The Star Formation History of the Universe: An Infrared Perspective

A simple and versatile parameterized approach to the star formation historyallows a quantitative investigation of the constraints from far infrared andsubmillimetre counts and background intensity measurements. The models include four spectral components: infrared cirrus (emission frominterstellar dust), an M82-like starburst, an Arp220-like starburst and an AGNdust torus. The 60 $\mu$m luminosity function is determined for each chosenrate of evolution using the PSCz redshift data for 15000 galaxies. Theproportions of each spectral type as a function of 60 $\mu$m luminosity arechosen for consistency with IRAS and SCUBA colour-luminosity relations, andwith the fraction of AGN as a function of luminosity found in 12 $\mu$msamples. The luminosity function for each component at any wavelength can thenbe calculated from the assumed spectral energy distributions. With assumptionsabout the optical seds corresponding to each component and, for the AGNcomponent, the optical and near infrared counts can be accurately modelled. A good fit to the observed counts at 0.44, 2.2, 15, 60, 90, 175 and 850$\mu$m can be found with pure luminosity evolution in all 3 cosmological modelsinvestigated: $\Omega_o$ = 1, $\Omega_o$ = 0.3 ($\Lambda$ = 0), and $\Omega_o$= 0.3, $\Lambda$ = 0.7. All 3 models also give an acceptable fit to the integrated backgroundspectrum. Selected predictions of the models, for example redshiftdistributions for each component at selected wavelengths and fluxes, are shown. The total mass-density of stars generated is consistent with that observed,in all 3 cosmological models.