A new model for infrared and submillimetre counts

A new model for source counts from 8 to 1100 mu m is presented, which agrees well with source-count data and the observed background spectrum. The model assumes different evolution for each of the four infrared template types used. The evolution is modified in two ways compared to my 2001 model: (i) the exponential factor is modified so that it tends to a constant value at late times and (ii) the power-law factor is modified so that it tends to zero at redshift z(f), rather than zero as assumed previously. I find strong evidence from the 850 and 1100 mu m counts, and from the infrared background, that z(f) = 4-5, with some preference for a value at the low end of the range, implying that star-forming galaxies at z > 5 are not significant infrared emitters, presumably due to a low opacity in dust at these early epochs. The model involves zero or even negative evolution for starbursts and active galactic nuclei at low redshifts (< 0.2), suggesting that the era of major mergers and strong galaxy-galaxy interactions is over.