Galactic magnetic fields 9262020 MRI or SNdriven dynamo

Galactic magnetic fields: 9/26/2020 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: cosmic ray injection (Hanasz et al. 2004) thermal energy input (Gressel et al. 2008) 9/26/2020 Global models: mean field models ( ………. since 1960 ) artificial velocities (Gissinger et al. 2008) cosmic ray injection by SN (Hanasz et al. 2009)

SN driven dynamo 9/26/2020 • 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

SN-driven dynamo How it works: downward pumping equal upward fountain flow pumping acts only on the mean field 9/26/2020 fountain flow transports all the magnetic field

SN driven dynamo Omega dependence 9/26/2020 0. 6

Dependence of turbulence on SN-rate For 0. 01 < / 0 < 1 ~ 3( / 0 )1/2 9/26/2020 r ~ 6( / 0 )1/2 ~ 2( / 0)1/2 uz ~ 15 ( / 0)0. 4 z

Dependence of turbulence on SN-rate C ~ H C ~ 3/2 H 2 -1/2 Cpum ~ -1/2 H Cw ~ -1/2 Dynamo number: D ~ 5/2 H 3 -1/2 Pitch angle: P ~ -1/2 H-1 1/2 9/26/2020 Regular field decreases with increasing SF Pitch angle increases

9/26/2020 Energy densities in NGC 6946 Cold clouds: Vturb = 7 km/s, T=50 K, H=100 pc Ionized gas: T=104 K, fv=0. 05, H=1 kpc (Beck 2007) =0. 1 =0. 2 D=0. 06 but Ereg=0. 3

SN driven dynamo Rotation curve with small turnover radius have high C models up to =25 more far out Size of pitch angle: to small up to 35 Radial profile pitch angle: constant decreasing Radial extend: 9/26/2020 observed

SN-driven dynamo Mean field model: No fountain flow, No pumping Growth time: 0. 7 Gys 9/26/2020 Final field strength: Br = 0. 6 Beq B = 20 Beq Pitch angle: 10

SN-driven dynamo Mean field model: Growth time: 0. 25 Gys 9/26/2020 Final field strength: Br = 0. 3 Beq B = 1. 1 Beq Pitch angle: 160

9/26/2020 SN-driven dynamo

Increasing scale-height Mean field model: D ~ 5/2 H 3 -1/2 P ~ -1/2 H-1 1/2 m 7 a 4 b 2 i 174 Increase of scale height Growth time: 0. 7 Gys 9/26/2020 Final field strength: Br = 0. 6 Beq B = 20 Beq

9/26/2020 Central dominated wind

SN-driven dynamo Mean field model: Sr 5 tt 2 a-pol. avi Growth time: 0. 7 Gys 9/26/2020 Final field strength:

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) 9/26/2020 From simulations we get − 0. 38 (± 0. 01)

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 ) 9/26/2020 difficult to see a strong amplification, because of small scales for weak fields.

Magneto-rotational instability Limits for MRI by other sources of turbulence: Kitchatinov et al. (2004) 0. 7 /H < va < 0 H ----> 0. 7 < C = 0 H 2/ 0. 7 < S < C 9/26/2020 for turbulent : 100 - 3 G > B > 0. 5 G (observed above the disk)

MRI 9/26/2020 nt 3 Disk field at 1. 6 Gys Polarisation at 1. 6 Gys

MRI 1<r<10 -5< z < 5 512 x 256 x 128 9/26/2020 time 2. 8 Gys growth time 0. 9 Gys Efinal/Einit=24

9/26/2020 MRI growth only linearly? 0. 17 0. 035

MRI: dynamo or primordial field ? Total magnetic energy growth is ~ t 2 n potential for large pitch angle but for maximal fields only ? n smooth polarisation maps n halo fields are strong n Strong fields in the outer parts without starformation 9/26/2020 n

Dynamos have to explain Pitch angle right size and profile (for real rotation curve) MRI ? n Turbulent field scales with starformation n Regular component is independent of starformation n Field geometry (also MRI but is it consistent with RM ? ) n Strong fields in the outer parts and halo MRI ? n Fast growth 9/26/2020 n

Summary 9/26/2020 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 / < 100 Mys Dynamo ? SN driven turbulent dynamo needs star formation and strong rotation pitch angle up to 20 o for galactic values growth time of order 100 Mys

Outlook Role of cosmic rays for the SN driven dynamo n New insight from LOFAR for halo magnetic fields n Combined models with SN-dynamo and MRI n Other sources of turbulence in the outer part for a dynamo n Strong field dynamos n Magnetic instabilities for more complicated field configurations n Galaxy formation models n magnetic fields in dwarf galaxies 9/26/2020 n

9/26/2020 Thank you!