Dynamic simulations of the Galactic Dynamo based on supernova-driven ISM turbulence

Transcription

1 Dynamic simulations of the Galactic Dynamo based on supernova-driven ISM turbulence Oliver Gressel, Axel Brandenburg Astrophysics group, NORDITA, Stockholm Abhijit Bendre, Detlef Elstner, Udo Ziegler & Günther Rüdiger MHD group, AIP, Potsdam May 13-17, 2013 MKSP meeting, Sant Antioco O. Gressel LOFAR MKSP meeting, Sant Antioco 1/27

2 contents 1 Context The galactic dynamo MF-MHD in a nutshell 2 Measuring dynamo tensors Non-linear quenching 3 Description of disc model Preliminary results O. Gressel LOFAR MKSP meeting, Sant Antioco 2/27

3 contents 1 Context The galactic dynamo MF-MHD in a nutshell 2 Measuring dynamo tensors Non-linear quenching 3 Description of disc model Preliminary results O. Gressel LOFAR MKSP meeting, Sant Antioco 2/27

4 contents 1 Context The galactic dynamo MF-MHD in a nutshell 2 Measuring dynamo tensors Non-linear quenching 3 Description of disc model Preliminary results O. Gressel LOFAR MKSP meeting, Sant Antioco 2/27

5 The galactic dynamo MF-MHD in a nutshell observations What is the origin of regular galactic magnetic fields? primoridial field, (i.e. frozen-in fossil record of galaxy formation) dynamo-generated field, (i.e. dynamically replenished) Andrew Fletcher/Rainer Beck, SuW and Hubble Heritage Team, STScI/AURA Beck of the envelope galactic rotation winds-up B φ τ Ω 2π/25 kpc 1 km s Myr turbulent diffusion τ d (0.5 kpc) 2 /0.5 kpc km s Myr large observed pitch angle strongly favours dynamo O. Gressel LOFAR MKSP meeting, Sant Antioco 3/27

6 The galactic dynamo MF-MHD in a nutshell observations What is the origin of regular galactic magnetic fields? primoridial field, (i.e. frozen-in fossil record of galaxy formation) dynamo-generated field, (i.e. dynamically replenished) Andrew Fletcher/Rainer Beck, SuW and Hubble Heritage Team, STScI/AURA Beck of the envelope galactic rotation winds-up B φ τ Ω 2π/25 kpc 1 km s Myr turbulent diffusion τ d (0.5 kpc) 2 /0.5 kpc km s Myr large observed pitch angle strongly favours dynamo O. Gressel LOFAR MKSP meeting, Sant Antioco 3/27

7 The galactic dynamo MF-MHD in a nutshell supernova-driven turbulence play interstellar medium highly turbulent energy deposited by supernovae, CRs, MRI, stellar winds, protostellar jets, SNe per century in our own Milky Way small-scale dynamo is simple (in a way...) but how amplify regular fields in a turbulent environment? rotation + stratification mean-field dynamo O. Gressel LOFAR MKSP meeting, Sant Antioco 4/27

8 Context The galactic dynamo MF-MHD in a nutshell supernova-driven turbulence interstellar medium highly turbulent energy deposited by supernovae, CRs, MRI, stellar winds, protostellar jets, SNe per century in our own Milky Way play small-scale dynamo is simple (in a way...) but how amplify regular fields in a turbulent environment? rotation + stratification mean-field dynamo O. Gressel LOFAR MKSP meeting, Sant Antioco 4/27

9 Context The galactic dynamo MF-MHD in a nutshell supernova-driven turbulence interstellar medium highly turbulent energy deposited by supernovae, CRs, MRI, stellar winds, protostellar jets, SNe per century in our own Milky Way play small-scale dynamo is simple (in a way...) but how amplify regular fields in a turbulent environment? rotation + stratification mean-field dynamo O. Gressel LOFAR MKSP meeting, Sant Antioco 4/27

10 The galactic dynamo MF-MHD in a nutshell the α effect dynamo Key mechanisms helical α effect in a single expanding SNR play breaking the homogeneity of the turbulence play rotation (and/or shear) field-line stretching helical flow component avoid cancellation due to anti-parallel field α effect couples the poloidal and toroidal field components reconnection restore original field-line topology O. Gressel LOFAR MKSP meeting, Sant Antioco 5/27

11 The galactic dynamo MF-MHD in a nutshell the α effect dynamo Key mechanisms helical α effect in a single expanding SNR play breaking the homogeneity of the turbulence play rotation (and/or shear) field-line stretching helical flow component avoid cancellation due to anti-parallel field α effect couples the poloidal and toroidal field components reconnection restore original field-line topology O. Gressel LOFAR MKSP meeting, Sant Antioco 5/27

12 The galactic dynamo MF-MHD in a nutshell the big picture encapsulate the effect of the supernovae model the evolution of the large-scale field O. Gressel LOFAR MKSP meeting, Sant Antioco 6/27

13 The galactic dynamo MF-MHD in a nutshell modelling the dynamo process Mean-field approach: split into mean + fluctuation U = U + u and B = B + b derive mean-field equation t B = ( U B + Ē η B) turbulent EMF Ē = u b Parametrise small-scale effects Ē as a functional of U, B, f (u) for sufficient scale separation Ē i = α ij B j η ij ε jkl k B l Andrew Fletcher/Rainer Beck, SuW / Hubble Heritage Team, STScI/AURA O. Gressel LOFAR MKSP meeting, Sant Antioco 7/27

14 The galactic dynamo MF-MHD in a nutshell modelling the dynamo process Mean-field approach: split into mean + fluctuation U = U + u and B = B + b derive mean-field equation t B = ( U B + Ē η B) turbulent EMF Ē = u b Parametrise small-scale effects Ē as a functional of U, B, f (u) for sufficient scale separation Ē i = α ij B j η ij ε jkl k B l Andrew Fletcher/Rainer Beck, SuW / Hubble Heritage Team, STScI/AURA O. Gressel LOFAR MKSP meeting, Sant Antioco 7/27

15 Measuring dynamo tensors Non-linear quenching 1 Context The galactic dynamo MF-MHD in a nutshell 2 Measuring dynamo tensors Non-linear quenching 3 Description of disc model Preliminary results O. Gressel LOFAR MKSP meeting, Sant Antioco 7/27

16 Measuring dynamo tensors Non-linear quenching the nirvana code Udo Ziegler s NIRVANA-III conservative (dual energy) MHD grid code 2nd-order central scheme + constrained transport (CT) 3rd-order Runge-Kutta time integrator block structured adaptive mesh refinement (AMR) efficiently MPI-parallel (space-filling-curve techniques for AMR) O. Gressel LOFAR MKSP meeting, Sant Antioco 8/27

17 Measuring dynamo tensors Non-linear quenching local box simulations Model geometry: local patch of interstellar medium, up to 1.6 kpc on edge ( 10 pc) vertical stratification up to ±6 kpc sheared galactic rotation Physical ingredients: non-ideal MHD (+ heat conduction) optically thin radiative heating/cooling localised thermal energy input modelling the supernovae play Korpi, Brandenburg, Shukurov, Tuominen & Nordlund (1999) O. Gressel LOFAR MKSP meeting, Sant Antioco 9/27

18 Measuring dynamo tensors Non-linear quenching local box simulations Model geometry: local patch of interstellar medium, up to 1.6 kpc on edge ( 10 pc) vertical stratification up to ±6 kpc sheared galactic rotation Physical ingredients: non-ideal MHD (+ heat conduction) optically thin radiative heating/cooling localised thermal energy input modelling the supernovae play Korpi, Brandenburg, Shukurov, Tuominen & Nordlund (1999) O. Gressel LOFAR MKSP meeting, Sant Antioco 9/27

19 Measuring dynamo tensors Non-linear quenching α-profiles dynamo effect α R, α φ 3 km s 1 diamagn. pumping γ z 7 km s 1 directed inward α : γ consistent w/ SOCA results effect of galactic wind ū z balanced by turb. pumping Gressel, Elstner, Ziegler & Rüdiger (2008), A&A 486, L35 O. Gressel LOFAR MKSP meeting, Sant Antioco 10/27

20 Measuring dynamo tensors Non-linear quenching η-profiles turb. diffusivity 2 kpc km s 1 coherence time τ 3 Myr non-vanishing Ω J effect δ z 0.5 kpc km s 1 add shear dynamo O. Gressel LOFAR MKSP meeting, Sant Antioco 11/27

21 Measuring dynamo tensors Non-linear quenching it really works! O. Gressel LOFAR MKSP meeting, Sant Antioco 12/27

22 Measuring dynamo tensors Non-linear quenching scaling relations (brute force) dynamo effect as function of supernova rate ˆσ = σ/σ 0 rotation frequency ˆΩ = Ω/Ω 0 midplane density ˆρ = ρ/ρ 0 scaling relations: α = 2 km s 1 ˆσ 0.4 ˆΩ0.5 ˆρ 0.1 γ z = 12 km s 1 ˆσ 0.5 ˆΩ 0.2 ˆρ 0.3 η = 2 kpc km s 1 ˆσ 0.4 ˆΩ0.25 ˆρ 0.4 where σ 0 = 30Myr 1 kpc 2, Ω 0 = 25Gyr 1, ρ 0 = 1 cm 3 O. Gressel LOFAR MKSP meeting, Sant Antioco 13/27

23 Measuring dynamo tensors Non-linear quenching 1 Context The galactic dynamo MF-MHD in a nutshell 2 Measuring dynamo tensors Non-linear quenching 3 Description of disc model Preliminary results O. Gressel LOFAR MKSP meeting, Sant Antioco 13/27

24 Measuring dynamo tensors Non-linear quenching magnetic field saturation O. Gressel LOFAR MKSP meeting, Sant Antioco 14/27

25 Measuring dynamo tensors Non-linear quenching a lingering catastrophe Quenching scenarios: (a) classic: flow quenching due to Lorentz force (b) catastrophic: helicity conservation inhibits growth (c) similar to scenario (b) but alleviated by small-scale helicity removal Test possible realisations: quenching sets-in... (a)... at B B eq (b)... at B B eq /Rm (c)... at B B eq l 0 /L 0 Suppression of wind: (c) (b) O. Gressel LOFAR MKSP meeting, Sant Antioco 15/27

26 Measuring dynamo tensors Non-linear quenching extracting quenching functions quenching quadratic in β B/B eq magnetic Reynolds number, Rm u rms (k f η) scale separation ratio, l 0 /L kpc/1 kpc = 10 Gressel, Bendre & Elstner (2013), MNRAS 429, 967 O. Gressel LOFAR MKSP meeting, Sant Antioco 16/27

27 Description of disc model Preliminary results 1 Context The galactic dynamo MF-MHD in a nutshell 2 Measuring dynamo tensors Non-linear quenching 3 Description of disc model Preliminary results O. Gressel LOFAR MKSP meeting, Sant Antioco 16/27

28 Description of disc model Preliminary results the disc model Based on flaring HI disc Kalberla & Dedes (2008) expon. + power-law density profile NFW-type DM halo + stellar disc / bulge self-consistent rotation curve Goal: perform fully-dynamical MHD + MFD simulations momentum equation with turbulent viscosity will capture Parker / Tayler / MRI on long wavelengths O. Gressel LOFAR MKSP meeting, Sant Antioco 17/27

29 Description of disc model Preliminary results disc surface density Kalberla & Dedes (2008) O. Gressel LOFAR MKSP meeting, Sant Antioco 18/27

30 Description of disc model Preliminary results disc surface density Kalberla & Dedes (2008) O. Gressel LOFAR MKSP meeting, Sant Antioco 18/27

31 Description of disc model Preliminary results 1 Context The galactic dynamo MF-MHD in a nutshell 2 Measuring dynamo tensors Non-linear quenching 3 Description of disc model Preliminary results O. Gressel LOFAR MKSP meeting, Sant Antioco 18/27

32 Description of disc model Preliminary results emerging dynamo mode Dynamo solution initial NVF leads to transient A0, S0 mode S0+A0 produces one-sided vertical field Mao, Gaensler et al. (2010) O. Gressel LOFAR MKSP meeting, Sant Antioco 19/27

33 Description of disc model Preliminary results model overview O. Gressel LOFAR MKSP meeting, Sant Antioco 20/27

34 Description of disc model Preliminary results radial pitch angle radial fall-off in pitch angle (agrees with observations) explained conveniently by flaring disc Fletcher (2010) O. Gressel LOFAR MKSP meeting, Sant Antioco 21/27

35 Description of disc model Preliminary results saturated field profile radial scale length 4 kpc for saturated B outer disc essentially unmagnetised O. Gressel LOFAR MKSP meeting, Sant Antioco 22/27

36 Description of disc model Preliminary results MHD w/o mean-field dynamo MRI-only simulation MRI with dissipation from SNe O. Gressel LOFAR MKSP meeting, Sant Antioco 23/27

37 Description of disc model Preliminary results MHD w/o mean-field dynamo MRI-only simulation MRI with dissipation from SNe O. Gressel LOFAR MKSP meeting, Sant Antioco 23/27

38 Description of disc model Preliminary results combined MHD / mean-field dynamo Parker modes & MRI in outer disc pronounced loop structures undulating mode with m = 3 radial scale length 10 kpc inner disc dominated by m = 0 O. Gressel LOFAR MKSP meeting, Sant Antioco 24/27

39 Description of disc model Preliminary results synthetic polarisation maps O. Gressel LOFAR MKSP meeting, Sant Antioco 25/27

40 summary of results Measuring dynamo coefficients 1D mean-field model matches simulations quantitative scaling relations for sub-grid physics Non-linear saturation quenching functions obtained indications for the presence of helicity constraints suppression of wind threatens saturation level Global mean-field models first fully quantitative global dynamo models dynamic momentum equation MRI / Parker / Tayler parametrisation of small-scale effects is essential O. Gressel LOFAR MKSP meeting, Sant Antioco 26/27

41 summary of results Measuring dynamo coefficients 1D mean-field model matches simulations quantitative scaling relations for sub-grid physics Non-linear saturation quenching functions obtained indications for the presence of helicity constraints suppression of wind threatens saturation level Global mean-field models first fully quantitative global dynamo models dynamic momentum equation MRI / Parker / Tayler parametrisation of small-scale effects is essential O. Gressel LOFAR MKSP meeting, Sant Antioco 26/27

42 summary of results Measuring dynamo coefficients 1D mean-field model matches simulations quantitative scaling relations for sub-grid physics Non-linear saturation quenching functions obtained indications for the presence of helicity constraints suppression of wind threatens saturation level Global mean-field models first fully quantitative global dynamo models dynamic momentum equation MRI / Parker / Tayler parametrisation of small-scale effects is essential O. Gressel LOFAR MKSP meeting, Sant Antioco 26/27

43 workshop announcement Galactic Magnetism in the Era of LOFAR and SKA: Developing Tools for Synthetic Polarization Maps Sept , 2013 AlbaNova, Stockholm organising committee Sui Ann Mao (NRAO/UWisconsin) Cathy Horellou (Chalmers) Bryan Gaensler (Sydney) Andrew Fletcher (Newcastle) Axel Brandenburg (Nordita) Michael Bell (Garching) Rainer Beck (Bonn) O. Gressel LOFAR MKSP meeting, Sant Antioco 27/27