Galaxy formation in WMAP1 and WMAP7 cosmologies

Abstract

Using the technique of Angulo & White, we scale the Millennium and Millennium II simulations of structure growth in a Λ cold dark matter (ΛCDM) universe from the cosmological parameters with which they were carried out (based on first-year results from the Wilkinson Microwave Anisotropy Probe, WMAP1) to parameters consistent with the seven-year WMAP data (WMAP7). We implement semi-analytic galaxy formation modelling (SAM) on both simulations in both cosmologies to investigate how the formation, evolution and clustering of galaxies are predicted to vary with cosmological parameters. The increased matter density Ωm and decreased linear fluctuation amplitude σ8 in WMAP7 have compensating effects, so that the abundance and clustering of dark haloes are predicted to be very similar to those in WMAP1 for z ≤ 3. As a result, local galaxy properties can be reproduced equally well in the two cosmologies by slightly altering galaxy formation parameters. The evolution of the galaxy populations is then also similar. In WMAP7, structure forms slightly later. This shifts the peak in cosmic star formation rate to lower redshift, resulting in slightly bluer galaxies at z = 0. Nevertheless, the model still predicts more passive low-mass galaxies than are observed. For rp < 1 Mpc, the z = 0 clustering of low-mass galaxies is weaker for WMAP7 than for WMAP1 and closer to that observed, but the two cosmologies give very similar results for more massive galaxies and on large scales. At z > 1 galaxies are predicted to be more strongly clustered for WMAP7. Differences in galaxy properties, including clustering, in these two cosmologies are rather small out to z ∼ 3. Given that there are still considerable residual uncertainties in galaxy formation models, it is very difficult to distinguish WMAP1 from WMAP7 through observations of galaxy properties or their evolution.