1999A&A...343..389F

Deep 6.75µm mid-IR ISOCAM observations were obtained from the Canada-France Redshift Survey (CFRS) 1415+52 field with the Infrared Space Observatory. The identification of the sources with optical counterparts is described in detail, and a classification scheme is devised which depends on the S/N of the detection and the inverse probability of chance coincidence. 83% of the 54 ISOCAM sources are identified with I_AB<23.5 counterparts. The (I-K)_AB colors, radio properties, spectrophotometric properties and frequency of nuclear activity of these counterparts differ on average from those of typical CFRS galaxies. CFRS spectra are available for 21 of the sources which have I_AB≤22.5 (including 7 stars). Most of the strongest sources are stars or AGN. Among the non-stellar counterparts with spectra, 40% are AGNs, and 53% are galaxies that display star formation activity and/or significant contributions of A stars. The ISOCAM sources also display an IR excess, even when compared with heavily-reddened local starburst galaxies. An upper limit of 30% of extragalactic ISO sources could be at z>1 of the 44 S_6.75µm>150µJy sources which are non-stellar (7 "spectroscopic" and 3 "photometric" stars excluded).