The sloan digital sky survey: The cosmic spectrum and star formation history

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 it 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 in which the star formation rate is ∝(1 + z)^β out to z = 1, then we find that a β of 2-3 is still the most likely model and that there is no evidence for current surveys to be missing large amounts of star formation at high redshift. In particular, "fossil cosmology" of the local universe gives measures of star formation history that are consistent with direct observations at high redshift. Using the photometry of the Sloan Digital Sky Survey, we are able to derive the cosmic spectrum in absolute units (i.e., units of W Å^-1 Mpc^-3) at 2-5 Å resolution and find good agreement with published broadband luminosity densities. For a Salpeter initial mass function, the best-fit stellar mass-to-light ratio is 3.7-7.5 M⊙/L⊙ in the r band (corresponding to Ω _starsh = 0.0025-0.0055), and from both the stellar emission history and the Ha luminosity density independently we find a cosmological star formation rate of 0.03-0.04 h M⊙ yr^-1 Mpc^-3 today.