Constraints on cosmic star-formation history from the cosmic spectrum

Astronomy and Astrophysics – Astronomy

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Scientific paper

We present the first results on the history of star formation in the Universe based on the `cosmic spectrum', in particular, the volume-averaged 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 a luminosity density based estimator. 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 scales as (1+z)β with a strong upper limit of β <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 SFR at z>1: e.g., if β >2 then the SFR between z=1 and 5 scales as (1+z)α with α <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<β <2.9 and -1.0<α <0.7. KG and IKB acknowledge generous funding from the David and Lucille Packard foundation.

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