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 μm in one of the deeper ELAIS repeatedly observed regions. The extragalactic objects in this sample have 15‐μm 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 ( z med ∼ 0.2) ; four objects are active galactic nuclei. We have used the 15‐μm, Hα 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‐μm estimates are in good agreement, showing that our galaxies are forming stars at a median rate of ∼40 M ⊙ yr −1 , the raw Hα‐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 Hα/Hβ 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‐μm objects, suggesting that the average extinction suffered by galaxies increases with infrared luminosity.