The magnetic cosmic web

Challenging simulations
Starting from two large allocations of computing time at the CSCS supercomputing centre in Lugano, we produced the largest cosmological simulations of extragalactic magnetic fields to-date, using the ENZO code. 
Now we will use them to make real discoveries!

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What is the origin of extragalactic magnetic fields?

The origin of the magnetic fields observed in galaxy clusters is unknown. 
Given the existing data, they may  be equally well explained assuming a “primordial" scenario (i.e.  the fields follow the  small-scale dynamo amplification of primordial weak fields already in place at the CMB) or an “astrophysical” scenario (i.e. the fields in clusters have been injected by  winds/jets from AGN at lower redshift). 
With our MHD  cosmological simulations we can allow future radio surveys   to probe cluster outskirts and cosmic filaments, where old traces of magnetogenesis should still still be present. 

"primordial scenario"

"astrophysical scenario"

Related works​   
Public repository of data

main contributors: F. Vazza C. Gheller M. Brüggen , P. Wang

Can we detect the magnetic cosmic web?

We know the signal from the magnetic cosmic web must be weak, because it eluded detection so far. However, the incoming generation of radio telescopes (e.g. LOFAR , MWA , ASKAP , MEERKAT ...and   SKA  on top of them all) should be able to detect at least the tip of the iceberg of it. With the assistance of big simulations (combining the complexity of physics and of radio observing procedures) we will enable detecting such weak signal, either suggesting the best places where to hunt for it, or the best statistical procedures (e.g. by stacking analysis).
Related works​   
  • "Filaments of the radio cosmic web: opportunities and challenges for SKA" ( Vazza et al. 2015 PoS )
  •  "Forecasts for the detection of the magnetised cosmic web from cosmological simulations" ( Vazza et al. 2015 A&A )
  • "Unravelling the origin of large-scale magnetic fields in galaxy clusters" ( Bonafede et al. 2015 PoS )
    and beyond through Faraday Rotation Measures with the SKA
  • "Detecting the cosmic web with radio surveys" ( Vazza et al. 2015 PoS )
Public repository of data

main contributors: F. Vazza , C. Ferrari  , C. Gheller M. Brüggen A. Bonafede

Magnetic fields in galaxy clusters & particle acceleration

The intracluster medium is highly turbulent and the evolution of magnetic fields within it is complex.
Understanding how cluster dynamics forces magnetic fields to evolve is crucial to understand the non-thermal properties of galaxy clusters.
Moreover, the relation between the direction of magnetic fields and shock waves can in turn affect the way in which cosmic rays are accelerated and may become visible (through their radio emission in case of electrons or through gamma emission in case of protons). We are using our MHD simulations to link for the first time the acceleration of cosmic rays to the obliquity of shocks, and predict the observational consequences of this. 
Related works​   
  • "Electron and proton acceleration efficiency by merger shocks in galaxy clusters" ( Vazza et al. 2015 MNRAS )
  • "Unravelling the origin of large-scale magnetic fields in galaxy clusters" ( Bonafede et al. 2015 PoS )
    and beyond through Faraday Rotation Measures with the SKA
  • "Non-thermal emission from galaxy clusters: feasibility study with SKA" ( Ferrari et al. 2015 PoS )
  • "Radio haloes in Sunyaev-Zel'dovich-selected clusters of galaxies: the making of a halo?" ( Bonafede et al. 2015 MNRAS )
  • "Constraining the efficiency of cosmic ray acceleration by cluster shocks ( Vazza et al. 2016 MNRAS )
  • "A shock front at the radio relic of Abell 2744" ( Eckert et al. 2016 MNRAS )
  • "The impact of the SZ effect on cm-wavelength (1-30 GHz) observations of galaxy cluster radio relics" ( Basu et al. 2016 MNRAS )
  • "Testing cosmic-ray acceleration with radio relics: a high-resolution study using MHD and tracers" ( Wittor, Vazza & Bruggen 2016 MNRAS )
  • "Sardinia Radio Telescope observations of Abell 194 - the intra-cluster magnetic field power spectrum" ( Govoni et al. 2017 A&A)
  • "Observations of the galaxy cluster CIZA J2242.8+5301 with the Sardinia Radio Telescope" ( Loi et al. 2017 A&A)
  • "On the Connection between Turbulent Motions and Particle Acceleration in Galaxy Clusters" (Eckert et al. 2017 ApJ)

main contributors: F. Vazza , M. Brüggen , D. Wittor , A. Bonafede , D. Eckert , C.Ferrari, K. Basu

Propagation of Ultra-high energy cosmic rays

An unsolved mystery in astrophysics is the origin of ultra-high energy cosmic rays, which are charged particles that travel at the speed of light and can deliver an enormous energy (up to ~1e21 electron volt) when impact on the outer shell of the Earth’atmosphere.  These particles should originate from outside the Milky Way, and their direction of propagation can be deflected by magnetic fields  outside our galaxy. Only if the magnetic fields outside the Milky Way are below a certain level, the trajectories of ultra-high energy cosmic rays remain rectilinear enough to point towards their exact sources.  But predicting the exact strength and topology of these magnetic fields is non trivial, as it requires advanced numerical simulations. 

We are using our new simulations in order to predict the deflection of UHECRs by
possible extragalactic magnetic fields, combined with the CRPropa code.
Related works​   
  • "Propagation of Ultra High Energy Cosmic Rays in Extragalactic Magnetic Fields: A view from cosmological simulations ​" ( Hackstein et al. 2016 MNRAS )
  • Bachelor Thesis by Stefan Hackstein at Hamburg University (2015)

main contributors: S. Hackstein,  F. Vazza , M. Brüggen , G. Sigl , A. Dundovic

The oscillation of Axionlike particles


An important candidate for dark matter is represented by " Axion-like particles " (ALPs). On of their most interesting properties is that they couple with the magnetic fields, and can oscillate into high-energy photons (and back). This would explain the lack of IR absorption of the spectra of TeV sources at high redshift.
With our simulations we can explore the space of parameters allowed by current model of ALPs, and see if the observed spectra can be explained by ALPs oscillations in presence of realistic magnetic fields on cosmic scales.

Related works​   
  • "Enhancing the spectral hardening of cosmic TeV photons by the mixing with axion-like particles in the magnetised cosmic web" , Montanino,Vazza,Mirizzi & Viel, 2017 PRL

Public repository of data

main contributors: ​ D. Montanino ,   F. Vazza , M. Viel A. Mirizzi