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On the Origin and Growth of Cosmic Magnetic Fields
Dr. Alexander M. Beck(University Observatory Munich)
Ljubljana, October 2016
Collaborators: H. Lesch, K. Dolag, P. Kronberg & S. Borgani
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Dunja Fabjan
Long-term simulation challenges
The larger our ignorance -- the stronger the magnetic field. L. Woltjier, 1967
Andrew Fletcher & Rainer Beck (MPIfR Bonn)
Magnetic fields in galaxies
M51: R. Beck & MPIfR Bonn
M31: R. Beck & MPIfR Bonn (6cm) NGC 4569: K. Chyzy (6cm)
NGC 4631: M. Krause(3.6 cm)
NGC 6946: R. Beck & MPIfR Bonn6cm + H-alpha
High-redshift rotation measures
Beck & Dolag 2016
Formation of cosmic structures
Credit: ESA
Magnetic fields theory
Plasma motions are the only source of field amplification Topological changes due to
magnetic diffusionMagnetic seeding term
This equation has to be solved in the context of a galaxy
Galaxy formation ???? Magnetic fields1.) Do the first structures form within magnetized or unmagnetized gas?2.) Can a generation of high-redshift protogalaxies magnetise the IGM?
Proto Galactic magnetic fields
⇒ Galactic magnetic fields are a result of the galaxy formation process !
Magnetic fields of stellar origin
Extend induction equation
Seeding term
Supernova events
Supernova mass
Injected magnetic amplitude
α, β from IMF (Salpeter 1955)
SN-Radius: 5pc – Bubble-Radius: 25pc – Injection-Radius: 1 kpcAmplitude: 10^{-4} G – SN-Events: 10^{6} - Seeding rate of: 10^{-9} G/Gyr
Beck et al., 2013
MHD simulation set-up
Induction equation:
Beck et al., 2013
Cosmological initial conditions
Re-Simulated a MW-like dark matter haloVelocity curve peaks at 220 km/s and virial radius is 270 kpcInitial conditions introduced by Stoehr et al. 2002 / 2003 / 2006
WMAP satellite (CMB)
Millenium simulation
High- & Low-res regions
Expanding universe
Fluctuations
MW halo gas density
Beck et al., 2013
MW halo magnetic field
Beck et al., 2013
MW halo Faraday rotation measure
Patchy at high redshift
Beck et al., 2013
Comparison with observations
Rotation measure time evolution
Rotation measure distribution
What causes the observed RM’s?What causes the simulated RM’s?
Beck et al., 2013Beck & Dolag, 2016
Offtopic: A run with primordial seed fieldsD
ensi
tyM
agne
tic fi
eld
Beck et al., 2012
The growth of energy densities (primordial run)
Dip
Amplification
Decay
Equipartition
Beck et al., 2012
The supernova contribution
Seeding only is not sufficient in yielding μG amplitude→ The distribution and amplitude of the seed field depend on the strength and distribution of star formation→ The supernova seed field can be constructed stand-alone or in addition to possible primordial fields
Beck et al., 2013
Field growth in simulations of minor mergers
Field growth and saturation in galaxy minor mergers Geng, Beck et al., 2012
Analytical toy model
Cosmological turbulent small-scale dynamo
Ansatz
Evolution equation
Truncation
Growth rate
Kulsrud & Anderson, 1992Malyshkin & Kulsrud, 2002Beck et al. 2012
Belyanin, Sokoloff & Shukurov, 1993
Kulsrud et al., 1997 (Kolmogorov turbulence)
Beck et al., 2012
Agreement of toy model and simulations
Length scale: 25 kpcVelocity scale: 75 km/s
Excellent agreement !!Beck et al., 2012
Growth curves of analytical toy model
Beck et al., 2012
Small-scale dynamo
Inertial forces >> Viscous forces ---- Very complex physical processes - Turbulent and instable hydrodynamical flow with Re >> Recrit - Study small magnetic perturbations - Magnetic field following the velocity field (weak-field / frozen in)
Van Dyke et al., 1982
Transfer of magnetic energy
1.) Amplification of field via small-scale dynamo2.) Saturation of dynamo at energy equipartition Schekochihin et al., 2002
Summary● Magnetic fields are rarely followed in state-of-art
cosmological simulations of galaxies and beyond
● MF add to the complexity of physical (subgrid) prescriptions and are difficult in implement
● MF in collapsed cosmic objects are a result of the very basic process of structure/galaxy/star formation
● We have limited knowledge about the impact of MF on the formation of structures
● Non-thermal components (such as comic rays protons and electrons) need a good MF description
Non-thermal cosmological simulations: a promising and growing research field
Dr. Alexander M. Beck(University Observatory Munich)