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 ?
galaxy clusters?
clusters?
Bright radio synchrotron "ring"
(Effelsberg)
Gießübel, PhD 2012
Berkhuijsen et al. 2003
The spiral field of M31 is coherent and axisymmetric (small spiral pitch angle)
Fletcher et al. 2004
Inward-directed field along magnetic arms: Superposition of two dynamo modes (m=0 + m=2) ?
RM 5/8 GHz VLA+Effelsberg (Beck 2007)
Fletcher 2011
(from synchrotron intensity, assuming equipartition between energy densities of magnetic fields and cosmic rays) Total (mostly turbulent) field in spiral arms: 20 - 30 μG Ordered field in interarm regions: 5 - 15 μG Total field in circum-nuclear rings: 40 – 100 μG 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
Effelsberg 8 GHz Total intensity + B-vectors (Krause 2007) X-shaped magnetic field in the halo: Signature of outflows
1.4 GHz SINGS survey (WSRT)
Braun et al. 2010
Maximum PI always on the approaching major axis - observed in galaxies with inclinations ≤ 60°
Braun et al. 2010
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
Jansson & Farrar 2012
The Milky Way is similar to external galaxies – except for two field reversals
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
Neronov & Vovk 2010
Blazars: Detected at TeV γ-rays by HESS, but not in the GeV range by FERMI:
Brightness temperature
emission absorption B=0 B=0.8 nG
Strong field: no absorption ! Schleicher et al. 2009 absorption emission
Three main cosmological phases in simulations of hierarchical galaxy formation: Phase 1 (z ≈ 40-20): Formation of low-density dark halos with M ≈ 1010 Msun Phase 2 (z ≈ 20-10): Merging of sub-halos (e.g. Wise & Abel 2007) Phase 3 (z ≈ 10-2): Formation of large baryonic disks
Mayer & Governato 2008 Kaufmann et al. 2007 60 kpc 20 kpc
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
The evolution of magnetic fields is coupled to the evolution of galaxies
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: ≈ 3 108 Gyr Amplitude: ≈ 10-5 G
Equipartition with turbulent energy is reached within ≈ 108 yr , almost independent of the seed field
2012
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: ≈ 1-2 109 Gyr
There is no alternative theory to explain regular fields
“alpha-effect” and magnetic diffusivity
Ionized gas + differential rotation + turbulence + seed field
Phase 1 Phase 2 Phase 3
GD – giant disk galaxy (>15 kpc) MW – Milky Way type galaxy (≈ 10 kpc) DW – dwarf galaxy (≈ 3 kpc)
Arshakian et al. 2009
Small-scale dynamo Spherical mean-field dynamo Disk mean- field dynamo Seed field
Hanasz et al. 2009
(moderate / high differential rotation)
Axisymmetric spiral field Spiral field with large-scale reversals Moss et al. 2012
(box of 1x1x2 kpc3) Total field Regular field Turbulent field Gent et al. 2012
Bi-helical fields reveal characteristic features in
depending on RM
Brandenburg & Stepanov, arXiv 2014
Balbus & Hawley 1998 Jim Stone, Princeton
Machida et al. 2013
Magnetic fields are dynamically unimportant (β >1)
Many large-scale field reversals along radius and height
No large-scale field
Pakmor & Springel 2013
Magnetic fields are dynamically important (β ≈ 1)
Many large-scale field reversals along radius and height
No large-scale field
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
→ Total synchrotron emission from
young galaxies can be observed at z < 10
→ Polarized radio emission and some Faraday rotation
from normal and dwarf galaxies can be observed at z < 3
Milky Way-type galaxies at z < 0.5
→ Large-scale patterns of Faraday rotation can be observed
at z < 0.5
fully coherent fields
turbulent field strength (Moss et al., in prep.) Arshakian et al. 2009
Murphy 2009
due to Inverse Compton loss with the CMB background
Magnetic fields must still be strong in distant galaxies: B > BCMB = 3.25 μG (1+z)2
increase relative to FIR
redshifts Murphy 2009 q: ratio of FIR/radio luminosities
correlation breakdown gives information on the field evolution: B ~ (1+z) ξ
Schleicher & Beck
2013
JVLA 5 GHz
Taylor et al., in prep. FRII FRI
Normal galaxies
10-pointing mosaic, 60h integration time, 1μJy rms noise (see talk by Jeroen Stil)
Stil & Taylor, unpubl.
SKA1
POSSUM
Vollmer et al. 2007
Field compression by interaction
At least 10 RM values per galaxy needed
Can be applied to galaxies out to ≈100 Mpc distance
A few 1000 RM values per galaxy needed
Can be applied to galaxies out to ≈10 Mpc distance
Stepanov et al. 2008
(simulation by Bryan Gaensler) ≈10000 polarized sources shining through M31
Faraday depolarization
by NGC 1310 (VLA)
Fomalont et al. 1989
Population of intervening galaxies towards polarized quasars: Faraday rotation at high frequencies (≈5-10 GHz) is stronger for more distant quasars (z>1)
Finding signatures of intergalactic fields needs much deeper
Kronberg et al. 2008, Bernet et al. 2008
(Bernet et al. 2012, Hammond et al. 2013)
(in front of an extended homogeneous source)
Bernet et al. 2012 Model: anisotropic Faraday screen with filling factor f=0.5, 8 random cells within the beam (Main component: FD of the Milky Way foreground)
If resolved by RM Synthesis, Faraday depolarization is reduced What is the optimum frequency range for RM grids ?
Magnetized outflows
Intervenors may be frequent
Bernet et al. 2013
The evolution of galactic magnetic fields can be measured Deep SKA1 observations:
Diffuse polarization:
SKA1-MID (deep)
SKA1-MID high band (deep)
RMs of polarized background sources:
SKA1-SUR (survey)
SKA1-MID (survey)
SKA1-MID (deep)
SKA1-SUR (survey)