The interplay between cosmology and galaxy formation

Abstract

The standard ΛCDM model of cosmology has been very successful in matching a large set of observational constraints and describes accurately the evolution of our Universe. Within this framework, the gravitational collapse of cold dark matter structures depends solely on the cosmological background. The formation of galaxies inside these haloes is thought to be determined by complex baryonic process and we rely on numerical techniques to model their effects.

Here, we investigate the impact of the cosmological background on galaxy formation. We take the advantage of state of the art cosmological hydrodynamical simulations from the eagle suite to vary the cosmological parameters, in particular, the cosmological constant, to test its effect on the efficiency of star formation. We use this set of new simulations to calculate the likelihood of the observed value of the cosmological constant, given a measure of the multiverse. We discuss the implication of our results in the context of the anthropic principle.

We use this framework to develop a fully analytic model of galaxy formation that connects the growth of dark matter haloes in a cosmological background, with the build-up of stellar mass within these haloes. The model identifies the physical processes that drive the Galaxy-Halo co-evolution through cosmic time. Despite the complexity of the baryonic processes involved, galaxy formation is revealed as a remarkably simple process, where the instantaneous star formation efficiency within halos is only a function of their virial temperature and can be described with a ‘single’ differential equation. We find that the model reproduces self-consistently the shape and evolution of the cosmic star formation rate density, the specific star formation rate of galaxies, and the galaxy stellar mass function, both at the present time and at high redshift.

Finally, we use the merger rate of supermassive black holes in the eagle simulations to estimate the expected event rate of gravitational wave signals that could be resolved by future space-based gravitational wave detectors. We discuss the power of these detections to provide information about the origin of supermassive black holes and the initial mass distribution of black hole seeds.