On the nature of the ISO-selected sources in the ELAIS S2 region

We have studied the optical, near-infrared and radio properties of a complete sample of 43 sources detected at 15 ’¦Ìm in one of the deeper ELAIS repeatedly observed regions. The extragalactic objects in this sample have 15-’¦Ìm flux densities in the range of 0.4-10 mJy, where the source counts start diverging from no evolution models. About 90 per cent of the sources (39 out of 43) have optical counterparts brighter than I= 21 mag. Eight of these 39 sources have been identified with stars on the basis of imaging data; for another 22 sources, we have obtained optical spectroscopy, reaching a high identification percentage (30/43, ~70 per cent). All but one of the 28 sources with flux density >0.7 mJy are identified. Most of the extragalactic objects are normal spiral or starburst galaxies at moderate redshift (zmed~ 0.2); four objects are active galactic nuclei. We have used the 15-’¦Ìm, H’¦Á and 1.4-GHz luminosities as indicators of star formation rate and we have compared the results obtained in these three bands. While 1.4-GHz and 15-’¦Ìm estimates are in good agreement, showing that our galaxies are forming stars at a median rate of ~40 Msolar yr-1, the raw H’¦Á-based estimates are a factor of ~5-10 lower and need a mean correction of ~2 mag to be brought on the same scale as the other two indicators. A correction of ~2 mag is consistent with what suggested by the Balmer decrements H’¦Á/H’¦Â and by the optical colours. Moreover, it is intermediate between the correction found locally for normal spirals and the correction needed for high-luminosity 15-’¦Ìm objects, suggesting that the average extinction suffered by galaxies increases with infrared luminosity.