Magnetic Fields in Evolving Spiral Galaxies and their Observation - PowerPoint PPT Presentation

Magnetic Fields in Evolving Spiral Galaxies and their Observation with the SKA Rainer Beck MPIfR Bonn

Fundamental magnetic questions  When and how were the first fields generated ?  Did significant fields exist before galaxies formed ?  How and how fast were fields amplified in galaxies and galaxy clusters?  How did fields affect the evolution of galaxies and clusters?  Is intergalactic space magnetic ?

M 31: Total and polarized synchrotron emission at 5 GHz (Effelsberg) Gießübel, PhD 2012 Bright radio synchrotron "ring"

Faraday rotation in M 31: The dynamo is working Berkhuijsen et al. 2003 Fletcher et al. 2004 The spiral field of M31 is coherent and axisymmetric (small spiral pitch angle)

NGC 6946 RM 5/8 GHz VLA+Effelsberg (Beck 2007) Inward-directed field along magnetic arms: Superposition of two dynamo modes (m=0 + m=2) ?

Magnetic modes in galaxies Fletcher 2011

Magnetic field strengths (from synchrotron intensity, assuming equipartition between energy densities of magnetic fields and cosmic rays) Total (mostly turbulent) field in spiral arms: 20 - 30 μ G Ordered field in interarm regions: 5 - 15 μ G Total field in circum-nuclear rings: 40 – 100 μ G Total field in galactic center filaments: ≤ 1 mG The magnetic energy density is in equipartition with the kinetic energy density of the turbulent gas motions

NGC 891 Effelsberg 8 GHz Total intensity + B-vectors (Krause 2007) X-shaped magnetic field in the halo: Signature of outflows

Polarization asymmetry 1.4 GHz SINGS survey (WSRT) Braun et al. 2010 Maximum PI always on the approaching major axis - observed in galaxies with inclinations ≤ 60 °

Dipolar and quadrupolar-type halo fields Braun et al. 2010

Evidences for large-scale dynamos in galaxies Magnetic and turbulent energy densities are similar  Spiral patterns exist in almost all galaxies  Large-scale coherent fields exist in the disks of many galaxies  Axisymmetric fields dominate  Coherent fields exist in many galactic halos 

Magnetic field model for the Milky Way Jansson & Farrar 2012 The Milky Way is similar to external galaxies – except for two field reversals

Generation and amplification of cosmic magnetic fields Stage 1 : Field seeding Primordial (intergalactic), Biermann battery, Weibel instability; ejection by supernovae, stellar winds or jets Stage 2 : Field amplification MRI, shock fronts, compressing flows, shearing flows, turbulent flows, small-scale dynamo Stage 3 : Coherent field ordering in galaxies Large-scale (mean-field) dynamo

Particle scattering by IGM magnetic fields Blazars: Detected at TeV γ - rays by HESS, but not in the GeV range by FERMI: B IGM ≥ 10 - 17… -16 G Neronov & Vovk 2010

Primordial fields in the Epoch of Reionization Brightness temperature Schleicher et al. 2009 emission B=0.8 nG emission B=0 absorption absorption Strong field: no absorption ! Strong impact on predicted HI spectra

Formation of galaxies Three main cosmological phases in simulations of hierarchical galaxy formation: Phase 1 (z ≈ 40 -20): Formation of low-density dark halos with M ≈ 10 10 M sun 60 kpc Phase 2 (z ≈ 20 -10): Merging of sub-halos (e.g. Wise & Abel 2007) Phase 3 (z ≈ 10 -2): Formation of large baryonic disks 20 kpc Mayer & Governato 2008 Kaufmann et al. 2007

Evolution of magnetic fields (1) The evolution of magnetic fields is coupled to the evolution of galaxies Phase 1 (z ≈ 40-20): Formation of halos Generation of seed magnetic fields by the Biermann battery or the Weibel instability or plasma fluctuations Amplitude: ≈ 10 -18 G - 10 -6 G (locally) Medvedev et al. 2004

Evolution of magnetic fields (2) Phase 2 (z ≈ 20-10): Merging of halos and virialization: Turbulence is driven by accretion shocks and SN explosions Amplification of seed fields by the turbulent (small-scale) dynamo (Schleicher et al. 2010, A. Beck et al. 2012) Timescale of amplification: ≈ 3 10 8 Gyr Amplitude: ≈ 10 -5 G

Simulation of a small-scale dynamo in young galaxies A. Beck et al. 2012 Equipartition with turbulent energy is reached within ≈ 10 8 yr , almost independent of the seed field

Evolution of magnetic fields (3) Phase 3 (z ≈ 10-2): Formation of disks: Turbulence is driven by SN explosions and MRI in the disk Field ordering (stretching) by shear Field ordering (regular fields) by the mean-field (large-scale) dynamo No further amplification needed Timescale of ordering: ≈ 1-2 10 9 Gyr There is no alternative theory to explain regular fields other than the mean-field dynamo

Large-scale (mean-field) dynamo Microphysics approximated by the average parameters:  “alpha - effect” and magnetic diffusivity Needed:  Ionized gas + differential rotation + turbulence + seed field 3-D structure: helical field, 2-D projection: spiral  Outflow needed to ensure the preservation of magnetic helicity 

Magnetic field amplification by galactic dynamos Arshakian et al. 2009 Phase 1 Phase 2 Phase 3 Disk mean- field dynamo Spherical mean-field dynamo Small-scale dynamo Seed field GD – giant disk galaxy (>15 kpc) MW – Milky Way type galaxy (≈ 10 kpc) DW – dwarf galaxy (≈ 3 kpc)

Global cosmic-ray driven dynamo model Hanasz et al. 2009

Model with injection of random seed fields (moderate / high differential rotation) Moss et al. 2012 Axisymmetric spiral field Spiral field with large-scale reversals

High-resolution dynamo simulation (box of 1x1x2 kpc 3 ) Gent et al. 2012 Total field Regular field Turbulent field

The large-scale dynamo generates bi-helical fields Bi-helical fields reveal characteristic features in λ 2 space, depending on RM Brandenburg & Stepanov, arXiv 2014

Magneto-rotational instability (MRI): source of turbulence in outer galaxies Balbus & Hawley Jim Stone, 1998 Princeton

MHD-MRI model of spiral galaxies (1) Magnetic fields are dynamically  unimportant ( β >1) Machida et al. 2013 Many large-scale field reversals  along radius and height No large-scale field 

MHD-MRI model of spiral galaxies (2) Pakmor & Springel 2013 Magnetic fields are  dynamically important ( β ≈ 1) Many large-scale field  reversals along radius and height No large-scale field 

MHD-MRI model of spiral galaxies (3) Pakmor & Springel 2013 The magnetic field affects the galaxy evolution: Spiral arms are more  patchy after 2 Gyr Star-formation rate is  ≈30% lower Vertical outflows are  driven

Predictions of the dynamo model Arshakian et al. Strong turbulent magnetic fields at z < 10  2009 → Total synchrotron emission from young galaxies can be observed at z < 10 Strong but " spotty " regular fields at z < 3  → Polarized radio emission and some Faraday rotation from normal and dwarf galaxies can be observed at z < 3 Large-scale coherent regular magnetic fields in dwarf or  Milky Way-type galaxies at z < 0.5 → Large-scale patterns of Faraday rotation can be observed at z < 0.5 Large galaxies (>15 kpc) may not yet have generated  fully coherent fields Major mergers may disrupt regular fields, but increase the  turbulent field strength (Moss et al., in prep.)

Detecting total emission from distant galaxies with SKA2 Murphy 2009

The radio-FIR correlation: Tracing magnetic fields in distant galaxies Radio synchrotron emission should break down at some redshift z  due to Inverse Compton loss with the CMB background FIR/radio ratio should increase with z  q: ratio of FIR/radio luminosities This is not observed:  Magnetic fields must still be strong in distant galaxies: B > B CMB = 3.25 μ G (1+z) 2 Synchrotron emission seems to even  increase relative to FIR But this cannot hold at very high  redshifts Murphy 2009

Magnetic fields in distant starburst galaxies The critical redshift of  correlation breakdown gives information on the field evolution: B ~ (1+z) ξ Schleicher & Beck 2013

Polarized source counts in deep surveys JVLA 5 GHz Taylor et al., in prep. 10-pointing mosaic, 60h integration time, 1 μ Jy rms noise FRII FRI Normal galaxies (see talk by Jeroen Stil)

Detecting polarized emission from distant galaxies with SKA1 SKA1 Stil & Taylor, unpubl. POSSUM

Virgo polarization survey VLA 5 GHz Vollmer et al. 2007 Field compression by interaction

RM Grids: Resolving field patterns with help of polarized background sources Stepanov et al. 2008 Recognition of field patterns: At least 10 RM values per galaxy needed  Can be applied to galaxies out to ≈ 100 Mpc distance  3-D reconstruction of field patterns: A few 1000 RM values per galaxy needed  Can be applied to galaxies out to ≈ 10 Mpc distance 

SKA2: RM grids of galaxies (simulation by Bryan Gaensler) ≈10000 polarized sources shining through M31

Faraday screens: Probing galactic magnetism in distant galaxies Faraday depolarization of Fornax A by NGC 1310 (VLA) Fomalont et al. 1989