The evolution of the far infrared galaxy population and the predicted far infrared background

Direct estimates of the rate of evolution of IRAS galaxies from redshift surveys are reviewed. There is now good evidence for strong evolution of the starburst galaxy population and this is supported by the interpretation of the 60 μm source‐counts. The sub‐mJy radio source population at 1.4 GHz has also been found to be essentially identical to the IRAS starburst galaxy population and faint radio source‐counts support luminosity evolution models. A new model is presented for source‐counts and integrated background radiation from radio to optical wavelengths. For normal spirals, only 30% of their energy, on average, is emitted in the far infrared, so they must be optically thin. For starburst galaxies we estimate that 95% of their optical and uv radiation is absorbed by dust and reemitted at far infrared wavelengths. Our predicted far infrared integrated background intensity lies well below the DIRBE upper limits, but interestingly close to the FIRAS limit at 500 μm. It also lies in the range estimated from absorption of Tev γ‐rays in AGN.