Near- and mid-infrared colours of star-forming galaxies in European Large Area ISO Survey fields

We present J- and K-band near-infrared (near-IR) photometry of a sample of mid-infrared (mid-IR) sources detected by the Infrared Space Observatory (ISO) as part of the European Large Area ISO Survey (ELAIS) and study their classification and star-forming properties. We have used the Preliminary ELAIS Catalogue for the 6.7-mum (LW2) and 15-mum (LW3) fluxes. All of the high-reliability LW2 sources and 80 per cent of the LW3 sources are identified in the near-IR survey reaching K~ 17.5 mag. The near-IR/mid-IR flux ratios can effectively be used to separate stars from galaxies in mid-IR surveys. The stars detected in our survey region are used to derive a new accurate calibration for the ELAIS ISOCAM data in both the LW2 and LW3 filters. We show that near- to mid-IR colour-colour diagrams can be used to classify galaxies further, as well as to study star formation. The ELAIS ISOCAM survey is found mostly to detect strongly star-forming late-type galaxies, possibly starburst-powered galaxies, and it also picks out obscured active galactic nuclei. The ELAIS galaxies yield an average mid-IR flux ratio LW2/LW3 = 0.67 +/- 0.27. We discuss the fnu(6.7 mu m)/fnu(15 mu m) ratio as a star formation tracer using ISO and IRAS data of a local comparison sample. We find that the fnu(2.2 mu m)/fnu(15 mu m) ratio is also a good indicator of activity level in galaxies and conclude that the drop in the fnu(6.7 mu m)/fnu(15 mu m) ratio seen in strongly star-forming galaxies is a result of both an increase of 15-mum emission and an apparent depletion of 6.7-mum emission. Near-IR together with the mid-IR data make it possible to estimate the relative amount of interstellar matter in the galaxies.