ISO deep far-infrared survey in the "Lockman Hole" II. Power spectrum analysis: evidence of a strong evolution in number counts

We investigate the characteristics of FIR brightness fluctuations at 90 mu m and 170 mu m in the Lockman Hole, which were surveyed with ISOPHOT aboard the Infrared Space Observatory (ISO). We first calculated the angular correlation function of each field and then its Fourier transform (the angular Power Spectral Density: PSD) over the spatial frequency range of f = 0.05-1 arcmin(-1). The PSDs are found to be rather flat at low spatial frequencies (f less than or equal to 0.1 arcmin(-1)), slowly decreasing toward higher frequencies. These spectra are unlike the power-law ones seen in the IR cirrus fluctuations, and are well explained by randomly distributed point sources. Furthermore, point-to-point comparison between 90 mu m and 170 mu m brightness shows a linear correlation between them, and the slope of the linear fit is much shallower than that expected from the IR cirrus color, and is consistent with the color of galaxies at low or moderate redshift (z < 1). We conclude that the brightness fluctuations in the Lockman Hole are not caused by the IR cirrus, but are most likely due to faint star-forming galaxies. We also give the constraints on the galaxy number counts down to 35 mJy at 90 mu m and 60 mJy at 170 Gem, which indicate the existence of a strong evolution down to these fluxes in the counts. The galaxies responsible for the fluctuations also significantly contribute to the cosmic infrared. background radiation.