September 2005 Magnetic field excitation in galaxies

September 2005 Outline  The problem of large scale fields  Large scale flows  Meanfield dynamos (alpha effect due to …)  The role of cosmic rays  MRI  The problem of large scale fields  Large scale flows  Meanfield dynamos (alpha effect due to …)  The role of cosmic rays  MRI

September 2005 Observations Large scale magnetic fields Axisymmetric mode dominates Large pitch angle Strongly (anti) correlated with optical arms Strength is compatible to the small scale component Seldomly reversals

September 2005 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 A lot of processes inject small scale fields in the ISM  stellar dynamos combined with winds  shock-turbulence  plasma instabilities  small scale dynamos  No problem to explain the magnetic field energy in galaxies But which processes bring coherent magnetic fields on a scale of several kpc? But which processes bring coherent magnetic fields on a scale of several kpc?  No problem to explain the magnetic field energy in galaxies But which processes bring coherent magnetic fields on a scale of several kpc? But which processes bring coherent magnetic fields on a scale of several kpc? Balsara et al. 2004

September 2005 How to do ?  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 flows amplifying a primordial field  inverse turbulent cascade  formation of stronger fields during galaxy mergers and a more rapid decay of small scale structures

September 2005 Large scale flow Rotation without diffusivity (ideal MHD) (50 rotations)  In a Hubble time amplification by a factor of 10  Strong seed field is neccassary (10 -7  G) on large scale  Pitch angle would be less than 1 o  A lot of radial reversals  Adding turbulent diffusion one needs a source for B r Rotation without diffusivity (ideal MHD) (50 rotations)  In a Hubble time amplification by a factor of 10  Strong seed field is neccassary (10 -7  G) on large scale  Pitch angle would be less than 1 o  A lot of radial reversals  Adding turbulent diffusion one needs a source for B r

September 2005 Large scale flow  B r from nonuniform radial motions in spirals  reversals or positive pitch angles  B r from nonuniform radial motions in spirals  reversals or positive pitch angles Otmianowska-Mazur & Chiba 1995

September 2005 Inverse cascade  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  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 Mean field language: Alpha-effect

September 2005 Parker ´55 Steenbeck, Krause & Rädler ´66 BC: curl B = 0 outside Dynamo Theory div B = 0

September 2005  Differential rotation  max = 50 Gy -1  turbulence defined by u´ and   u´= 10 km s -1 and  = 0.01 Gy  = 1 kpc 2 Gy -1 and  = 10 km s -1  induction equation with  -quenching  Differential rotation  max = 50 Gy -1  turbulence defined by u´ and   u´= 10 km s -1 and  = 0.01 Gy  = 1 kpc 2 Gy -1 and  = 10 km s -1  induction equation with  -quenching stationary axisymmetric quadrupolar field  field strength several  G  pitch angle about 20 o  maximal field strength at maximal  stationary axisymmetric quadrupolar field  field strength several  G  pitch angle about 20 o  maximal field strength at maximal  Dynamo Theory

September 2005 Mean field dynamo Dynamo numbers: C  =  H  -1 C  =   H 2  -1 Pitch angle: tan p =   C   C  Independent of  But with quenching tan p =   C  crit   C  = 0.4    with     km kpc s -1 p=20 o tan p =   C  crit   C  = 0.4    with     km kpc s -1 p=20 o

September 2005 Single supernova explosions Ferrière, 1998 Single supernova explosions Ferrière, 1998 Pitch angle : 0.1 o - 1 o Turbulence from SN

September 2005 Super Bubbles Ferrière, 1998 Super Bubbles Ferrière, 1998 Pitch angle : 1 o - 30 o Turbulence from SN

September 2005 Mean flow Rotation law:  =   (1+(r/r   n ) -1/n (n=2) r  =2 r  =5

September 2005 Density wave flows Model: Brandt law of rotation, r   and C  =41, C  =4.7 u r =  u 0 cos ( m p (  p t) + K r/R max ) f(r) u  =u 0 sin ( m p (  p t) + K r/R max ) f(r)

September 2005 Density wave flows 2D stationary density wave flows disturb the dynamo

September 2005 Pitch angle is maximal at field minimum Density wave flows

September 2005

MHD instabilities Supernova explosions provide enough energy for the turbulence in the ISM Supernova explosions provide enough energy for the turbulence in the ISM Magnetic field instabilities contribute to the turbulence MRI in galaxies Parker instability together with cosmic rays Observational hint: Turbulence in low star forming regions

September 2005 Parker instability Hanasz et al Parker instability largescale B r Rotation coalesence largescale B 

September 2005 Cosmic rays and Parker instability Hanasz et al SN-explosions with cosmic rays Fast diffusion of cosmic rays + Parker instability Fast expansion of pressure disturbances More effective creation of B r

September 2005 Cosmic rays and Parker instability

September 2005 Magneto rotational instability (MRI) Dziourkevitch 2004 Weak magnetic field V a <  H is unstable

September 2005 MRI Dziourkevitch 2004

September 2005 MRI Dziourkevitch 2004

September 2005 MRI Dziourkevitch 2004

September 2005 MRI Dziourkevitch 2004

September 2005 MRI Dziourkevitch 2004

September 2005 Open problems Turbulence in the ISM – simulations in the box Simulation with Superbubbles Role of magnetic instabilities Multicomponent gas - role of hot phase External temporal gravitational disturbances

September 2005 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 What is the influence of a more complicated mean flow in a high Reynolds number regime What determines the correlation between optical arms and magnetic arms What determines the correlation between optical arms and magnetic arms How important is the galaxy evolution for magnetic field properties

September 2005 Questions to Observers Can we see coherent structures on the large scale during galaxy formation process Can we see coherent structures on the large scale during galaxy formation process Detailed observations of smaller scales 3D field topology in SNR Detailed observations of smaller scales 3D field topology in SNR Correlation to rotation curves and spiral flows

September 2005 Thank you