Observations of Magnetic Fields in normal Galaxies Rainer Beck

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Observations ofMagnetic Fields in (normal)
Galaxies Rainer BeckElly Berkhuijsen, Marita
Krause, Wolfgang Reich, Richard Wielebinski,
Maik Wolleben (Bonn)Ralf-Jürgen Dettmar, Volker
Heesen (Bochum)Chris Chyzy, Marian Soida, Marek
Urbanik (Krakow)Andrew Fletcher, Anvar Shukurov
(Newcastle) Dmitry Sokoloff, Vladimir Shoutenkov
(Moscow)Peter Frick, Igor Patrickeyev
(Perm)Matthias Ehle (Madrid) Julienne Harnett
(South Pole)
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Fundamental questions

• STRUCTURE
• What are the strength and structure of the
  magnetic fields in the interstellar and
  intergalactic medium ?
• What is the interplay between fields and gas ?
• EVOLUTION
• How were magnetic fields amplified and maintained
  and how did they evolve as galaxies evolve ?
• ORIGIN
• When and how were the first magnetic fields
  generated ?

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Motivations to study magnetic fields in galaxies

• Understand their dynamical importance
• Map gas flows
• Map ram pressure and interactions
• Understand cosmic-ray propagation
• Determine age of cosmic-ray electrons (see
  posters 10 and 59)
• Input to MHD models

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Outline

• Field strength energy density
• Structure of the regular field
• Magnetic fields in the Milky Way
• Halo fields
• (see next talk by Ralf-Jürgen Dettmar)

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Observation of magnetic fields

• Optical polarization (absorption by aligned,
  rotating, paramagnetic dust grains)
• Infrared polarization (emission from aligned dust
  grains)
• Zeeman effect
• Radio synchrotron emission
• Faraday rotation

(remember Dick Crutchers talk)
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NGC6946 Optical polarization (Fendt et al. 1998)

• Distorted
• by scattered
• light !

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VLA
Effelsberg
ATCA
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M51 Total intensity (Fletcher, Beck et al. 2005)

• Total
• synchrotron
• intensity traces
• the total
• magnetic
• field

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M51 Polarized intensity (Fletcher, Beck et al.
2005)

• Polarized
• synchrotron
• intensity traces
• the regular
• magnetic field

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Chapter OneHow strong are interstellar
magnetic fields ?
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(Total field)2 (turbulent field)2
(regular field)2
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Measuring total field strength

• Equipartition between energy densities of
  magnetic fields and total cosmic rays
• Input
• Synchrotron intensity
• Ratio K of proton/electron number densities at
  GeV energies (K100 for shock acceleration)
• Energy spectral index of dominating protons,
  represented by the radio spectral index
• Problems
• Electrons suffer from energy losses which modify
  their spectrum and hence K
• Textbook formula uses integration of the radio
  spectrum between two frequencies
• Textbook formula is wrong !

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Revised equipartition formula(based on
integration of the proton spectrum)

• Beq,- ? ( Isync (K1) / L ) 1/(3a)
• Isync Synchroton intensity
• K Proton/electron ratio (at GeV energies)
• L Pathlength through source
• a Synchrotron spectral index (S ? ?-? )

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Beck Krause (2004)
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Equipartition field estimates

• The classical estimate is too high for radio
  spectral indices of 0.7 and field strengths of
  gt10µG.
• The popular choice of K0 or K1 (e.g. for radio
  lobes and clusters) is not justified, so that the
  eq. field estimate is too low by 3x.
• Energy losses of electrons further increase K, so
  that the eq. field estimate is too low.
• As electron propagation speed is limited, they
  may not illuminate the field outside star
  formation regions, where the eq. field estimate
  is too low.

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Synchrotron Ages

• tsyn 1 Gyr (B- /µG)-1.5 (?syn /GHz)-0.5
• An underestimate in
• B- leads to an
• overestimate of the
• synchrotron age
• ( an underestimate
• of the propagation
• speed)

?syn
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There are magnetic fields inside and outside of
the ring !
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Total field strengths

• Niklassurvey of 74 spiral galaxies
• ltBtotgt 9 µG

Niklas 1995
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Equipartition magnetic field strengths in M51
Fletcher,Beck, et al. 2005
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Synchrotron losses of cosmic-ray electrons in M51
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Magnetic fields in the inner spiral arms of M51

• Arm Inter-arm
• Total magnetic field 25 ?G 20?G
• Regular magnetic field 15 ?G 15
  ?G
• Turbulent magnetic field 20 ?G 13
  ?G
• Turbulent fields are strongest in spiral arms
• Regular fields are strong in arm interarm
  regions
• Interarm fields are underestimated due to
  synchrotron loss of the electrons

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NGC6946 (Beck Hoernes 1996)
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Energy densities in NGC6946

• (Beck 2004)
• Emagn Eturb (inner disk)
• Emagn gt Eturb (outer disk)
• Emagn Etherm (everywhere)
• ß Etherm/Emagnlt 1

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Radial scale lengths in NGC6946

• Cold gas 4 kpc
• Turbulent motions 4 kpc
• Warm gas 4 kpc
• Cosmic rays 8 kpc
• Magnetic field 16 kpc

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Magnetic fields (and cosmic rays) extend far
beyond the star formation regions but there is
no supernova-driven turbulence - evidence for
magneto-rotational instability (MRI) ?
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NGC1097 Center Beck et al. 2004
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NGC7552 Center Harnett et al. 2004
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Equipartition field strengths in starbursts

• Starburst galaxies 30 - 50µG
• NGC 1097 (ring knots) 60µG
• NGC 7552 (ring knots) 100µG
• The strongest extended fields detected so far
  in spiral galaxies
• (still lower limits due to bremsstrahlung loss)

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Mass inflow by magnetic stress

• dM/dt - h/O (ltbr bFgt Br BF)
• (Balbus Hawley 1998)
• NGC1097
• h100pc, v450km/s, brbF60µG
• dM/dt 1 Mo / yr

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Open questions - I

• On which scales in space time is equipartition
  between fields and cosmic rays valid ?
• How far out do the magnetic fields extend in
  galaxies ?
• Can the fields affect gas rotation in the
  outermost regions of galaxies ?

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Comparison of M51 maps
Greenawalt et al. 1998
Fletcher et al. 2005
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Comparison of M51 maps
Roussel et al. 2001
Fletcher et al. 2005
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Wavelet scale-by-scale correlation in M51(using
isotropic 2-D wavelets)
15?m against total radio intensity
Patrickeyev, Fletcher, et al. (2004)
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The radio infrared correlation

• One of the tightest correlations in astronomy !
• The correlation may be the result of field
  coupling in gas clouds Btot ? ?0.5
• (where ? is the gas density averaged over a
  large volume)
• and of the Schmidt law SFR ? ?1.4
• (Niklas Beck 1997)

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Parker 1972
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Open questions - II

• How does the coupling between fields and gas work
  ? Which gas component takes care of this relation
  ?
• Does the radio-IR correlation break down at low
  SFR ?
• Does the correlation break down at small spatial
  and/or time scales ?
• Does the correlation break down at high redshifts
  (very young galaxies in the early Universe)?

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Chapter TwoStrength and structure of regular
magnetic fields
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Measuring regular fields

• Polarized emission (and angles)
• I ? ? nCR B-1a dl
• Faraday rotation measures of the diffuse
  polarized emission
• RM ? ? ne B dl
• RM grid of polarized background sources (see
  poster 36 by Bryan Gaensler)

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The regular field has two components

• Anisotropic turbulent field with
  frequent reversals
• (due to shear, compression or the
• magneto-rotational instability)
• Coherent field
• with constant direction
• (generated by the large-scale,
• mean-field dynamo)

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A regular magnetic field is not always a coherent
(mean) field
Polarization weak strong
strong ? regular field no yes
yes
Faraday rotation weak strong
weak ? coherent field no yes
no
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Regular magnetic fields prefer spiral patterns
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NGC4414 (Soida et al. 2002)

• Flocculent galaxies
• spiral field without
• spiral arms

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NGC4449 (Chyzy et al. 2000)

• Large
• Irregulars
• some
• traces of
• spiral field

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IC10 (Chyzy et al. 2005)

• Small
• Irregulars
• no spiral
• field

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NGC1097 Center (Beck et al. 2004)
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Regular magnetic fields prefer quiet regions
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NGC6946 (Beck Hoernes 1996)

• Magnetic arms

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Regular fieldsfollow the density-wave spiral arms
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Magnetic field and molecular gas
Polarized intensity (EffelsbergVLA) and BIMA CO
data (Regan et al. 2001)
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• Resolution
• 4
• (200pc)

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Inner spiral arms of M51
TWO components of the regular field !
gas flow
Green CO Blue radio polarization
3?
inter-arm
arm
Patrickeyev, Fletcher, et al. (2004)
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Regular fields follow the shearing gas
flowaround massive bars
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NGC1097 (Beck et al. 2004)
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NGC1097 (Beck et al. 2004)
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NGC1365 (Beck et al. 2004)
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Regular fields are impressed by external forces
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NGC 4254 Polarized Intensity B-Vectors
NGC4254 (Chyzy et al.)
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NGC3627 (Soida et al. 2001)
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The Antennae (Chyzy Beck 2004)
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Regular fields results

• Field lines form spiral patterns (even in
  flocculent and irregular galaxies and in
  circumnuclear rings)
• Two populations of strong regular fields
• (1) Magnetic arms in interarm regions of
  galaxies with prominent spiral arms
• (M51, M81, NGC6946,...)
• (2) on or inside massive molecular spiral arms of
  galaxies with strong density waves (M51)
• Field lines follow the gas flow around bars
• Field lines may be strongly distorted by tidal
  forces or ram pressure

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Faraday rotation is the key to detect coherent
fields and to test dynamos
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M31 very regular (coherent) field revealed by
rotation measures
Fletcher et al. 2004
The coherent magnetic field in M31 is the best
evidence so far for dynamo action !
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M51 chaotic rotation measures
Fletcher et al. 2005
150 rad m-2 0 rad m-2 -150 rad m-2
The regular magnetic field in M51 is not a
coherent field!
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Large-scale modesof the coherent field

• Simple RM patterns, i.e. a single dominant
  axisymmetric (m0) mode are rare.
• (M31, IC342, LMC see poster 35 by Bryan
  Gaensler)
• Dominating bisymmetric (m1) modes are even rarer
  (M81 is the only candidate).
• Magnetic arms can be described as a superposition
  of the m0 and m2 modes.
• A superposition of 3 (or more) modes is needed in
  most cases, but these cannot be resolved by
  present-day data.

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Preferred direction ?
F.Krause Beck (1998)
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Open questions - III

• Does the regular magnetic field affect the gas
  flow ?
• Do magnetic fields help to form spiral arms ?
• Is the radial component of spiral fields
  preferably directed inwards ?
• Do shear or compression generate anisotropic
  turbulent fields ?
• Can dynamos explain the coherent fields ?

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Do dynamos work in galaxies ?

• YES
• Spiral fields occur almost everywhere, even
• in irregular galaxies and central rings
• Pitch angles are as predicted
• Magnetic arms occur between gas arms
• Large-scale coherent fields exist
• There is at least one case of a
• dominating axisymmetric mode (M31)

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Do dynamos work in galaxies ?

• NO
• - Single dominating modes are rare
• (nonlinear dynamos?)
• - Coherent fields are surprisingly weak in
• galaxies with strong density waves (M51)
• (strong compression and/or shear?)
• - Spiral fields extend well into the centers
• - Dynamos cannot explain the preferred
• inward direction (large-scale seed fields?)
  
• - Fields are still strong in outer galaxies
• (magneto-rotational instability?)

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Finally, Chapter ThreeMagnetic fields in the
Milky Way
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Galactic center (La Rosa et al. 2000)

• (see talks by
• Giles Novak
• and
• Farhad Yusef-Zadeh)

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21cm Stockert Villa Elisa all-sky survey
(Reich Reich 1986)
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Equipartition fields in the Galaxy (Berkhuijsen,
priv. comm.)
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• Galactic polarization
• shows the details

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21cm DRAOVilla Elisa all-sky polarization
survey (Wolleben et al. 2004)
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Synchrotron Emission from the Milky Way (Perseus
- Auriga)
b4
Galactic polarization opens a new domain to study
small-scale magnetic fields
b-4
l150
l166
Total emission
Polarized emission
Effelsberg 21cm (Reich et al 2003)
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21cm ATCA southern Galactic plane survey
(Gaensler et al. 2001)
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11cm Effelsberg Galactic plane survey (Duncan et
al. 1999)
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Large-scale fields in the Milky Way
?
Han et al. 2001
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Future needs

• Higher sensitivity
• Higher angular resolution
• Wider frequency bands
• More attention to magnetic fields in the
  astronomical community

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The Square Kilometer Array ( S K A )

• The future of nonthermal radio astronomy

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SKA Concepts
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SKA Key Science

• - Testing Theories of Gravitation with pulsars
• - The Dark Ages
• Epoch of re-ionisation, first black holes
• - The Cradle of Life
• Protoplanets, biomolecules, SETI
• - Evolution Large-scale Structure Galaxies,
  Hubble Flow Dark Energy
• - Cosmic Magnetism (poster 36)

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Rotation Measures in the Milky Way
Pulsars to be detected with the SKA (Cordes 2001)
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RMs Through Galaxies
RMs of 21 polarized sources shining through M31
(Han et al 1998)
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TRACE 2000 (Sun in X-rays)
SKA 2015 (galaxies in radio)
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Fundamental questions

• STRUCTURE v
• EVOLUTION
• ORIGIN (enjoy the Friday session !)
• You are invited to attend the conference
• Origin Evolution of Cosmic Magnetism
• Bologna, 2005 Aug 29 - Sep 2