Power spectrum of the cosmic infrared background at 60 and 100 microns with IRAS

Based on a power spectrum analysis of the IRAS ISSA maps, we present the first detection of the Cosmic far-Infrared Background (CIB) fluctuations at 60 and 100 microns. The power spectrum of 12 low cirrus emission regions is characterized by a power excess at spatial frequencies higher than k~0.02 arcmin^{-1}. Most of this excess is due to noise and to nearby point sources with a flux stronger than 1 Jy. But we show that when these contributions are carefully removed, there is still a power excess that is the signature of the CIB fluctuations. The power spectrum of the CIB at 60 and 100 microns is compatible with a Poissonian distribution, at spatial frequencies between 0.025 and 0.2 arcmin^{-1}. The fluctuation level is ~1.6x10^3 Jy^2/sr and ~5.8x10^3 Jy^2/sr at 60 and 100 microns respectively. The levels of the fluctuations are used in a larger framework, with other observationnal data, to constrain the evolution of IR galaxies (Lagache et al. 2002). The detections reported here, coupled with the level of the fluctuations at 170 microns, give strong constraints on the evolution of the IR luminosity function. The combined results at 60, 100 and 170 microns for the CIB and its fluctuations allows, on the CIB at 60 microns, to put a firm upper limit of 0.27 MJy/sr and to give an estimate of 0.18 MJy/sr.