The evolution of black holes in cosmological simulations

Abstract

We investigate the growth of black holes and their effects on the evolution of galaxies through cosmic time in the ΛCDM cosmology by using fully hydrodynamical
simulations of structure formation. Gas accretion onto black holes is modelled and improved via a subgrid model
that takes into account the circularisation and subsequent viscous transport of infalling material. We incorporate the black hole accretion model in hydrodynamical simulations of relatively small size. The model broadly matches the observed stellar mass fractions in haloes and reproduces the expected correlation between the stellar velocity dispersion and the black hole mass. The distribution of black hole accretion rates is also compatible with observations. Additionally, we use a state-of-the-art hydrodynamic simulation that is designed to produce a virtual Universe that closely matches the observed properties of galaxies such as the galaxy stellar mass function and the relation between the black hole mass and the stellar mass at the present day. The critical part to
reproduce the galaxy stellar mass function is the subgrid models of AGN feedback
and black hole growth that are based on the model investigated above. We find
that the simulation reproduces the black hole mass function at the present day.
We investigate the predicted relations between the black hole mass and the
stellar mass and the black hole mass and the parent halo mass and their evolution
through cosmic time. We find that there is no evolution in approximately the last
10 and a half Giga years (z < 2), while at early times the most massive galaxies were inhabited by more massive black holes. The evolution of these relations are different in large halos and small halos. This can be explained in terms of self-regulation. Black holes living in massive haloes (< 10^12 M ) today self-regulate their growth via AGN feedback that quenches black hole accretion rates and star
formation, while the black holes in small haloes rapidly grow without affecting
the growth of the galaxy. By looking at the relations between the gas properties
and the parent halo mass, we compare the scatter of these relations to the ratio
of cumulative accreted mass of black holes to halo mass. We speculate that there
is a range of halos that frames the region where black holes start to grow by
self-regulation.
Finally, we explore the predicted evolution of the AGN luminosity functions
in X-ray bands predicted in this simulation. We find remarkable agreement with
observations. In addition, we find that the observed downsizing effect of AGNs
is well reproduced in the simulation as a natural consequence of reproducing
the AGN luminosity functions. We also explore AGN activity in different halos.
We find that the massive haloes are inhabited by AGNs with low or non activity
while low mass haloes are inhabited by AGNs with high activity that contribute
to the hard X ray luminosity function across time.