Transcript Magnetic field excitations in galaxies

September
2005
Magnetic field excitation
in galaxies
Outline
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The problem of large scale fields
Large scale flows
Meanfield dynamos (alpha effect due to …)
The role of cosmic rays
MRI
Observations
Large scale magnetic fields
Strength is compatible to the
small scale component
Axisymmetric mode dominates
Seldomly reversals
Large pitch angle
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Strongly (anti) correlated with
optical arms
The problem
A lot of processes inject small scale fields in the ISM
 stellar dynamos combined with winds
 shock-turbulence
 plasma instabilities
 small scale dynamos
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No problem to explain the magnetic
field energy in galaxies
But which processes bring coherent
magnetic fields on a scale of several
kpc?
Balsara et al. 2004
How to do ?
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large scale flows amplifying a primordial field
inverse turbulent cascade
formation of stronger fields during galaxy mergers
and a more rapid decay of small scale structures
Large scale flow
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Rotation without diffusivity (ideal MHD) (50 rotations)
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In a Hubble time amplification by a factor of 10
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Strong seed field is neccassary (10-7G) on large
scale
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Pitch angle would be less than 1o
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A lot of radial reversals
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Adding turbulent diffusion one needs a source for Br
Large scale flow
Br from nonuniform radial motions in spirals
 reversals or positive pitch angles
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Otmianowska-Mazur &
Chiba 1995
Inverse cascade
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inverse cascade in the turbulence
concept of the turbulent dynamo
still no simulations for a global galaxy possible
Questions about early saturation (catastrophic
quenching)
turbulence in the multiphase ISM
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Mean field language: Alpha-effect
Dynamo Theory
B
  curl E
t
 curl u  B    B  

div B = 0
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BC: curl B = 0 outside
T
curl B

Parker ´55
Steenbeck, Krause &
Rädler ´66
Dynamo Theory
 Differential rotation max = 50 Gy-1
 turbulence defined by u´ and 
 u´= 10 km s-1 and  = 0.01 Gy
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 = 1 kpc2 Gy-1 and  = 10 km s-1
 induction equation with -quenching
stationary axisymmetric quadrupolar field
 field strength several G
 pitch angle about 20o
 maximal field strength at maximal 
Mean field dynamo
Dynamo numbers:
C= H -1 C=0 H2 -1
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Pitch angle:
But with
quenching
tan p =  C / C
Independent of 
tan p =  Ccrit / C = 0.4 /0
with
0  1 km kpc s-1
p=20o
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Turbulence from SN
Single supernova explosions
Ferrière, 1998
Pitch angle : 0.1o - 1o
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Turbulence from SN
Super Bubbles
Ferrière, 1998
Pitch angle : 1o - 30o
Mean flow
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Rotation law:
=0 (1+(r/r
r=2
-1/n
n
)
(n=2)
r=5
Density wave flows
Model: Brandt law of rotation, r3 and C=41, C=4.7
ur=cu0cos(mp(f  pt) + K r/Rmax) f(r)
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uf=u0sin(mp(f  pt) + K r/Rmax) f(r)
Density wave flows
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2D stationary density wave flows disturb the dynamo
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Density wave flows
Pitch angle is maximal at field minimum
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MHD instabilities
Supernova explosions provide
enough energy for the turbulence in the ISM
Magnetic field instabilities contribute to the turbulence
Parker instability together with cosmic rays
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MRI in galaxies
Observational hint: Turbulence in low star forming regions
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Parker instability
Parker instability
largescale B
Rotation
Hanasz et al. 2002
coalesence
largescale Br
Cosmic rays and Parker instability
SN-explosions with cosmic rays
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Fast diffusion of cosmic rays
+ Parker instability
Fast expansion of pressure disturbances
More effective creation of Br
Hanasz et al. 2004
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Cosmic rays and Parker instability
Magneto rotational instability (MRI)
Weak magnetic field
Va <  H
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is unstable
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
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MRI
Dziourkevitch 2004
Open problems
Turbulence in the ISM – simulations in the box
Simulation with Superbubbles
Multicomponent gas - role of hot phase
Role of magnetic instabilities
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External temporal gravitational disturbances
Open problems
Up to which scale are magnetic fields dynamical important
What is the influence of a more complicated mean flow
in a high Reynolds number regime
How important is the galaxy evolution for magnetic field properties
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What determines the correlation between optical arms
and magnetic arms
Questions to Observers
Detailed observations of smaller scales
3D field topology in SNR
Correlation to rotation curves and spiral flows
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Can we see coherent structures on the large scale
during galaxy formation process
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Thank you