Nature of Field Galaxies in the (z<0.3) Universe from the CFRS Survey

We present the first detailed spectroscopic study of field galaxies up to z=0.3 from the I-magnitude limited CFRS sample. In this complete sample, we find that about half of the objects are blue emission-line galaxies. Using line ratio diagram and photoionization models, we show that in roughly half of these emission-line galaxies, ionization by hot stars cannot account for the observed line ratios. Their spectra have properties intermediate between Seyfert 2 galaxies and low-ionization nuclear emission-line regions (LINERs). Their number density (about 20%) relative to the total number of galaxies up to z=0.3, is much larger than the number density (2%) of active galactic nuclei (AGN) in the local Universe. Thus, the observed excess of blue galaxies in counts down to B=24 cannot be mainly due to starburst galaxies, as was previously thought. This result can shed a new light on the nature of the overabundant population of blue galaxies at low redshifts.