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Register a new userSpin evolution and feedback of supermassive black holes in cosmological simulations
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Sebastian Bustamante
Publication date
2019
fields
Physics
and research's language is
English
Authors
Sebastian Bustamante HITS
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It is well established that the properties of supermassive black holes and their host galaxies are correlated through scaling relations. While hydrodynamical cosmological simulations have begun to account for the co-evolution of BHs and galaxies, they typically have neglected the BH spin, even though it may play an important role in modulating the growth and feedback of BHs. Here we introduce a new sub-grid model for the BH spin evolution in the moving-mesh code {small AREPO} in order to improve the physical faithfulness of the BH modelling in galaxy formation simulations. We account for several different channels of spin evolution, in particular gas accretion through a Shakura-Sunyaev $alpha$-disc, chaotic accretion, and BH mergers. For BH feedback, we extend the IllustrisTNG model, which considers two different BH feedback modes, a thermal quasar mode for high accretion states and a kinetic mode for low Eddington ratios, with a self-consistent accounting of spin-dependent radiative efficiencies and thus feedback strength. We find that BHs with mass $M_{rm{bh}}lesssim 10^{8}, {rm M}_odot$ reach high spin values as they typically evolve in the coherent gas accretion regime. On the other hand, BHs with mass $M_{rm{bh}}gtrsim 10^{8}, {rm M}_odot$ have lower spins as BH mergers become more frequent, and their accretion discs fragment due to self-gravity, inducing chaotic accretion. We also explore the hypothesis that the transition between the quasar and kinetic feedback modes is mediated by the accretion mode of the BH disc itself, i.e.~the kinetic feedback mode is activated when the disc enters the self-gravity regime. We find excellent agreement between the galaxy and BH populations for this approach and the fiducial TNG model with no spin evolution. Furthermore, our new approach alleviates a tension in the galaxy morphology -- colour relation of the original TNG model.
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We develop a simple evolutionary scenario for the growth of supermassive black holes (BHs), assuming growth due to accretion only, to learn about the evolution of the BH mass function from $z=3$ to 0 and from it calculate the energy budgets of different modes of feedback. We tune the parameters of the model by matching the derived X-ray luminosity function (XLF) with the observed XLF of active galactic nuclei. We then calculate the amount of comoving kinetic and bolometric feedback as a function of redshift, derive a kinetic luminosity function and estimate the amount of kinetic feedback and $PdV$ work done by classical double Fanaroff-Riley II (FR II) radio sources. We also derive the radio luminosity function for FR IIs from our synthesized population and set constraints on jet duty cycles. Around 1/6 of the jet power from FR II sources goes into $PdV$ work done in the expanding lobes during the time the jet is on. Anti hierarchical growth of BHs is seen in our model due to addition of an amount of mass being accreted on to all BHs independent of the BH mass. The contribution to the total kinetic feedback by active galaxies in a low accretion, kinetically efficient mode is found to be the most significant at $z<1.5$. FR II feedback is found to be a significant mode of feedback above redshifts $zsim 1.5$, which has not been highlighted by previous studies.
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(Abridged) We present a method that tracks the growth of supermassive black holes (BHs) and the feedback from AGN in cosmological simulations. Our model is a substantially modified version of the one by Springel et al. (2005). Because cosmological simulations lack both the resolution and the physics to model the multiphase interstellar medium, they tend to strongly underestimate the Bondi-Hoyle accretion rate. To allow low-mass BHs to grow, it is therefore necessary to increase the predicted Bondi-Hoyle rates in dense gas by large, ad-hoc factors. We explore the physical regimes where the use of such factors is reasonable, and through this introduce a new prescription for gas accretion. Feedback from AGN is modeled by coupling a fraction of the rest-mass energy of the accreted gas thermally into the surrounding medium. We describe the implementation as well as the limitations of the model and motivate all the changes relative to previous work. We investigate the robustness of the predictions for the cosmic star formation history, the redshift zero cosmic BH density, BH scaling relations, and galaxy specific star formation rates. We find that the freedom introduced by the need to increase the predicted accretion rates, the standard procedure in the literature, is the most significant source of uncertainty. Our simulations demonstrate that supermassive BHs are able to regulate their growth by releasing a fixed amount of energy for a given halo mass, independent of the assumed efficiency of AGN feedback, which sets the normalization of the BH scaling relations. Regardless of whether BH seeds are initially placed above or below the BH scaling relations they grow onto the same relations. AGN feedback efficiently suppresses star formation in high-mass galaxies.
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An extraordinary recent development in astrophysics was the discovery of the fossil relationship between central black hole mass and the stellar mass of galactic bulges. The physical process underpinning this relationship has become known as feedback. The Chandra X-ray Observatory was instrumental in realizing the physical basis for feedback, by demonstrating a tight coupling between the energy released by supermassive black holes and the gaseous structures surrounding them. This white paper discusses how a great leap forward in X-ray collecting area and spectral resolution will allow a qualitatively new way of studying how feedback from black holes influenced the growth of structure.
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