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 MJy sr-1 the formula based on IRAS data (Helou & Beichman [CITE]) 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 m. We verified that the confusion noise scales as , independent of the wavelength and confirmed for 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 ([CITE]) 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 (-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 mJy and mJy at 90 and 170 μm, respectively. We obtained a CFIRB surface brightness of MJy sr-1 ( = nW m-2 sr-1) at 170 μm and an upper limit of 1.1 MJy sr-1 ( = 37 nW m-2 sr-1) at 90 μm.