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)