(university observatory munich) dr. alexander m. beck...
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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
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Long-term simulation challenges
The larger our ignorance -- the stronger the magnetic field. L. Woltjier, 1967
Andrew Fletcher & Rainer Beck (MPIfR Bonn)
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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
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High-redshift rotation measures
Beck & Dolag 2016
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Formation of cosmic structures
Credit: ESA
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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?
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Proto Galactic magnetic fields
⇒ Galactic magnetic fields are a result of the galaxy formation process !
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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
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MHD simulation set-up
Induction equation:
Beck et al., 2013
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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
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MW halo gas density
Beck et al., 2013
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MW halo magnetic field
Beck et al., 2013
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MW halo Faraday rotation measure
Patchy at high redshift
Beck et al., 2013
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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
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Offtopic: A run with primordial seed fieldsD
ensi
tyM
agne
tic fi
eld
Beck et al., 2012
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The growth of energy densities (primordial run)
Dip
Amplification
Decay
Equipartition
Beck et al., 2012
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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
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Field growth in simulations of minor mergers
Field growth and saturation in galaxy minor mergers Geng, Beck et al., 2012
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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
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Agreement of toy model and simulations
Length scale: 25 kpcVelocity scale: 75 km/s
Excellent agreement !!Beck et al., 2012
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Growth curves of analytical toy model
Beck et al., 2012
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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
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Transfer of magnetic energy
1.) Amplification of field via small-scale dynamo2.) Saturation of dynamo at energy equipartition Schekochihin et al., 2002
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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)