The Dependence of Galaxy Formation on Cosmological Parameters: Can we distinguish the WMAP1 and WMAP3 Parameter Sets?

TL;DR

This study combines simulations and physical models to assess how changes in cosmological parameters between WMAP1 and WMAP3 affect galaxy formation predictions, finding subtle differences that are hard to distinguish with current data.

Contribution

It demonstrates that current galaxy formation models cannot easily differentiate between WMAP1 and WMAP3 cosmologies due to compensating effects and uncertainties.

Findings

Galaxy formation delays in WMAP3 due to lower initial fluctuations.

Galaxy clustering predictions are similar across cosmologies.

Current data cannot reliably distinguish between the two cosmologies.

Abstract

We combine N-body simulations of structure growth with physical modelling of galaxy evolution to investigate whether the shift in cosmological parameters between the 1-year and 3-year results from the Wilkinson Microwave Anisotropy Probe affects predictions for the galaxy population. Structure formation is significantly delayed in the WMAP3 cosmology, because the initial matter fluctuation amplitude is lower on the relevant scales. The decrease in dark matter clustering strength is, however, almost entirely offset by an increase in halo bias, so predictions for galaxy clustering are barely altered. In both cosmologies several combinations of physical parameters can reproduce observed, low-redshift galaxy properties; the star formation, supernova feedback and AGN feedback efficiencies can be played off against each other to give similar results. Models which fit observed luminosity functions predict projected 2-point correlation functions which scatter by about 10-20 per cent on large scale and by larger factors on small scale, depending both on cosmology and on details of galaxy formation. Measurements of the pairwise velocity distribution prefer the WMAP1 cosmology, but careful treatment of the systematics is needed. Given current modelling uncertainties, it is not easy to distinguish the WMAP1 and WMAP3 cosmologies on the basis of low-redshift galaxy properties. Model predictions diverge more dramatically at high redshift. Better observational data at z>2 will better constrain galaxy formation and perhaps also cosmological parameters.