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Astron. Astrophys. 360, 813-832 (2000)

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Far infrared extragalactic background radiation

I. Source counts with ISOPHOT

M. Juvela 1, K. Mattila 2 and D. Lemke 3

1 Helsinki University Observatory, Tähtitorninmäki, P.O. Box 14, 00014 University of Helsinki, Finland (mjuvela@astro.helsinki.fi)
2 Helsinki University Observatory, Tähtitorninmäki, P.O. Box 14, 00014 University of Helsinki, Finland
3 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received 9 November 1999 / Accepted 8 February 2000


As a part of the ISOPHOT CIRB (Cosmic Infrared Background Radiation) project we have searched for point-like sources in eight fields mapped at two or three wavelengths between 90 µm and 180 µm. Most of the 55 sources detected are suspected to be extragalactic and cannot be associated with previously known objects. It is probable, also from the far-infrared (FIR) spectral energy distributions, that dust-enshrouded, distant galaxies form a significant fraction of the sources.

We present a tentative list of new extragalactic FIR-sources and discuss the uncertainties involved in the process of extracting point sources from the ISOPHOT maps. Based on the analyzed data we estimate the number density of extragalactic sources at wavelengths 90 µm, 150 µm and 180 µm and at flux density levels down to 100 mJy to be 1[FORMULA]105 sr-1, 2[FORMULA]105 sr-1, and 3[FORMULA]105 sr-1, respectively.

Strong galaxy evolution models are in best agreement with our results, although the number of detections exceeds most model predictions. No-evolution models can be rejected at a high confidence level.

Comparison with COBE results indicates that the detected sources correspond to [FORMULA]20% of the extragalactic background light at 90 µm. At longer wavelengths the corresponding fraction is [FORMULA]10%.

Key words: galaxies: evolution – galaxies: starburst – cosmology: observations – infrared: galaxies

Send offprint requests to: M. Juvela

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000