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Black holes and their host galaxies: coevolution across cosmic time

Periodic Reporting for period 3 - COEVOLUTION (Black holes and their host galaxies: coevolution across cosmic time)

Reporting period: 2018-09-01 to 2020-02-29

Galaxy formation is one of the most fascinating yet challenging fields of astrophysics. The desire to understand galaxy formation has led to the design of ever more sophisticated telescopes which show a bewildering variety of galaxies in the Universe. However, the degree to which an interpretation of this wealth of data can succeed depends critically on having accurate and realistic theoretical models of galaxy formation. While cosmological simulations of galaxy formation provide the most powerful technique for calculating the non-linear evolution of cosmic structures, the enormous dynamic range and poorly understood baryonic physics are main uncertainties of present simulations. This impacts on their predictive power and is the major obstacle to our understanding of observational data. The objective of this proposal is to drastically improve upon the current state-of-the-art by i) including more realistic physical processes, such as those occurring at the sphere of influence of a galaxy’s central black hole and ii) greatly extending spatial dynamical range with the aid of a novel technique I have developed. With this technique I want to address one of the major unsolved issues of galaxy formation: “How do galaxies and their central black holes coevolve?” Specifically, I want to focus on three crucial areas of galaxy formation: a) How and where the very first black holes form, what are their observational signatures, and when is the coevolution with host galaxies established? b) Is black hole heating solely responsible for the morphological transformation and quenching of massive galaxies, or are other processes important as well? c) What is the impact of supermassive black holes on galaxy clusters and can we calibrate baryonic physics in clusters to use them as high precision cosmological probes?
Overall the progress of the ERC project has been excellent. Two postdocs have been hired promptly at the start of the grant (Colin DeGraf and Martin Bourne) and a third one has been hired a year later (Davide Fiacconi). The COEVOLUTION team is well balanced and has the necessary expertise in all areas of the ERC grant. Postdocs are leading different research areas of the grant, but also closely collaborating as anticipated. The team also collaborates with a number of PhD students (10), master students (2) and a number of external collaborators (all supported by other sources). The team produced 26 publications so far in high impact journals and was very active in disseminating scientific results, attending or organizing over 50 international conferences supported by the ERC grant to present the results. The team was heavily involved in organizing a local scientific workshop (funding provided by other sources) which brought international experts to the team's host institution as well as co-organizing a Special Session at EWASS in 2018. Also the team has been very successful in winning high performance computing grants on UK national facilities as well as on European PRACE HPC facilities. The team was heavily engaged with public outreach as well, reaching over 1500 people each year during the Cambridge Science Festival.

Specifically in terms of the key scientific achievements,Colin DeGraf has mainly worked on Area A of the ERC grant. Recently, he has developed novel black hole seed models and demonstrated their impact in state-of-the-art simulations. This is very relevant for the cosmic black hole merger rates that future gravitational wave detectors will be able to probe (IPTA, LISA), and this work allowed us to join the LISA consortium. The results of this work are completed now and will be summarized in a publication expected shortly. Colin DeGraf has also worked on Area B of the ERC grant, exploring the co-evolution of black holes and galaxies. This resulted in two published papers. In DeGraf & Sijacki, 2017, MNRAS, 466, 3331 we have explored black hole clustering taking advantage of state-of-the-art Illustris simulation and found that the observationally inferred clustering-predicted host halo masses are significantly overestimated leading to the systematic bias. In this work we have provided accurate calibration to avoid this bias which will be a useful tool for future observational campaigns.

Martin Bourne has mainly worked on Area C of the ERC grant. He has developed an entirely novel method of black hole heating via jets based on the work by Curtis & Sijacki, 2015, MNRAS, 454, 3445. Given the timeliness he has applied this method to compare against the Hitomi (former Astro-H) mission and to constrain the properties of galaxy clusters, heating and turbulence. This work has been published in Bourne & Sijacki, 2017, MNRAS, 472, 4707 and Martin has started two follow up projects which exploit this method which are both now in a mature state and we expect to publish them in a few months. Martin Bourne has done further work related to Area B of the ERC grant exploring the interplay between star formation in galaxies and AGN feedback. This resulted in 4 published papers where specifically emphasis has been placed on the nature of AGN outflows. This together with our efforts with external collaborators (Costa et al. 2018, MNRAS, 473, 4197) has put us in the leading position of theoretically interpreting observational data and currently there is an ongoing fruitful collaboration there.

Davide Fiacconi has developed a novel method to incorporate the effect of black hole spin and its imprint on black hole accretion and feedback. This model is needed for diverse parts of Area A, B and C. The model is fully functional now and a paper has been published (Fiacconi, Sijacki, Pringle, 2018, MNRAS, 477, 3807). Note that this model is especially timely given the recent detection of gravitational waves, as it will also permit to simulate merging black holes with a spin. In fact, Davide is completing a paper on this now where we will be able to place testable predictions for the LISA space mission. During this project a master student was involved whom Davide helped supervising. Moreover, Davide has worked on a new model to incorporate the evolution of black hole spins and jets self-consistently and the theoretical basis of this model have been fully developed. Davide also worked on the evolution of galaxies in the early Universe and on the nature of X-ray sources pertinent to Area B of the ERC grant which resulted in 2 published papers.

Debora Sijacki together with her PhD student Mike Curtis has worked extensively on the development of super-Lagrangian refinement technique around supermassive black holes (Curtis & Sijacki, 2015, MNRAS, 454, 3445, Curtis & Sijacki, 2016, MNRAS, 457L, 34, Curtis & Sijacki, 2016, MNRAS, 463, 63) which is now completed and which constitutes a back bone to many of the projects performed or in development of this ERC grant. Together with the PhD student Matthew Smith, Debora Sijacki developed a novel model for star formation and supernovae feedback (Smith, Sijacki & Shen, 2018, MNRAS, 478, 302) which will also be a very useful tool for a number of forthcoming projects in Area A and B. Matthew Smith has tested this model on extremely high resolution cosmological simulations of dwarf galaxies which are reported in a recent paper submitted to MNRAS (arXiv:1807.04288) which helped to clarify the role of supernovae in small mass galaxies. Matthew Smith has further worked on improving this model and incorporating it with different black hole feedback schemes to tackle problems listed in Area B and C of the grant. Together with external collaborators Zephyr Penoyre, Benjamin Moster and Tiago Costa, Debora Sijacki further worked on the Area B of the ERC grant (Penoyre et al., 2017, MNRAS, 468, 3883, Costa et al., 2018, MNRAS, 473, 4197).

Shaoran Hu worked together with Debora Sijacki to understand the nature of spiral structures of disc galaxies which is pertinent to the Area B of the grant. In particular, Shaoran has in a very careful study examined the role of cosmological satellities in driving disk perturbations such as spirals and warps. This study consists of highest resolution simulations performed to date and allowed us to conclude that only a handful of encounters with massive subhaloes are sufficient for regenerating and sustaining spiral structures in discs over their entire lifetime.
The COEVOLUTION team has developed completely new numerical techniques to study the evolution of galaxies and their central black holes which are at the forefront of the field. The team has already proven in a number of publications that these techniques are not only innovative and more accurate than previous methods, but that they change the physical understanding of galaxy formation in several important ways. The team is currently applying these techniques on a broad range of topics, which will permit to shed new light on the formation of the very first galaxies and black holes, on their successive evolution through the majority of their lifetime and on the on their ultimate faith in the largest virialized objects, galaxy clusters. This has already produced a significant impact in the scientific community mirrored in a large number of conference talks that our team members have given. The team has additionally produced a more broader socio-economic impact by actively participating in the University of Cambridge Science Festival for the past 3 consecutive years. More than 1200 visitors (each year) came to our institute where the team members have engaged with the public (including many children) explaining in layman terms the science supported by this ERC grant. This has been accomplished through posters, video displays, interactive computer simulations as well as hands on experiments for all ages.