A new model for infrared and submillimetre counts

A new model for source counts from 8-1100 $\mu$m is presented, which agrees well with source-count data and the observed background spectrum. The model is similar to that of Rowan-Robinson (2001), but with different evolution for each of the four assumed infrared template types. The evolution is modified in two ways; (i) the exponential factor is modified so that it tends to a constant value at late times, (ii) the power-law factor is modified so that it tends to zero at redshift z_f, rather than 0 as assumed by Rowan-Robinson (2001). I find strong evidence from the 850 and 1100 mum 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 AGN at low redshifts (<0.2), suggesting that the era of major mergers and strong galaxy-galaxy interactions is over.