such galaxies is at least 8 per cent of the field galaxy population at 2 less than or equal to 0.3.