We present a determination of the `Cosmic Optical Spectrum' of the Universe, i.e. the ensemble emission from galaxies, as determined from the red-selected Sloan Digital Sky Survey main galaxy sample and compare with previous results of the blue-selected 2dF Galaxy Redshift Survey. Broadly we find good agreement in both the spectrum and the derived star-formation histories. If we use a power-law star-formation history model where star-formation rate \∝ (1+z)^\β out to z=1, then we find that \β of 2 to 3 is still the most likely model and there is no evidence for current surveys missing large amounts of star formation at high redshift. In particular `Fossil Cosmology' of the local universe gives measures of star-formation history which are consistent with direct observations at high redshift. Using the photometry of SDSS we are able to derive the cosmic spectrum in absolute units (i.e.W \⁻¹$ Mpc$⁻³) at 2--5\ resolution and find good agreement with published broad-band luminosity densities. For a Salpeter IMF the best fit stellar mass/light ratio is 3.7--7.5\/$ in the r-band (corresponding to $\ h = 0.0025$--0.0055) and from both the stellar emission history and the H$\α$ luminosity density independently we find a cosmological star-formation rate of 0.03--0.04 h $$ yr$⁻¹$ Mpc$⁻³$ today.
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Glazebrook et al. (2003) studied this question.
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