Thierry Morel

The European Large Area ISO Survey — IV. The preliminary 90-μm luminosity function

We present the luminosity function of 90-mum-selected galaxies from the European Large Area ISO Survey (ELAIS), extending to z=0.3. Their luminosities are in the range 109 =100mJy data base, we have found optical, 15-mum or 1.4-GHz identifications for 24 (65 per cent). We have obtained 2dF and UK Schmidt FLAIR spectroscopy of 89 per cent of identifications to rigid multivariate flux limits. We construct a luminosity function assuming that (i) our spectroscopic subset is an unbiased sparse sample, and (ii) there are no galaxies that would not be represented in our spectroscopic sample at any redshift. We argue that we can be confident of both assumptions. We find that the luminosity function is well described by the local 100-mum luminosity function of Rowan-Robinson, Helou & Walker. Assuming this local normalization, we derive luminosity evolution of (1+z)2.45+/-0.85 (95 per cent confidence). We argue that star formation dominates the bolometric luminosities of these galaxies, and we derive comoving star formation rates in broad agreement with the Flores et al. and Rowan-Robinson et al. mid-infrared-based estimates.

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.

The European Large Area ISO Survey — VI. Discovery of a new hyperluminous infrared galaxy

We report the discovery of the first hyperluminous infrared galaxy (HyLIG) in the course of the European Large Area ISO Survey (ELAIS). This object has been detected by ISO at 6.7, 15 and 90μm, and is found to be a broad-line, radio-quiet quasar at a redshift [formmu4]z=1.099. From a detailed multicomponent model fit of the spectral energy distribution, we derive a total IR luminosity L_IR [formmu5](1-1000μm)~1.0×10^13h65-2L_solar [formmu6](q_0=0.5), and discuss the possible existence of a starburst contributing to the far-IR output. Observations to date present no evidence for lens magnification. This galaxy is one of the very few HyLIGs with a X-ray detection. On the basis of its soft X-ray properties, we suggest that this broad-line object may be the face-on analogue of narrow-line, Seyfert-like HyLIGs.