A multi-wavelength study of the IRAS Deep Survey galaxy sample - I. Optical observations. The redshift distribution

Context. The IRAS Deep Survey (IDS) of the North Ecliptic Pole region remains one of the deepest complete far-infrared-selected galaxy samples available. ISOCAM observations in the LW3 filter (12-18 mu m) of 3.'2 x 3.'2 fields centered on the nominal positions of 94 out of the 98 IDS sources yielding 116 ISOCAM detections. The resulting sample has allowed us to assess the source reliability, identify confusion effects, and pinpoint optical counterparts. Aims. To fully exploit the potential of this sample for investigating galaxy evolution, we carried out optical observations of candidate identifications to 100 out of the 116 ISOCAM sources. Methods. Optical imaging was acquired for 88 galaxies in 54 IDS fields: 44 galaxies were imaged in both the B- and the R-band, 33 only in the R-band, and 11 only in the B-band. These observations aimed at extending the MIR/FIR spectral energy distribution to the optical region and achieving morphological information. In addition, spectra were obtained for 65 IDS fields, 20 of which had not been observed before. Redshifts were measured for the 85 galaxies found within the positional error circles of 77 ISOCAM sources detected in those IDS fields. Results. These observations allowed us to acquire spectroscopic redshifts of all counterparts that until now lacked them, so that, in combination with the results by Ashby et al. (1996), we now have redshifts for all IDS/ISOCAM galaxies in the field and, in particular, for the complete sub-sample of IDS galaxies defined by Mazzei et al. (2001). Conclusions. In several cases we find evidence of multiple galaxies with very close redshifts within the error circles around ISOCAM positions, hinting at an important role for interactions in triggering the star formation activity. Although the z distribution of the complete sub-sample has a tail extending up to z similar or equal to 0.37, our results are inconsistent with the extreme cosmological evolution advocated by some models.