Growth of massive seed black holes and their impact on their host galaxies
Eastwood, Daniel Shaun
The recent discovery of an 800 million solar mass black hole powering a quasar at a redshift corresponding to 690 million years after the Big Bang is the latest in a growing list of observations of super-massive black holes (SMBHs) that were most likely seeded with masses larger than those expected from the remnants of the first generation of stars. This thesis investigates the consequences of the seeding of SMBHs by massive black holes on galaxy evolution through a combination of analytic modelling techniques. Firstly, I analytically model the growth of gravitational instabilities in an isolated proto-galaxy disc as it progresses into a fully-formed galaxy in the presence of a massive seed black hole formed directly through isothermal collapse. The model shows for the first time how the gravitational effects of a seed black hole lead to an increase in the stability of the disc and an increase in the star formation timescale in the region of the disc close to the black hole. This gravitational imprint of the black hole on the galactic disc has the effect of suppressing star formation. To investigate if this has a lasting effect on the properties of seed hosting galaxies, I evolve the disc galaxy model from the epoch of seed formation down to z ~ 6. I show how star formation in seed hosting galaxies is further regulated by a combination of gravitational stability and the accretion of gas onto the black hole, leading to a scenario where the resulting ratio of black hole to stellar mass at z = 6 is significantly higher than observed in the local universe. I also investigate how the growth of massive seed black holes is regulated by the mass and momentum transport in the disc. The inward accretion of gas towards the central massive black hole and therefore the accretion of gas onto the black hole itself is a function of the stability of the disc. The stabilising effect of the black hole therefore has the potential to regulate the inflow of gas, depending on the relative masses of the galaxy disc and black hole. I find that even in the regime where the inflow rate is not affected by the presence of the black hole, viscosity driven accretion is too inefficient for even massive seeds to grow into a SMBHs by z ~ 6. This indicates the isolated growth of SMBHs is not possible. Indeed, merger events or other processes which are efficient at dissipating angular momentum are required to provide the necessary rapid accretion of gas for SMBHs to form. Finally, I study the dynamical heating of a dark matter halo through the accretion of massive black hole systems. Modelled as both a black hole embedded in its own subhalo and as a naked black hole, the infall of the black hole system acts as a perturber to the density of the central halo, converting potential energy to heat the gas of the central halo through dynamical friction. The timescale over which this infall occurs decreases with a larger perturber mass relative to the central halo mass. The total energy released through black hole accretion during the period of infall is strongly dependent on the black hole growth model. Generally, the total energy from black hole growth exceeds the total energy from dynamical friction. However, the energy released through dynamical friction reaches a maximum when the perturber's proximity to the centre of the central halo is minimised, generally after black hole accretion has ceased. There is therefore a period of time where dynamical friction is the primary source of heating within a halo, potentially contributing to the luminosities of some distant quasars and leading to over-estimates in inferred black hole masses.