We present the first results on the history of star formation in the Universe based on the ‘ cosmic spectrum ’ , in particular , the volume-averaged , luminosity-weighted , stellar absorption line spectrum of present day galaxies from the 2dF Galaxy Redshift Survey .
This method is novel in that unlike previous studies it is not an estimator based on total luminosity density .
The cosmic spectrum is fitted with models of population synthesis , tracing the history of star formation prior to the epoch of the observed galaxies , using a method we have developed which decouples continuum and spectral-line variations and is robust against spectrophotometric uncertainties .
The cosmic spectrum can only be fitted with models incorporating chemical evolution and indicates there was a peak of star-formation rate in the past of at least three times the current value and that the increase back to z = 1 , assuming it scales as ( 1 + z ) ^ { \beta } , has a strong upper limit of \beta < 5 .
We find in the general case there is some model degeneracy between star formation at low and high redshift .
However , if we incorporate previous work on star formation at z < 1 we can put strong upper limits on the star-formation rate at z > 1 : e.g. , if \beta > 2 then the SFR for 1 < z < 5 scales as ( 1 + z ) ^ { \alpha } with \alpha < 2 .
This is equivalent to stating that no more than 80 % of stars in the Universe formed at z > 1 .
Our results are consistent with the best-fit results from compilations of cosmic SFR estimates based on UV luminosity density , which give 1.8 < \beta < 2.9 and -1.0 < \alpha < 0.7 , and are also consistent with estimates of \Omega _ { stars } based on the K -band luminosity density .