The Canada - France Redshift Survey XII : 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%) spectra exhibit at least Hα in emission, the remaining 21 (15%) are purely absorption-line spectra. We focus our analysis on spectra with Hα and Hβ in emission, accounting for about half of this lowz 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% 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% of the field galaxy population at z ≤ 0.3.