Sky confusion noise in the far-infrared: Cirrus, galaxies and the cosmic far-infrared background

Abstract

We examined the sky confusion noise in 40 sky regions by analysing 175 far-infrared (90-200 μm) maps obtained with ISOPHOT, the photometer on-board the Infrared Space Observatory. For cirrus fields with> 5 MJy sr-1 the formula based on IRAS data (Helou & Beichman 1990) predicts confusion noise values within a factor of 2 of our measurements. The dependence of the sky confusion noise on the surface brightness was determined for the wavelength range 90 ≤ λ ≤ 200 μm. We verified that the confusion noise scales as N ∝1.5, independent of the wavelength and confirmed N ∝ λ2.5 for λ ≥ 100 μm. The scaling of the noise value at different separations between target and reference positions was investigated for the first time, providing a practical formula. Since our results confirm the applicability of the Helou & Beichman (1990) formula, the cirrus confusion noise predictions made for future space missions with telescopes of a similar size can be trusted. At 90 and 170 μm a noise term with a Poissonian spatial distribution was detected in the faintest fields (≤ 3-5 MJy sr-1), which we interpret as fluctuations in the Cosmic Far-Infrared Background (CFIRB). Applying ratios of the fluctuation amplitude to the absolute level of 10% and 7% at 90 and 170 μm, respectively, as supported by model calculations, we achieved a new simultaneous determination of the fluctuation amplitudes and the surface brightness of the CFIRB. The fluctuation amplitudes are 7 ± 2 mJy and 15 ± 4 mJy at 90 and 170 μm, respectively. We obtained a CFIRB surface brightness of B0 = 0.8 ± 0.2 MJy sr-1 (νIν = 14 ± 3 nW m-2 sr-1) at 170 μm and an upper limit of 1.1 MJy sr-1 (νIν = 37 nW m-2 sr-1) at 90 μm.