“magnetic fields in young galaxies” · 2013. 3. 14. · jennifer schober zentrum für...

Jennifer Schober Zentrum für Astronomie Heidelberg

“Magnetic Fieldsin Young Galaxies”

Collaborators:

Ralf Klessen, Simon Glover (Heidelberg, Germany)Dominik Schleicher, Stefano Bovino (Göttingen, Germany)

Christoph Federrath (Melbourne, Australia)Robi Banerjee (Hamburg, Germany)

MFU IV, Playa del Carmen 2013

“ B-Fields in Young

Galaxies”

1. Introduction

2. Small-Scale Dynamo - Theory

3. Magnetic Fields in Primordial Halos

4. Conclusion

1. Introduction

J. Schober(ZAH Heidelberg)

1. Introduction



4. Conclusion

The First Stars and Galaxies

[http://www.wpclipart.com/space/stars_universe/universe_timeline_light.jpg.html]


Galaxies”


1. Introduction



4. Conclusion

● turbulence driven by accretion and SN explosions

● weak magnetic seed fields

- phase transitions in the early Universe: [QCD phase transition, 10 Mpc comoving scale,e.g. Sigl et al. 1997]

- battery processes: [Biermann battery, kpc scale, e.g. Xu et al. 2008]

The First Stars and Galaxies

Fig.: Mach number in Primordial Halo[Greif et al. 2008]

=> dynamo action is possible


Galaxies”


2. The Small-Scale Dynamo


Galaxies”

1. Introduction



4. Conclusion


1. Introduction



4. Conclusion

Small-Scale Dynamo

● definition: “small-scale dynamo”

Mechanism that converts turbulent kinetic energy into magnetic energy.

● depends strongly on environment:

- magnetic Prandtl number

- type of turbulence (in inertial range)

Kolmogorov Burgers (incompressible) (highly compressible)


Galaxies”


Stretch-Twist-Fold Dynamo

1. Introduction



4. Conclusion

→ fastest on length scale with shortest eddy time scale = viscous scale (smallest scale of inertial range)


Galaxies”


Kinematic Phase

1. Introduction



4. Conclusion


Galaxies”

● evolution of the spectrum (schematically):



● magnetic field growth:

(from Kazantsev theory)

● growth rate on :

[see also: Bovino et al. 2013]

Kinematic Phase

1. Introduction



4. Conclusion

[Schober et al. 2012a]

[Schober et al. 2012c]


Galaxies”


Non-Linear Phase


1. Introduction



4. Conclusion


Galaxies”


Non-Linear Phase


● Fokker-Planck model (see e.g. Schekochihin 2002) with assumption that peak scale shifts on eddy time scale

● no more dependence on

● magnetic field growth:

[Schleicher et al. 2013]

1. Introduction



4. Conclusion


Galaxies”


3. Magnetic Fieldsin Young Galaxies


Galaxies”

1. Introduction



4. Conclusion


● spherical core

● radius:

● density:

● temperature:

→ gas is ionised

● sources of turbulence:

- accretion

- SNe (not in this talk)

worst case assumption: compressive turbulence ( )

1. Introduction



4. Conclusion

Simple Model for Primordial Halo

Fig.: Density in the center of a protogalaxy. [Latif et al. 2012]


Galaxies”


● viscosity:

● resistivity:

● sound speed:

● turbulent velocity:

Mach number percentage that goes into turbulence

(motivated by simulation of protogalaxies by Latif et al. 2012)

● Reynolds numbers:

(hydrodynamic)

(magnetic)

1. Introduction



4. Conclusion

Simple Model for Primordial Halo“ B-Fields in Young

Galaxies”


Kinematic Phase

● kinematic growth of magnetic energy (on viscous scale):

● maximum field strength:

from equipartition efficiency factor for with kinetic energy Mach 2 and compressive forcing (Federrath et al. 2011) scaling velocity down to viscous scale

1. Introduction



4. Conclusion


Galaxies”



Kinematic Phase

● kinematic growth of magnetic energy (on viscous scale):

● maximum field strength:

1. Introduction



4. Conclusion


Galaxies”

Non-Linear Phase

● non-linear growth of magnetic energy :

● maximum field strength on forcing scale:

from equipartition efficiency factor for with kinetic energy Mach 2 and compressive

forcing (Federrath et al. 2011)

1. Introduction



4. Conclusion


Galaxies”


Non-Linear Phase

● non-linear growth of magnetic energy :

● maximum field strength on forcing scale:

1. Introduction



4. Conclusion


Galaxies”


4. Conclusions


Galaxies”

1. Introduction



4. Conclusion


1. Introduction



4. Conclusion

● dynamo amplification in kinematic phase:

● dynamo amplification in non-linear phase:

● small-scale dynamo in formation of first stars & galaxies:

Dynamical important magnetic fields can be generated on small time scales compared to free-fall time.

=> magnetisation of the primordial ISM

Conclusion“ B-Fields in Young

Galaxies”


Thanks for your attention!

Further questions/comments?→ [email protected]

“magnetic fields in young galaxies” · 2013. 3. 14. · jennifer schober zentrum für...

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