Krakow 2010 Galactic magnetic fields: MRI or SN-driven dynamo? Detlef Elstner Oliver Gressel Natali...

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Galactic magnetic fields:

MRI or SN-driven dynamo?

Detlef Elstner

Oliver Gressel

Natali Dziourkevich

Alfio Bonanno

Günther Rüdiger

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Magnetic field amplificationDynamos by SN-driven turbulence are numerically feasible

Box models: cosmic ray injection (Hanasz et al. 2004) thermal energy input (Gressel et al. 2008)

Global models: mean field models ( ………. since 1960 ) artificial velocities (Gissinger et al. 2008) cosmic ray injection by SN (Hanasz et al.2009)

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SN driven dynamo• SN energy is injected by thermal energy explosions • State of the art simulations of the ISM with heating, cooling• Wind or fountain flow develops (essential for helicity transport)• Clustering is essential

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SN-driven dynamoHow it works: downward pumping equalupward fountain flow

pumping acts only onthe mean field

fountain flow transports all the magnetic field

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SN driven dynamo

Omega dependence

0.6

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Dependence of turbulence on SN-rate

~ 3(/0)1/2

r ~ 6(/0)1/2

~ 2(/0)1/2

For 0.01 < /0 < 1

uz ~ 15 (/0)0.4 z

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Dependence of turbulence on SN-rateC ~ H C ~ 3/2H2 -1/2

Dynamo number: D ~ 5/2 H3 -1/2

Pitch angle: P ~ -1/2H-1 1/2

Cpum ~ -1/2H Cw ~ -1/2

Regular field decreases with increasing SF

Pitch angle increases

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Energy densities in NGC6946(Beck 2007)

Cold clouds:Cold clouds:

VVturbturb = 7 km/s, T=50 K, = 7 km/s, T=50 K,

H=100 pc H=100 pc Ionized gas: Ionized gas:

T=10T=1044 K, f K, fvv=0.05, H=1 kpc=0.05, H=1 kpc

=0.1 =0.2 D=0.06

but Ereg=0.3

=0.1 =0.2 D=0.06

but Ereg=0.3

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SN driven dynamoRotation curve with small turnover radius have high C

models observed

Radial extend: up to =25 more far out

Size of pitch angle: to small up to 35

Radial profile pitch angle: constant decreasing

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SN-driven dynamoMean field model:

No fountain flow, No pumping

Growth time: 0.7 Gys

Final field strength: Br = 0.6 Beq

B= 20 Beq

Pitch angle: 10

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SN-driven dynamoMean field model: Growth time: 0.25 Gys

Final field strength: Br = 0.3 Beq

B= 1.1 Beq

Pitch angle: 160

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SN-driven dynamo

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Increasing scale-height Mean field model:

m7a4b2i 174 Increase of scale height

Growth time: 0.7 Gys

Final field strength: Br = 0.6 Beq

B= 20 Beq

D ~ 5/2 H3 -1/2D ~ 5/2 H3 -1/2

P ~ -1/2H-1 1/2P ~ -1/2H-1 1/2

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Central dominated wind

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SN-driven dynamoMean field model:

Sr5tt2a-pol.avi

Growth time: 0.7 Gys

Final field strength:

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SN driven dynamoField regularity as IR based star formation rate for NGC 4254

(Chyzy 2008)

log (Breg: Btur) =

−0.32 (±0.01) log SFR

−0.90 (±0.03)

From simulations we get

−0.38 (±0.01)

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Magnetic field amplificationMagnetic instabilities as source for the dynamo

- Parker instability (may be with cosmic ray support) - Tayler instability ( for strong fields in the outer parts)

- Magneto rotational instability (in low starforming regions )

difficult to see a strong amplification, because of small scales for weak fields.

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Magneto-rotational instabilityLimits for MRI by other sources of turbulence: Kitchatinov et al. (2004)

0.7 /H < va < 0H ----> 0.7 < C = 0 H2/

0.7 < S < C

for turbulent : 100 - 3 G > B > 0.5 G (observed above the disk)

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MRI

nt3nt3

Disk field at 1.6 GysDisk field at 1.6 Gys Polarisation at 1.6 GysPolarisation at 1.6 Gys

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MRI

nt5nt5

512x256x128

time 2.8 Gys

growth time 0.9 Gys

Efinal/Einit=24

1<r<10

-5< z < 5

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MRI growth only linearly?

0.170.17 0.0350.035

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MRI: dynamo or primordial field ? Total magnetic energy growth is ~ t2

potential for large large pitch angle

but for maximal fields only ? smooth polarisation maps halo fields are strong Strong fields in the outer parts without starformation

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Dynamos have to explain Pitch angle right size and profile (for real rotation curve) MRI ? Turbulent field scales with starformation Regular component is independent of starformation Field geometry (also MRI but is it consistent with RM ?) Strong fields in the outer parts and halo MRI ? Fast growth

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SummaryMRI during disk formation in a uniform vertical field

large pitch angle, smooth PM

dynamic influence on the gas disk

fast growth time of the unstable mode 2/ < 100Mys

Dynamo ?

SN driven turbulent dynamo

needs star formation and strong rotation

pitch angle up to 20o for galactic values

growth time of order 100Mys

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Outlook Role of cosmic rays for the SN driven dynamo New insight from LOFAR for halo magnetic fields Combined models with SN-dynamo and MRI Other sources of turbulence in the outer part for a dynamo Strong field dynamos Magnetic instabilities for more complicated field configurations Galaxy formation models magnetic fields in dwarf galaxies

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Thank you!