Dr Ian Lewis

Abstract:

We compare the amplitudes of fluctuations probed by the 2dF Galaxy Redshift Survey (2dFGRS) and by the latest measurements of the cosmic microwave background (CMB) anisotropies. By combining the 2dFGRS and CMB data, we find the linear-theory rms mass fluctuations in 8 h-1 Mpc spheres to be σ8m = 0.73 ± 0.05 (after marginalization over the matter density parameter Ωm and three other free parameters). This normalization is lower than the COBE normalization and previous estimates from cluster abundance, but it is in agreement with some revised cluster abundance determinations. We also estimate the scale-independent bias parameter of present-epoch Ls = 1.9L* APM-selected galaxies to be b(Ls, z = 0) = 1.10 ± 0.08 on comoving scales of 0.02 < k < 0.15 h Mpc-1. If luminosity segregation operates on these scales, L* galaxies would be almost unbiased, b(L*,z = O) ≈ 0.96. These results are derived by assuming a flat ACDM Universe, and by marginalizing over other free parameters and fixing the spectral index n = 1 and the optical depth due to reionization τ = 0. We also study the best-fitting pair (Ωm, b), and the robustness of the results to varying n and τ. Various modelling corrections can each change the resulting b by 5 - 15 per cent. The results are compared with other independent measurements from the 2dFGRS itself, and from the Sloan Digital Sky Survey (SDSS), cluster abundance and cosmic shear.

Abstract:

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 before the epoch of the observed galaxies, using a method we have developed that 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 it indicates that there was a peak in the star formation rate (SFR) in the past of at least 3 times the current value and that the increase back to z = 1, assuming it scales as (1 + z)β, has a strong upper limit of β < 5. We find, in the general case, that 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 β > 2, then the SFR for 1 < z < 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 yield 1.8 < β < 2.9 and - 1.0 < α < 0.7, and are also consistent with estimates of Ω stars based on the K-band luminosity density.