Black Hole-Stellar Mass Relation

This thesis has been submitted in fulfilment of the requirements for a postgraduate degree (e.g. PhD, MPhil, DClinPsychol) at the University of Edinburgh. Please note the following terms and conditions of use: This work is protected by copyright and other intellectual property rights, which are retained by the thesis author, unless otherwise stated. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the author. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the author. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given. The growth of massive seed black holes and their impact on their host galaxies Daniel S. Eastwood I V N E R U S E I T H Y T O H F G E R D I N B U Doctor of Philosophy The University of Edinburgh June 2019 Lay Summary Black holes around a billion times the mass of the Sun have been observed to have existed when the Universe was only a fraction of its current age, at less than a billion years old. Exactly how these super-massive objects formed within this, in cosmological terms, short timescale is not fully understood. The first generation of stars that formed in the Universe are thought to have had very short lifetimes before collapsing to form black holes. However, these black holes are thought to be too low in mass to grow to become super-massive within this first billion years. More exotic formation mechanisms are required to provide black holes which have higher initial masses and are therefore capable of reaching super-massive status. Such objects are known as massive seed black holes. One of the most promising mechanisms for forming seed black holes is known as direct collapse and results in the formation of a black hole with a mass of up to a million times the mass of the Sun. This thesis details my investigations into how the development of galaxies in the early universe is affected by massive seed black holes formed via the direct collapse mechanism. I focus on using analytic modelling techniques to study how the presence of these most massive of seed black holes influenced the growth of galaxies in the early universe. In Chapter 2, I create a simple model of a galaxy forming as a disc around a massive seed black hole. I follow the evolution of the system as the seed black hole grows from formation to become a super-massive black hole within the first billion years of the Universe. During this time the disc is growing, gaining more and more mass from gas falling into the system under gravity. As the disc grows in mass it will become more susceptible to collapsing under the gravitational force it exerts on itself, leading to the formation of stars from the collapsing gas. For the first time I show how the gravity of the black hole has a stabilising effect on the disc, preventing this collapse until the disc gains more mass. The black i hole therefore reduces the extent to which the disc can form stars, decreasing the overall rate of star formation. This can have a lasting effect on the evolution of the galaxy as the black hole grows and, in some circumstances, the black hole can prevent the formation of stars entirely. In Chapter 3, I adapt the model to test whether a super-massive black hole can grow from a seed fed solely through the inflow of gas from the galactic disc. The inflow of gas within the disc can be modelled as driven by viscosity which is brought on by the instability of the disc. I find that this viscosity-driven inflow of gas is not sufficient to feed the required black hole growth to form a super-massive black hole within the first billion years of the Universe. This indicates that more efficient inflow mechanisms are needed to grow super-massive black holes in the early universe. I show that, as the merging of two similar mass galaxies can lead to the total disruption of any disc, such an event can provide the required inflow of gas to feed the formation of a super-massive black hole. Both of these studies have lent some weight to an emerging theoretical picture for the potential growth of super-massive black holes seeded through direct collapse whereby the massive seeds originally form in separate haloes from their eventual host galaxies. Following their formation, the systems that host direct collapse seeds fall into the dark matter haloes of neighbouring star-forming galaxies which they subsequently merge with. In chapter 4, I investigate the process of a massive seed black hole system moving through a dark matter halo. As a massive system, the gravitational pull of the object falling in perturbs the background density to form a wake which drags behind it. This wake in turn has a gravitational pull and acts to slow the infalling object. This drag force is known as dynamical friction. As the massive seed black hole falls in, dynamical friction acts to convert the potential energy of the system into heating the gas of the halo. I find that the total energy released and the time it takes for the infalling system to reach the galaxy at the centre both depend on the mass of the infalling system. In certain circumstances the heating from dynamical friction is significant and can even be the primary mechanism for heating the gas in the halo while the massive seed black hole system falls in. This has potential consequences for black hole mass estimates based on brightness measurements. Such mass estimates are inferred by assuming that the power observed is solely generated by black hole growth. As dynamical friction heating may be providing a significant proportion of the observed power, black hole masses may be over- estimated in certain circumstances. ii Abstract 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, iii 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.

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