1 Introduction

The Cosmic Far-Infrared Background (CIB) was detected by Puget et al. (1996), exploiting COBE-FIRAS data, and has now been firmly established over a range of wavelengths (e.g. Dwek et al. 1998; Gispert et al. 2000; Hauser & Dwek 2001). The intensity is quite high with respect to predictions based on evolutionary models of star formation in galaxy populations inferred from optical data. Source counts obtained with SCUBA at 850 $\mu$m and ISO at 170 and 15 $\mu$m have partly resolved the CIB and shown a strong cosmological evolution. In the future, far-IR and sub-millimeter telescopes from ground and space will perform deep surveys over small areas, aimed at resolving a substantial fraction of the CIB and to shed light on the number density, luminosity and spectral evolution of the infrared galaxy populations. However, investigation of the clustering of these populations requires surveys over much larger areas. One way to tackle the limitation on the number of detected galaxies per field is to search for CIB fluctuations. So far, CIB fluctuations has only been observed at 170 $\mu$m in the FIRBACK fields (Lagache & Puget 2000) and at 90 and 170 $\mu$m in the Lockman hole (Matsuhara et al. 2000) surveys. These detections were probably dominated by the Poissonian contribution. By analyzing larger FIRBACK fields, power spectra seem to reveal correlated fluctuations, well above the cirrus contribution (Puget & Lagache 2002). These results are currently under further investigation. Above $170~\mu$m, only SCUBA observations might currently be able to give information on the fluctuations. However, no significant CIB correlations have been detected in the SCUBA maps (Peacock et al. 2000). The CIB anisotropies are mainly contributed by moderate to high redshift star-forming galaxies, whose clustering properties and evolutionary histories are currently unknown. Since the clustering strength depends on the bias at the relevant redshift, observing the CIB correlated fluctuations will provide valuable informations on bulge and elliptical formation, as well as potentially QSOs, thereby providing clues on the physical relations between dark matter and starburst galaxies.

In this paper we present the results of a power spectrum analysis of the IRAS 60 and 100 $\mu$m emission of 12 regions in the sky with very low interstellar emission. All these regions are characterized by an excess of power at high spatial frequencies with respect to the interstellar emission. We will show that this excess is not of instrumental origin and may be attributed to the CIB. The discovery of the CIB fluctuations at 60 and 100 $\mu$m could give constraints on the number counts below the IRAS point source detection limit. However, instead of extrapolating the number counts at 60 and 100 $\mu$m using the level of the fluctuations, which is of limited cosmological interest, we prefer to use the two detections in a more general framework of the modelisation of the IR galaxy evolution that combines all existing number counts, redshift distributions, and observations of the CIB and its fluctuations, in the whole IR and submm range Lagache et al. (2002).

The paper is organized as follow. After a presentation (Sect. 2) of the data used for this analysis we describe in Sect. 3 the power spectrum at 60 and 100 $\mu$m of the 12 fields selected. In Sect. 4 we present the method used to separate the Galactic and extra-Galactic contributions to the power spectrum and we discuss our results in Sect. 5.