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Galactic magnetic fields: MRI or SN-driven dynamo?

Galactic magnetic fields: MRI or SN-driven dynamo?. Detlef Elstner Oliver Gressel Natali Dziourkevich Alfio Bonanno Günther Rüdiger. Magnetic field amplification. Dynamos by SN-driven turbulence are numerically feasible Box models:

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Galactic magnetic fields: MRI or SN-driven dynamo?

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  1. Galactic magnetic fields: MRI or SN-driven dynamo? Detlef Elstner Oliver Gressel Natali Dziourkevich Alfio Bonanno Günther Rüdiger

  2. Magnetic field amplification Dynamos 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)

  3. 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 Missing: stellar winds, mass recycling, cosmic rays ……

  4. SN-driven dynamo How it works: downward pumping equal upward fountain flow pumping acts only on the mean field fountain flow transports all the magnetic field

  5. SN driven dynamo Omega dependence 0.6

  6. Dependence of turbulence on SN-rate For 0.01 </0 < 1  ~ 2(/0)1/2  ~ 3(/0)1/2 r ~ 6(/0)1/2 uz ~ 15 (/0)0.4 z

  7. Dependence of turbulence on SN-rate C ~ HC ~ 3/2H2 -1/2 Cpum ~ -1/2HCw ~ -1/2 Dynamo number: D ~ 5/2 H3 -1/2 Pitch angle: P ~ -1/2H-1 1/2 Regular field decreases with increasing SF Pitch angle increases

  8. Energy densities in NGC6946 (Beck 2007) =0.1 =0.2 D=0.06 but Ereg=0.3 Cold clouds: Vturb = 7 km/s, T=50 K, H=100 pc Ionized gas: T=104 K, fv=0.05, H=1 kpc

  9. SN driven dynamo Rotation 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

  10. SN-driven dynamo Mean 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

  11. SN-driven dynamo Mean field model: Growth time: 0.25 Gys Final field strength: Br = 0.3 Beq B= 1.1 Beq Pitch angle: 160

  12. SN-driven dynamo

  13. 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 -1/2 P ~ -1/2H-1 1/2

  14. Central dominated wind

  15. SN-driven dynamo Mean field model: Sr5tt2a-pol.avi Growth time: 0.7 Gys Final field strength:

  16. SN driven dynamo Field 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)

  17. Magnetic field amplification Magnetic 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.

  18. Magneto-rotational instability Limits 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)

  19. MRI nt3 Disk field at 1.6 Gys Polarisation at 1.6 Gys

  20. MRI 1<r<10 -5< z < 5 512x256x128 time 2.8 Gys growth time 0.9 Gys Efinal/Einit=24 nt5 Vertical cut

  21. MRI growth only linearly? 0.035 0.17

  22. 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

  23. 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

  24. Summary MRI 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

  25. 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

  26. Thank you!

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