During the preliminary deep imaging phase of our large spectroscopic survey of faint field galaxies (CFRS), one of our fields (10 arcmin × 10 arcmin) was chosen to coincide with the Fomalont et al (1991, A.J. 102,1258, hereafter FWKK) radio source field, including 36 S≃ 16μJy radio sources of their complete sample. All sources but two have been identified to V < 25 and/or IAB ≤ 24, and/or KAB ≤ 21.

The microJy population is mainly constituted of three distinct populations of galaxies with different redshift regimes: early-type galaxies at z > 0.75 with a low powered AGN in their cores, post-starburst galaxies at intermediate redshifts (z = 0.375 to z = 0.8 or slightly > 1), and emission-line galaxies at z < 0.45 containing AGNs. The fraction of μJy sources with z > 1 could be as high as 30%. Most of the μJy radio sources (> 50%) are likely associated to AGNs, conversely to what is found at mJy levels (mostly starburst galaxies, Benn et al, 1993, MNRAS, 263, 98). Only one galaxy in our sample has a classical starburst spectrum.

The strong decrease of the radio spectral index from sub-mJy to μJy counts appears to be due to a combination of three factors: (1) the emergence of an elliptical population at high redshifts with moderate radio emission (2) an increasing fraction of narrow emission-line AGNs (Seyfert 2 and LINER); (3) a higher contribution of the thermal radiation to the radio emission from spirals, and the almost complete disappearance of starburst galaxies. Details of the results summarized here can be found in Hammer et al (1995, MNRAS, 276, 1085).