Infrared colours of star-forming galaxies and a flux calibration of ISOCAM ELAIS catalogues

We present J and K-band near-infrared photometry of a sample of ISOCAM sources detected by the European Large Area ISO-Survey (ELAIS). All of the high-reliability LW2 (6.7 μm) sources and 80 per cent of the LW3 (15 μm) sources are identified in the near-IR survey reaching K â 17.5 mag. The near- to mid-IR flux ratios can effectively be used to separate stars from galaxies in mid-IR surveys: at 6.7 μm, 80 per cent of the identified ELAIS objects are stars while at 15 μm 80 per cent are galaxies. The stars are then used to perform an accurate new calibration of the ELAIS ISOCAM data at both 6.7 and 15 μm: we adopt values of 1.23 and 1.05 ADU/gain/s/mJy for the LW2 and LW3 filters, respectively. The ISOCAM ELAIS survey is found to mostly detect strongly star-forming late-type galaxies. We show that near to mid-IR colour-colour diagrams can be used to further classify galaxies, as well as study star-formation. In a [15/2.2] vs. [6.7/2.2] plot the Hubble type of a galaxy can be roughly estimated from its position along the diagonal ([6.7/15] = 1) and the star-forming efficiency from a galaxys departure from the diagonal (eg. [6.7/15] <1). The ELAIS galaxies yield an average mid-IR flux ratio LW2/LW3 =0.67 ± 0.27. We discuss this [6.7/15] ratio as a star formation tracer using ISO and IRAS data of a local comparison sample. The [2.2/15] ratio is also found to be a good indicator of activity level in galaxies and conclude that the drop in the [6.7/15] ratio seen in strongly star-forming galaxies is a result of both an increase of 15 μm emission and an apparent depletion of 6.7 μm emission.