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The origin of the Galactic magnetic field

Periodic Reporting for period 1 - ORIGAMI (The origin of the Galactic magnetic field)

Reporting period: 2017-06-01 to 2019-05-31

Magnetic fields can affect the evolution of turbulence, the flux of cosmic rays, the formation of the cold molecular gas, and the evolution of feedback regions; in essence, magnetic fields can affect all the internal processes that redistribute mass, energy, and momentum in a galaxy.

However, the value and the morphology of the interstellar magnetic field are largely unknown, because the observational methods available to measure it are inherently uncertain. Without a theoretical understanding of the history of the Galactic magnetic field and the complex interactions between the phases of the ISM, even the most accurate and complete observational studies remain inconclusive. It is the main objective of ORIGAMI to provide the theoretical foundations for a comprehensive understanding of galactic magnetic fields.

The most widely accepted theory for the growth and the long-term evolution of the galactic magnetic field is the galactic dynamo. In this scenario, primordial, tiny, seed magnetic fields grow inside a proto-galaxy as they are twisted by turbulence and differential rotation at large scales, and then re-ordered by diffusive reconnection at small scales. The primary goal of this project is to investigate the growth of the magnetic field through a galactic dynamo process, in the context of the current Galactic evolution models.

In the course of ORIGAMI, we developed numerical techniques that produced a groundbreaking result: the first direct, multi-physics simulations of galactic dynamos.

ORIGAMI has had a great impact on the community, the career of the ER, and the expertise of the host.
It offered to the community the first global models of Milky-Way-like galaxies with magnetization that directly demonstrate a dynamo.
This was the first discovery of a mean-field dynamo in multi-physics numerical simulations of spiral galaxies.
These models will have a tremendous impact on observational campaigns.
They are realizations of the galactic magnetic field that take into account the dynamical evolution of the galaxy and include the effects of stellar feedback. Therefore, they can be used to simulate the results of magnetic field observations in our galaxy and towards extragalactic sources. The fact that they represent an entire spiral galaxy also allows observers to test the effects of cosmic variance by changing the position of the observer.
ORIGAMI proceeded in two stages: a testing phase and a performance phase after we secured the necessary computational time.
In the first stage, we developed the modules for RAMSES that were necessary in order to perform global galaxy simulations with magnetization. We also tested the feasibility of including small-scale processes such as thermal conductivity and realistic magnetic diffusion. In the second stage, we performed a series of simulations aimed at reproducing a large-scale dynamo, while including the global dynamical evolution of the dark matter and stars. The simulations also included supernova feedback from newly formed stars.

The results from the first stage are published, while those of the second stage are either submitted or about to be submitted to high-impact, peer-reviewed journals.

1. Exploitation of the results:

* The raw data of the simulations will be hosted for at least five years at the servers of the HI (dedicated storage devices were obtained with project funds).
* The source code used for the numerical simulations is public, and the modules developed for the purposes of ORIGAMI are publically available.
* The models we developed are already used to predict the effect of the galactic magnetic field on observations of dust polarization (V.Pelgrims E. Ntormousi, K. Tassis et al., in prep.).

2. Dissemination of the results:

ORIGAMI resulted in the following peer-reviewed publications, all of which acknowledge funding from the European Commission:

* E. Ntormousi, K. Tassis, F. Del Sordo, F. Fragkoudi and R. Pakmor, “A large-scale dynamo amplifies the magnetic field of a Milky-Way-like galaxy”, Nature Astronomy, submitted
* G. Korkidis, V. Pavlidou, K. Tassis, E. Ntormousi, T. N. Tomaras, and K. Kovlakas, “Turnaround radius of galaxy clusters in N-body simulations”, 2019, A&A submitted, arXiv: 1912.08216
* E. Ntormousi, “Magnetic fields in massive spirals: The role of feedback and initial conditions”, 2019, 619L, 5N, arXiv: 1810.08450

The published papers are available for free to any individual who wishes to view them. We commit to ensuring that all forthcoming publications will also comply with our obligation to provide open access products.

The ER also presented the results of the project in international conferences and as an invited speaker at various institutes. She participated in three major outreach events, two of which were specifically targetted to increasing the participation of women in STEM.
Many numerical works so far have studied the evolution of galactic dynamos under different conditions. However, the models developed so far either represented only a section of a galaxy or included the effects of small-scale turbulence as a parameter.
ORIGAMI produced the first-ever detection of a mean-field dynamo in direct simulations of galaxy evolution that included both the large-scale effects of dark matter and stars and the small-scale effects of turbulence from supernova explosions (Ntormousi et al. 2020, submitted to Nature Astronomy). This first direct demonstration of a mean-field dynamo acting in entirely self-consistent models is an important precedent for the study of galactic magnetism.
ORIGAMI also produced the first discovery of the effect of magnetic fields on the dynamical structure of the galaxy (Ntormousi et al. 2020, in prep.)

In the immediate future, we expect the results of the peer review of our two submitted papers, and we are preparing four more papers for publication. Despite the formal end of ORIGAMI, our research on galactic magnetic fields is ongoing. The PRACE HPC project relevant to ORIGAMI and led by the PI expires in April, and many more models are currently under production. The resulting papers will acknowledge funding from the EC.

The transfer of knowledge from the ER to the undergraduate and graduate students of the HI was very successful and has far-reaching implications. The ER is an expert in MHD numerical simulations and she transferred this expertise to the students she supervised. She worked with a Master student on the post-processing of large-scale simulations (Korkidis et al., 2020, submitted). This student is now continuing this research as a doctorate candidate at the HI. The two undergraduate students who worked with the ER are both in the process of submitting their results to peer-reviewed journals and plan to continue their research using numerical simulations.
The career of the ER advanced through the recognition of her work on ORIGAMI. She was invited to apply for her current job as a researcher at the prestigious Scuola Normale Superiore thanks to the visibility of the first published work with the project (Ntormousi 2018).
The mentoring and outreach work performed by the ER as part of this project has made her into a strong role model for future female scientists. In particular, she gave two public talks aimed at encouraging girls and young women to pursue careers in STEM. She also became part of the Supernova Foundation, an international mentoring program for women in science.