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ERC

CRAGSMAN Report Summary

Project ID: 646955
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - CRAGSMAN (The Impact of Cosmic Rays on Galaxy and Cluster Formation)

Reporting period: 2016-04-01 to 2017-03-31

Summary of the context and overall objectives of the project

Understanding the physics of galaxy formation is arguably among the greatest problems in modern astrophysics. Recent cosmological simulations have demonstrated that "feedback" by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies, to slow down star formation to the small observed rates, to move gas and metals out of galaxies into the intergalactic medium, and to balance radiative cooling of the low-entropy gas at the centers of galaxy clusters. This progress still has the caveat that "feedback" was modeled empirically and involved tuning to observed global relations, substantially weakening the predictive power of hydrodynamic simulations. More problematic, these simulations neglected cosmic rays and magnetic fields, which provide a comparable pressure support in comparison to turbulence in our Galaxy, and are known to couple dynamically and thermally to the gas. Building on our previous successes in investigating these high-energy processes, we propose a comprehensive research program for studying the impact of cosmic rays on the formation of galaxies and clusters. To this end, we will study cosmic-ray propagation in magneto-hydrodynamic turbulence and improve the modeling of the plasma physics. This will enable us to perform the first consistent magneto-hydrodynamical and cosmic-ray simulations in a cosmological framework, something that has just now become technically feasible. Through the use of an advanced numerical technique that employs a moving mesh for calculating hydrodynamics, we will achieve an unprecedented combination of accuracy, resolution and physical completeness. We complement our theoretical efforts with a focused observational program on the non-thermal emission of galaxies and clusters, taking advantage of new capabilities at radio to gamma-ray wavelengths and neutrinos. This promises important and potentially transformative changes of our understanding of galaxy formation.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

* Preparational phase of the project (1 year before the start) until a few months into the project: initial code development of modelling cosmic rays in the cosmological code Arepo in collaboration with Drs. Pakmor and Simpson in the group of Prof. Springel at HITS. The initial four papers (Pakmor et al. 2016a,b, Simpson et al. 2016, Pfrommer et al. 2017a) have been published within the first months of the projects, thereby delivering the first key milstone “CR proton implementation” (sub-project 2) as identified in the Gantt chart of Sect 2.c of the Description of Action. Using this code, I was self-consistently simulating the growth of magnetic fields (sub-project 3) and the emergence and propagation of cosmic rays in forming galaxies. This enabled me to calculate for the first time non-thermal signatures of live galaxy models at radio and gamma-ray energies (sub-project 5). I am currently writing up the results in two Letters (Pfrommer et al 2017b,c).

* Matteo Pais used this implementation to successfully validate the code with cosmic-ray simulations of the process of diffusive shock acceleration at supernova remnants (sub-project 1) and the resulting gamma-ray morphologies. He is currently writing up the results in a series of three papers (Pais et al. 2017a,b,c) before he will move on to study the origin of galactic magnetism (sub-project 3) in the second part of his thesis. This work follows up on an exploratory study of magnetic dynamos in high-resolution cosmological zoomed simulations of forming Milky Way-like galaxies (Pakmor et al. 2017).

* Philipp Girichidis was actively working on 1.) cosmic ray transport in the interstellar medium and studying the driving of galactic winds (sub-project 2) and 2.) on an implementation of the spectral description of cosmic rays (sub-project 5) which will lead to a number of publications (Girichidis et al. 2017a,b,c).

* Kristian Ehlert in collaboration with Rainer Weinberger (PhD student of Prof. Springel, HITS) has implemented an improved model for relativistic jets from active galactic nuclei (AGNs) at the centers of galaxy clusters (sub-project 4) and study magnetic amplification and cosmic-ray transport. The first paper on this has been submitted and is in the referee process (Weinberger et al. 2017), the second is in an advanced state (Ehlert et al. 2017). A large comprehensive study (published in two major articles) of the physics of cosmic ray heating in cool core clusters demonstrated the viability of the proposed idea and the existence of a self-regulation cycle (Jacob & Pfrommer 2017a, b).

* Georg Winner is complementing the work on cosmic ray protons and develops and implements an algorithm that is able to follow the spectral distribution of cosmic-ray electrons in galaxies and clusters (leading to a first publication by the end of this year, Winner et al. 2017), which can be observed at radio, X-ray and gamma-ray energies (sub-project 5).

* In addition to work done directly within the group, we have successfully studied aspects of the project with external collaborators:
1. The first exploratory study of sub-project 2 that compares the (thermo-)dynamic impact of cosmic ray streaming and diffusion in simulations of galaxies have been published in Wiener, Pfrommer, Oh (2017).

2. A serendipitous discovery of a novel physical process that facilitates the heating of cooling centers of galaxy clusters (sub-project 4) was achieved in a joint project with our collaboration partners at the MIT, Dr. Kannan and Prof. Vogelsberger. Using cosmological simulations of zoomed high-resolution galaxy clusters, Kannan et al. (2017) demonstrated how AGN feedback can be assisted by anisotropic thermal conduction.

3. As regards sub-project 5 (Non-thermal signatures of galaxies and clusters) we accomplished a major step forward in understanding turbulence and particle acceleration in giant radio halos which was published in a recent paper (Pinzke, Oh, Pfrommer 2017).

4. Hydrody

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project has delivered an extremely innovative numerical algorithm and implementation: the world's first implementation of cosmic ray physics on an unstructured mesh, which is able to follow advective and anisotropic diffusive transport along the magnetic field. Moreover, we have indications that the addition of cosmic rays changes the growth of cosmic magnetism - which is highly interesting and will be followed up within the ERC group in the next years.

The original hypothesis that cosmic rays can actively modify the morphologies of galaxies and drive powerful outflows has been substantiated in a number of early results from the ERC group. At the scales of galaxy clusters, we found further confirmation that cosmic-ray heating can balance the fast radiative cooling at the centers of cooling galaxy clusters, thus mitigate star formation. This cosmic ray feedback could be one of the key processes that regulates and governs galaxy formation. We will follow up these encouraging early results with cosmological simulations in the years to come.

At the same time, this novel implementation enabled us for the first time to simulate non-thermal signatures of live galaxy models at radio and gamma-ray energies, thus discovering the cause of two fundamental relations of galaxies, the far-infrared--gamma-ray and far-infrared--radio correlations.
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