The 60-μm extragalactic background radiation intensity, dust-enshrouded AGNs and the assembly of groups and clusters of galaxies

Submillimetre (submm)-wave observations have revealed a cosmologically significant population of high-redshift dust-enshrouded galaxies. The form of evolution inferred for this population can be reconciled easily with COBE FIRAS and DIRBE measurements of the cosmic background radiation (CBR) intensity at wavelengths longer than about 100μm. At shorter wavelengths, however, the 60-μm CBR intensity reported by Finkbeiner, Davis & Schlegel is less easily accounted for. Lagache et al. have proposed that this excess CBR emission is a warm Galactic component, and the detection of the highest-energy γ-rays from blazars limits the CBR intensity at these wavelengths, but here we investigate possible sources of this excess CBR emission, assuming that it has a genuine extragalactic origin. We propose and test three explanations, each involving additional populations of luminous, evolving galaxies not readily detected in existing submm-wave surveys. First, an additional population of dust-enshrouded galaxies with hot dust temperatures, perhaps dust-enshrouded, Compton-thick AGN as suggested by recent deep Chandra surveys Secondly, a population of dusty galaxies with temperatures more typical of the existing submm-selected galaxies, but at relatively low redshifts. These could plausibly be associated with the assembly of groups and clusters of galaxies. Thirdly, a population of low-luminosity, cool, quiescent spiral galaxies. Hot AGN sources and the assembly of galaxy groups can account for the excess 60-μm background. There are significant problems with the cluster assembly scenario, in which too many bright 60-μm IRAS sources are predicted. Spiral galaxies have the wrong spectral energy distributions (SEDs) to account for the excess. Future wide-field far-IR surveys at wavelengths of 70 and 250μm using the SIRTF and Herschel space missions will sample representative volumes of the distant Universe, allowing any hot population of dusty AGNs and forming groups to be detected.