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Supermassive black holes and multiphase gas in early-type galaxies
Supermassive black holes and multiphase gas in early-type galaxies
In this thesis, we study the evolution of the multiphase gas in and around massive, quiescent early-type galaxies (ETGs), and how it is affected by their central supermassive black holes (SMBH). To model the physical processes acting on the gas, we perform hydrodynamical simulations with a modern smoothed particle hydrodynamics (SPH) code. We present simulations of an isolated, massive ETG that include models for gas cooling, star formation, stellar feedback (in the form of type Ia and type II supernovae (SNe) and winds from asymptotic giant branch stars), metal enrichment of the gas by the stellar feedback, and accretion onto and feedback (in kinetic and radiative form) from the central SMBH. We find that both forms of SMBH feedback together are necessary to keep the star formation rate (SFR) and the black hole growth within observational limits. We also find that the kinetic feedback of the SMBH is able to drive outflows of metal-rich gas into the circumgalactic medium of the ETG, enriching it out to a radius of ∼ 30 kpc. We then present high-resolution simulations of a dense, molecular circumnuclear disc (CND) in the centre of an ETG, which we compare to the observed CNDs of NGC 4429 (Davis et al., 2018) and similar systems. Besides the processes listed above, these simulations also include non-equilibrium cooling, hydrogen chemistry, interstellar UV radiation, shielding of the gas from it, cosmic ray (CR) ionisation, stellar photo-ionisation feedback, an improved star-formation model, and a new SMBH accretion model. We also implement a new “mechanical” SN feedback model. We find that, under a large range of conditions (different gravitational potentials, UV field strengths, CR ionisation rates, SN models, SMBH accretion and feedback), the simulated CND is more star-forming than the observed systems at equivalent gas surface densities. To prevent this, a physical mechanism (such as magnetic fields) is needed to stop the collapse of gas to high densities. Finally, we compare simulations of SMBH feedback in isolated ETGs with different feedback efficiencies, as well as different hydrodynamical solvers (two flavours of SPH, and meshless-finite-mass) to study the effect of the underlying hydrodynamical models on the results. We find that changing either the SMBH feedback efficiency or the hydrodynamical solver significantly alters the effect of the SMBH feedback on the structure, outflows, and SFR of the gas. While the dependence of the results on the efficiency is straightforward, that on the hydrodynamic solver shows a fundamental weakness in the numerical modelling. We conclude that results of hydrodynamical simulations with unresolved highly energetic processes (such as SMBH feedback) need to be interpreted carefully, taking into account their strong dependence on the simulation’s fundamentals.
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Eisenreich, Maximilian
2018
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Eisenreich, Maximilian (2018): Supermassive black holes and multiphase gas in early-type galaxies. Dissertation, LMU München: Faculty of Physics
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Abstract

In this thesis, we study the evolution of the multiphase gas in and around massive, quiescent early-type galaxies (ETGs), and how it is affected by their central supermassive black holes (SMBH). To model the physical processes acting on the gas, we perform hydrodynamical simulations with a modern smoothed particle hydrodynamics (SPH) code. We present simulations of an isolated, massive ETG that include models for gas cooling, star formation, stellar feedback (in the form of type Ia and type II supernovae (SNe) and winds from asymptotic giant branch stars), metal enrichment of the gas by the stellar feedback, and accretion onto and feedback (in kinetic and radiative form) from the central SMBH. We find that both forms of SMBH feedback together are necessary to keep the star formation rate (SFR) and the black hole growth within observational limits. We also find that the kinetic feedback of the SMBH is able to drive outflows of metal-rich gas into the circumgalactic medium of the ETG, enriching it out to a radius of ∼ 30 kpc. We then present high-resolution simulations of a dense, molecular circumnuclear disc (CND) in the centre of an ETG, which we compare to the observed CNDs of NGC 4429 (Davis et al., 2018) and similar systems. Besides the processes listed above, these simulations also include non-equilibrium cooling, hydrogen chemistry, interstellar UV radiation, shielding of the gas from it, cosmic ray (CR) ionisation, stellar photo-ionisation feedback, an improved star-formation model, and a new SMBH accretion model. We also implement a new “mechanical” SN feedback model. We find that, under a large range of conditions (different gravitational potentials, UV field strengths, CR ionisation rates, SN models, SMBH accretion and feedback), the simulated CND is more star-forming than the observed systems at equivalent gas surface densities. To prevent this, a physical mechanism (such as magnetic fields) is needed to stop the collapse of gas to high densities. Finally, we compare simulations of SMBH feedback in isolated ETGs with different feedback efficiencies, as well as different hydrodynamical solvers (two flavours of SPH, and meshless-finite-mass) to study the effect of the underlying hydrodynamical models on the results. We find that changing either the SMBH feedback efficiency or the hydrodynamical solver significantly alters the effect of the SMBH feedback on the structure, outflows, and SFR of the gas. While the dependence of the results on the efficiency is straightforward, that on the hydrodynamic solver shows a fundamental weakness in the numerical modelling. We conclude that results of hydrodynamical simulations with unresolved highly energetic processes (such as SMBH feedback) need to be interpreted carefully, taking into account their strong dependence on the simulation’s fundamentals.