1 Introduction

The European Space Agency's Infrared Space Telescope, ISO (Kessler et al. 1996; Kessler 2000) performed about 1000 programs between 1995 and 1998, including the deepest extragalactic observations ever made in the mid- and far-infrared range with an unprecedented sensitivity (for a review see Genzel & Cesarsky 2000). Most of these deep cosmological observations aim at probing galaxy formation and evolution, mainly by resolving the Cosmic Infrared Background (CIB) into discrete sources, but also by studying the CIB fluctuations.

Understanding and observing the sources contributing to the extragalactic background at all wavelengths has become one of the most rapidly evolving fields in observational cosmology since the discovery of the CIB (Désert et al. 1995; Puget et al. 1996). In particular, deep observations from space with ISO, and from the ground with SCUBA on the JCMT and MAMBO on the IRAM 30 m telescope, respectively in the infrared, submillimeter and millimeter range, together with observations at other wavelengths for source identification (in the radio and optical / NIR range), begin to provide a global view of galaxy evolution. The long wavelength observations reveal galaxies through their dust emission, providing a complementary and significantly different view to that of optical and UV observations.

The ISO legacy regarding galaxy evolution includes a number of significant studies. About a dozen deep surveys have been conducted in the mid infrared with ISOCAM (Cesarsky et al. 1996), reaching sensitivity levels of $30 \, \mu$Jy at 15 $\mu$m (Altieri et al. 1999; Elbaz et al. 1999; Aussel et al. 1999; Désert et al. 1999; Flores et al. 1999). The major results of the mid-infrared surveys involve source counts obtained by combining a number of surveys. These exhibit strong evolution with a steep slope up to $2.4 \pm 0.2$ (Elbaz et al. 1999) in the integral $\log N$-$\log S$ diagram. Multiwavelength identifications and redshift distributions constrain the nature of the sources (Flores et al. 1999; Aussel et al. 1999; Chary & Elbaz 2001): most of them are Luminous Infrared Galaxies, LIRG's, at a median redshift of 0.8.

In the far-infrared, the 60-240 $\mu$m spectral domain was explored using the imaging capabilities of ISOPHOT (PHT) (Lemke et al. 1996). As indicated in Fig. 1 of Gispert et al. (2000), this domain corresponds to the maximum emission of the extragalactic background . The main surveys published were carried out in the Lockman Hole on 1.1 sq. deg at 90 and 170 $\mu$m by Kawara et al. (1998), in the FIRBACKMarano field at 170 $\mu$m by Puget et al. (1999) and in the entire FIRBACKsurvey by Dole et al. (1999), in SA57 on 0.4 sq. deg at 60 and 90 $\mu$m by Linden-Vornle et al. (2000), and in 8 small fields covering nearly 1.5 sq. deg at 90, 120, 150 and 180 $\mu$m by Juvela et al. (2000). A shallower survey was performed over an area of 11.6 sq. deg at 90 $\mu$m by Efstathiou et al. (2000) as part of the ELAIS survey. The ISOPHOT Serendipity Survey at 170 $\mu$m (Stickel et al. 1998, 2000) took advantage of ISO slews between targets to detect about 1000 sources between 1 and 1000 Jy.

In the 60 to 120 $\mu$m spectral windows, the C_100 camera, with its $3 \times 3$ array of Ge:Ga detectors, was subject to strong transients and spontaneous spiking, limiting the sensitivity (which is a few times better than IRAS); fortunately, new attemps to overcome these problems with a physical model of the detector seem promising (Coulais et al. 2000; Lari & Rodighiero 2001). At 60 and 90 $\mu$m, no clear evolution in the source counts is observed, since both non-evolution and moderate evolution models can still fit the data (Linden-Vornle et al. 2000; Efstathiou et al. 2000). Furthermore, the K-correction (Fig. 1 from the model of Dole 2000 and Lagache et al. 2001) between 30 and 120 $\mu$m is not favorable for probing galaxy evolution up to redshifts $z\sim 1$. With the ELAIS survey, Serjeant et al. (2001) were able to derive the luminosity function of galaxies up to redshift $z \simeq 0.3$.

Figure 1: K-corrections at 15 (dot-dashed curve), 60 (dotted curve), 90 (dashed curve) and 170 $\mu$m (solid curve) for a LIRG (Dole 2000; Lagache et al. 2001). The wavelengths of cosmological interest are thus around 15 $\mu$m and above 150 $\mu$m where they benefit from the "negative K-correction effect'', increasing the sensitivity up to redshifts around unity.

At longer wavelengths (120-240 $\mu$m), the C_200 camera, a $2 \times 2$ array of stressed Ge:Ga detectors, is more stable and most of the detectors' behaviour can be characterized and, if needed, properly corrected (Lagache & Dole 2001). The K-correction at 170 $\mu$m (Fig. 1), as well as in the mid-infrared around 15 $\mu$m, is favorable and becomes optimal at redshifts around 0.7. The first analysis of deep surveys at 170 $\mu$m showed a large excess in source counts over predictions of no-evolution models at flux levels below 200 mJy (Kawara et al. 1998; Puget et al. 1999), suggesting strong evolution. Recent work by Juvela et al. (2000) is in agreement with this picture, and includes the far-infrared colors of the sources.

The FIRBACKsurvey (acronym for Far Infrared BACKground) was designed to broaden our understanding of galaxy evolution with its accurate source counts and its catalog allowing multiwavelength follow-up. It also enabled studies of the CIB fluctuations (first detected in the first area surveyed in the FIRBACK program by Lagache & Puget 2000). FIRBACKis one of the deepest surveys made at 170 $\mu$m and the largest at this depth. This survey used about 150 h of observing time, corresponding to the 8th largest ISO program (Kessler 2000).

The aim of this paper is to provide the catalogs and the source counts of the FIRBACKsurvey. Preliminary FIRBACKsource counts were published by Lagache et al. (1998) and Puget et al. (1999) on the 0.25 sq. deg Marano 1 field, and by Dole et al. (2000) on the entire survey. An overview of this paper is as follows. Section 2 presents the observational issues of the FIRBACKsurvey and Sect. 3 summarizes the data processing and the calibration (a complete description can be found in Lagache & Dole 2001). Section 4 explains the extensive simulations and the source extraction technique. Section 5 details the flux measurement by aperture photometry, analyses the photometric and astrometric noise of the sources and provides estimates of accuracies. In Sect. 6 we present the final FIRBACKcatalog ( $S > 4\sigma_{\rm s}$), and the complementary catalog ( $3\sigma_{\rm s} < S < 4\sigma_{\rm s}$) extracted for follow-up purposes. Section 7 describes the corrections that have been applied (completeness, Malmquist-Eddington effect) and presents the final FIRBACKsource counts at 170 $\mu$m. Section 8 compares our results to other observations as well as models, and discusses the cosmological implications of the FIRBACKsource counts: strong evolution and resolution of the CIB.