The European Large Area ISO survey: Optical identifications of 15-mu m and 1.4-GHz sources in N1 and N2

We present the multiwavelength properties and catalogue of the 15 mu m and 1.4 GHz radio sources detected in the European Large Area ISO Survey (ELAIS) areas N1 and N2. Using the optical data from the Wide Field Survey we use a likelihood ratio method to search for the counterparts of the 1056 and 691 sources detected at 15 mu m and 1.4 GHz, respectively, down to flux limits of S-15 = 0.5 mJy and S-1.4GHz = 0.135 mJy. We find that similar to 92 per cent of the 15 mu m ELAIS sources have an optical counterpart down to r' = 24. All mid-infrared (IR) sources with fluxes S-15 >= 3 mJy have an optical counterpart. The magnitude distribution of the sources shows a well-defined peak at relatively bright magnitudes r' similar to 18. The mid-IR-to-optical and radio-to-optical flux diagrams are presented and discussed in terms of actual galaxy models. About 15 per cent of the sources are bright galactic stars; of the extragalactic objects 65 per cent are compatible with being normal or starburst galaxies and 25 per cent active galactic nuclei (AGNs). Objects with mid-IR-to-optical fluxes larger than 100 are found, comprising 20 per cent of the sample. We suggest that that these sources are highly obscured luminous and ultraluminous starburst galaxies and AGNs.