The ELAIS Survey

People involved at OAB : Calabrese, Ciliegi, Comastri, Gruppioni, Mignoli, Pozzetti, Pozzi, Zamorani.

ELAIS is a large European project, involving 19 different institutes, initially aimed at studying the nature and evolution of the extragalactic sources detected by the Infrared Space Observatory (ISO) in selected areas of the sky. At present the collaboration has enlarged and the survey coverage has extended to other wavelengths, so that now the ELAIS southern area S1 (4 square degrees) is one of the best studied due to its extensive multiwavelength coverage.

In fact, this field is one of the main selected fields of the largest survey performed with ISO (ELAIS, covering 12 sq. deg. at 15 $\mu$m): its size is 4 sq. deg. and the 15 $\mu$m catalogue published by Lari et al. (2001) contains 329 extragalactic sources over the flux range 0.5-100 mJy. The entire area is covered by radio observations obtained with the ATCA down to $S_{\rm 1.4-GHz} \simeq 0.2$ mJy (Gruppioni et al. 1999), which C. Gruppioni, F. Pozzi, P. Ciliegi and G. Zamorani, in collaboration with C. Lari (IRA-CNR Bologna), F. La Franca (Università di Roma3) and I. Matute (MPI) have used to investigate and derive the radio-IR correlation for the first time at those flux densities and for a sample of that size ($\sim100$ ISOCAM-radio associations with measured $z$; Gruppioni, Pozzi, Zamorani, Ciliegi et al. 2003).

S1 is also covered by R-band CCD exposures reaching $R \sim 23$ (obtained at the ESO/Danish 1.5m telescope in collaboration with F. La Franca, I.  Matute, C. Lari et al.), where 82% of the IR extragalactic sample have a likely counterpart. Two main spectroscopic classes are found to dominate the MIR extragalactic population: star-forming galaxies mainly at $z < 0.5$, which account for $\approx$ 75% of the sources, and Active Galactic Nuclei (AGN; both type 1 and 2), which account for $\approx$ 25% of the sources (La Franca, Gruppioni et al. 2004)

Again in the framework of the follow-up of the ELAIS region, F. Pozzi, E. Calabrese, P. Ciliegi, C. Gruppioni, M. Mignoli and G. Zamorani (in collaboration with C. Lari, and P. Heraudeau (Groningen)) have studied the optical, near-IR and radio properties of the complete sample of 43 sources detected at 15 $\mu$m in the ELAIS field S2, where about 90% of the sources (39 out of 43) have optical counterparts brighter than $I=21$. The 15-$\mu$m, H$\alpha$ and 1.4-GHz luminosities have been used as different indicators of star-formation rate in galaxies (Pozzi et al. 2003).

F. Pozzi, C. Gruppioni and G. Zamorani, in collaboration with S. Oliver (Sussex Univ.), I. Matute, F. La Franca, C. Lari, A. Franceschini (Univ. of Padova) and M. Rowan-Robinson (ICSTM), have obtained the first direct determination of the 15-$\mu$m luminosity function and its cosmic evolution for galaxies from the ELAIS survey. The analysis is based on $\sim$150 ELAIS galaxies in the redshift interval $0.0 \leq z \leq 0.4$, covering a large flux density range, intermediate between IRAS and the deep ISOCAM surveys (0.5-50 mJy). Strong evolution (of the order of $(1+z)^{3.5}$ both in luminosity and in density) is suggested by our data for the starburst galaxy population, while normal spiral galaxies are consistent with no evolution. The model predictions have been compared with other observables, like source counts at all flux density levels (from 0.1 to 300 mJy) and redshift distributions and luminosity functions at high-$z$ ($0.7 < z < 1.0$ from HDF-N data), showing a remarkably good agreement. Using the evolutionary model found for the 15-$\mu$m galaxies and the data points from the $1/V_{max}$ LF analysis, we have estimated the star-formation rate density up to $z\sim1$. At $z \leq 0.4$ our model predictions are well consistent with other estimates derived from UV, optical and MIR data. At higher redshifts our model predictions are significantly higher than the UV extinction corrected data and lower by about a factor of two than the estimates derived from radio data (Pozzi, Gruppioni et al. 2004).

The ELAIS area S1 is one of the targets selected by the Spitzer legacy programme SWIRE (PI C. Lonsdale (Caltech)) and will be observed in June 2004. The SWIRE project is the largest Survey project that will be performed with Spitzer, covering a total of about 70 sq.deg. (10 of which on the S1 region) at all available wavelengths (from 3 to 200 $\mu$m). Our 15-$\mu$m ISOCAM data will be of extreme importance for calibrations and interpretation of the Spitzer data, since they will fill the gap between the short- (IRAC: 3.6-8$\mu$m) and long-wavelength (MIPS: 24-160$\mu$m) instruments on board Spitzer.
In the framework of the SWIRE collaboration, we have obtained deep $B$, $V$ and $R$ images with the WFI at the ESO 2.2m telescope (ESO Large Programme ESIS: PI A. Franceschini) down to about $B=24.7$, $V=24.0$ and $R=24.5$ and in the $I$ and $Z$ band with VIMOS-VLT. Moreover, about 1 sq. deg. of S1 has been covered by deep $K^{'}$ band exposures with SOFI at the ESO NTT telescope (ESO Large Programme: PI A. Cimatti (INAF-Arcetri Obs.)) and in the X-ray band with XMM-Newton (4 pointings of about 100 ksec each have been obtained in the central area of S1: PI F. Fiore (INAF-Rome Obs.)) with about 500 sources detected in the 0.5-10 keV band down to a flux of 2-3 $\times 10^{-15}$ cgs.
Recently, a new very deep radio observation at 1.4 GHz has been obtained with the Australia Telescope Compact Array (ATCA) (PI : Boyle, CoI: Ciliegi, Condon, Lonsdale) in the ELAIS region S1 down to an rms noise of 10 $\mu$Jy. The data have been collected in January and February 2004 and the data reduction is ongoing.
Finally, 30 hours at the ESO VLT telescope with the VIMOS spectrograph have been allocated in the ELAIS S1 field (PI: F. La Franca) with the aim of obtaining spectroscopic identification for the fainter 15-$\mu$m ISO sources, for the new XMM and $K^{'}$ sources and for the radio sources obtained with the new ATCA data.