Research Note

Determination of the cosmic far-infrared background level with the ISOPHOT instrument ^*,**

¹ Observatory, University of Helsinki, PO Box 14, 00014 Helsinki, Finland e-mail: mika.juvela@helsinki.fi
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary

Received: 15 November 2008
Accepted: 8 April 2008

Abstract

Context. The cosmic infrared background (CIRB) consists mainly of the integrated light of distant galaxies. In the far-infrared the current estimates of its surface brightness are based on the measurements of the COBE satellite. Independent confirmation of these results is still needed from other instruments.

Aims. In this paper we derive estimates of the far-infrared CIRB using measurements made with the ISOPHOT instrument aboard the ISO satellite. The results are used to seek further confirmation of the CIRB levels that have been derived by various groups using the COBE data.

Methods. We study three regions of very low cirrus emission. The surface brightness observed with the ISOPHOT instrument at 90, 150, and 180 μm is correlated with hydrogen 21 cm line data from the Effelsberg radio telescope. Extrapolation to zero hydrogen column density gives an estimate for the sum of extragalactic signal plus zodiacal light. The zodiacal light is subtracted using ISOPHOT data at shorter wavelengths. Thus, the resulting estimate of the far-infrared CIRB is based on ISO measurements alone.

Results. In the range 150 to 180 μm, we obtain a CIRB value of 1.08 ± 0.32 ± 0.30 MJy sr^-1 quoting statistical and systematic errors separately. In the 90 μm band, we obtain a 2-σ upper limit of 2.3 MJy sr^-1.

Conclusions. The estimates derived from ISOPHOT far-infrared maps are consistent with the earlier COBE results.