EDP Sciences
Free access
Volume 379, Number 3, December I 2001
Page(s) 1161 - 1169
Section Instruments, observational techniques and data processing
DOI http://dx.doi.org/10.1051/0004-6361:20011394
A&A 379, 1161-1169 (2001)
DOI: 10.1051/0004-6361:20011394

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

Cs. Kiss1, 2, P. Ábrahám1, 2, U. Klaas1, M. Juvela3 and D. Lemke1

1  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
2  Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary
3  Helsinki University Observatory, Tähtitorninmäki, PO Box 14, 00014 University of Helsinki, Helsinki

(Received 31 January 2001 / Accepted 26 September 2001)

We examined the sky confusion noise in 40 sky regions by analysing 175 far-infrared (90-200 $\mu$m) maps obtained with ISOPHOT, the photometer on-board the Infrared Space Observatory. For cirrus fields with $ \langle B \rangle > 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 $90 \le \lambda \le 200 \mu$m. We verified that the confusion noise scales as $N \propto \langle B \rangle ^{1.5}$, independent of the wavelength and confirmed $N \propto \lambda^{2.5}$ for $\rm\lambda \ge 100$ $\mu$m. 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 $\mu$m a noise term with a Poissonian spatial distribution was detected in the faintest fields ( $\langle B \rangle \le 3$-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 $\mu$m, 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 $7 \pm 2$ mJy and $15 \pm 4$ mJy at 90 and 170 $\mu$m, respectively. We obtained a CFIRB surface brightness of $B_0 = 0.8 \pm 0.2$ MJy sr-1 ( $\nu I_{\nu}$ = $14 \pm 3$ nW m-2 sr-1) at 170 $\mu$m and an upper limit of 1.1 MJy sr-1 ( $\nu I_{\nu}$ = 37 nW m-2 sr-1) at 90 $\mu$m.

Key words: methods: observational -- ISM: structure -- infrared: ISM: continuum

Offprint request: Cs. Kiss, pkisscs@konkoly.hu

© ESO 2001