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Observations of Magnetic Fields in (normal) Galaxies Rainer Beck & Elly 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): 

Observations of Magnetic Fields in (normal) Galaxies Rainer Beck & Elly 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)

Fundamental questions: 

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 ?

Motivations to study magnetic fields in galaxies: 

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

Outline: 

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)

Observation of magnetic fields: 

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 Crutcher´s talk)

Slide6: 

Distorted by scattered light ! NGC6946 Optical polarization (Fendt et al. 1998)

Slide7: 

Effelsberg VLA ATCA

Slide8: 

Total synchrotron intensity traces the total magnetic field M51 Total intensity (Fletcher, Beck et al. 2005)

Slide9: 

Polarized synchrotron intensity traces the regular magnetic field M51 Polarized intensity (Fletcher, Beck et al. 2005)

Chapter One: How strong are interstellar magnetic fields ?: 

Chapter One: How strong are interstellar magnetic fields ?

(Total field)2 = (turbulent field)2 + (regular field)2: 

(Total field)2 = (turbulent field)2 + (regular field)2

Measuring total field strength: 

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 (K≈100 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 !

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

Revised equipartition formula (based on integration of the proton spectrum) Beq,┴  ( Isync (K+1) / L ) 1/(3+α) Isync: Synchroton intensity K: Proton/electron ratio (at GeV energies) L: Pathlength through source α: Synchrotron spectral index (S  - )

Slide14: 

Beck & Krause (2004)

Equipartition field estimates: 

Equipartition field estimates The classical estimate is too high for radio spectral indices of ≥0.7 and field strengths of >10μG. The popular choice of K=0 or K=1 (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.

Synchrotron Ages: 

Synchrotron Ages tsyn ≈ 1 Gyr (B┴ /μG)-1.5 (νsyn /GHz)-0.5 An underestimate in B┴ leads to an overestimate of the synchrotron age (& an underestimate of the propagation speed) νsyn

Slide17: 

There are magnetic fields inside and outside of the “ring” !

Total field strengths: 

Total field strengths Niklas´survey of 74 spiral galaxies: <Btot> = 9 μG Niklas 1995

Equipartition magnetic field strengths in M51: 

Equipartition magnetic field strengths in M51 Fletcher,Beck, et al. 2005

Synchrotron losses of cosmic-ray electrons in M51: 

Synchrotron losses of cosmic-ray electrons in M51

Slide21: 

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

Slide22: 

NGC6946 (Beck & Hoernes 1996)

Energy densities in NGC6946: 

Energy densities in NGC6946 (Beck 2004) Emagn ≈ Eturb (inner disk) Emagn > Eturb (outer disk) Emagn » Etherm (everywhere) β = Etherm/Emagn< 1

Radial scale lengths in NGC6946: 

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

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) ?: 

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) ?

Slide27: 

NGC1097 Center Beck et al. 2004

Slide28: 

NGC7552 Center Harnett et al. 2004

Equipartition field strengths in starbursts: 

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)

Mass inflow by magnetic stress: 

Mass inflow by magnetic stress dM/dt = - h/Ω (<br bΦ> + Br BΦ) (Balbus & Hawley 1998) NGC1097: h=100pc, v=450km/s, br≈bΦ≈60μG : dM/dt ≈ 1 Mo / yr

Open questions - I: 

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 ?

Comparison of M51 maps: 

Comparison of M51 maps Greenawalt et al. 1998 Fletcher et al. 2005

Comparison of M51 maps: 

Comparison of M51 maps Roussel et al. 2001 Fletcher et al. 2005

Wavelet scale-by-scale correlation in M51 (using isotropic 2-D wavelets): 

Wavelet scale-by-scale correlation in M51 (using isotropic 2-D wavelets) 15m against total radio intensity Patrickeyev, Fletcher, et al. (2004)

The radio – infrared correlation: 

The radio – infrared correlation One of the tightest correlations in astronomy ! The correlation may be the result of field coupling in gas clouds: Btot  ρ≈0.5 (where ρ is the gas density averaged over a large volume) and of the Schmidt law: SFR  ρ≈1.4 (Niklas & Beck 1997)

Slide36: 

Parker 1972

Open questions - II: 

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)?

Chapter Two: Strength and structure of regular magnetic fields: 

Chapter Two: Strength and structure of regular magnetic fields

Measuring regular fields: 

Measuring regular fields Polarized emission (and angles): I  ∫ nCR B┴1+α 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)

The regular field has two components:: 

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)

A regular magnetic field is not always a coherent (“mean”) field :: 

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

Regular magnetic fields prefer spiral patterns : 

Regular magnetic fields prefer spiral patterns

Slide44: 

Flocculent galaxies: spiral field without spiral arms NGC4414 (Soida et al. 2002)

Slide45: 

Large Irregulars: some traces of spiral field NGC4449 (Chyzy et al. 2000)

Slide46: 

Small Irregulars: no spiral field IC10 (Chyzy et al. 2005)

Slide48: 

NGC1097 Center (Beck et al. 2004)

Regular magnetic fields prefer quiet regions: 

Regular magnetic fields prefer quiet regions

Slide50: 

“Magnetic arms” NGC6946 (Beck & Hoernes 1996)

Regular fields follow the density-wave spiral arms: 

Regular fields follow the density-wave spiral arms

Slide54: 

Magnetic field and molecular gas Polarized intensity (Effelsberg+VLA) and BIMA CO data (Regan et al. 2001)

Slide55: 

Resolution 4” (≈200pc)

Slide56: 

Inner spiral arms of M51 gas flow arm inter-arm 3 TWO components of the regular field ! Green: CO Blue: radio polarization Patrickeyev, Fletcher, et al. (2004)

Regular fields follow the shearing gas flow around massive bars: 

Regular fields follow the shearing gas flow around massive bars

Slide58: 

NGC1097 (Beck et al. 2004)

Slide59: 

NGC1097 (Beck et al. 2004)

Slide60: 

NGC1365 (Beck et al. 2004)

Regular fields are impressed by external forces: 

Regular fields are impressed by external forces

Slide62: 

NGC 4254 Polarized Intensity + B-Vectors NGC4254 (Chyzy et al.)

Slide63: 

NGC3627 (Soida et al. 2001)

Slide64: 

The Antennae (Chyzy & Beck 2004)

Regular fields: results : 

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

Slide66: 

Faraday rotation is the key to detect coherent fields and to test dynamos

M31: very regular (coherent) field revealed by rotation measures: 

M31: very regular (coherent) field revealed by rotation measures The coherent magnetic field in M31 is the best evidence so far for dynamo action ! Fletcher et al. 2004

M51: chaotic rotation measures: 

M51: chaotic rotation measures 150 rad m-2 0 rad m-2 -150 rad m-2 The regular magnetic field in M51 is not a coherent field! Fletcher et al. 2005

Large-scale modes of the coherent field: 

Large-scale modes of the coherent field Simple RM patterns, i.e. a single dominant axisymmetric (m=0) mode are rare. (M31, IC342, LMC – see poster #35 by Bryan Gaensler) Dominating bisymmetric (m=1) modes are even rarer (M81 is the only candidate). Magnetic arms can be described as a superposition of the m=0 and m=2 modes. A superposition of 3 (or more) modes is needed in most cases, but these cannot be resolved by present-day data.

Slide70: 

Preferred direction ? F.Krause & Beck (1998)

Open questions - III: 

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 ?

Do dynamos work in galaxies ?: 

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)

Do dynamos work in galaxies ?: 

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?)

Finally, Chapter Three: Magnetic fields in the Milky Way: 

Finally, Chapter Three: Magnetic fields in the Milky Way

Slide76: 

(see talks by Giles Novak and Farhad Yusef-Zadeh) Galactic center (La Rosa et al. 2000)

Slide77: 

21cm Stockert + Villa Elisa all-sky survey (Reich & Reich 1986)

Slide78: 

Equipartition fields in the Galaxy (Berkhuijsen, priv. comm.)

Slide79: 

Galactic polarization shows the details

Slide80: 

21cm DRAO+Villa Elisa all-sky polarization survey (Wolleben et al. 2004)

Synchrotron Emission from the Milky Way (Perseus - Auriga): 

Effelsberg 21cm (Reich et al 2003) Synchrotron Emission from the Milky Way (Perseus - Auriga) Total emission Polarized emission Galactic polarization opens a new domain to study small-scale magnetic fields l=166° l=150° b=-4° b=+4°

Slide82: 

21cm ATCA southern Galactic plane survey (Gaensler et al. 2001)

Slide83: 

11cm Effelsberg Galactic plane survey (Duncan et al. 1999)

Large-scale fields in the Milky Way: 

Large-scale fields in the Milky Way Han et al. 2001 ?

Future needs: 

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

The Square Kilometer Array ( S K A ): 

The Square Kilometer Array ( S K A ) The future of nonthermal radio astronomy

SKA Concepts: 

SKA Concepts

SKA Key Science: 

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)

Rotation Measures in the Milky Way: 

Rotation Measures in the Milky Way Pulsars to be detected with the SKA (Cordes 2001)

RMs Through Galaxies: 

RMs Through Galaxies RMs of 21 polarized sources shining through M31 (Han et al 1998)

Slide91: 

TRACE 2000 (Sun in X-rays) SKA 2015 (galaxies in radio)

Fundamental questions: 

Fundamental questions STRUCTURE √ EVOLUTION ORIGIN (enjoy the Friday session !) You are invited to attend the conference : “Origin & Evolution of Cosmic Magnetism” Bologna, 2005 Aug 29 - Sep 2

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