thanks for cooperation with dick manchester (atnf, australia), g.j. qiao (pku, china),

Magnetic fields in our GalaxyMagnetic fields in our GalaxyHow much do we know How much do we know

JinLin HanJinLin HanNational Astronomical ObservatoriesNational Astronomical Observatories

Chinese Academy of SciencesChinese Academy of Sciences Beijing, ChinaBeijing, [email protected]@bao.ac.cn

Thanks for cooperation with Dick ManchesterDick Manchester (ATNF, Australia), G.J. Qiao G.J. Qiao (PKU, China), A.G. LyneA.G. Lyne (Jodrell Bank, UK), K. Ferriere K. Ferriere (Obs. Midi-Pyr. France)

Galactic B field: Galactic B field: How much How much we we want to knowwant to know

B-StrengthB-Strength1. Random vs. ordered ( <δB>2/B2 )2. Local vs. large-scale B~ f(R)? B~ f(z)?

B-StructureB-Structure 1. Disk field: local 2. Disk: large stru

cture?3. Direction rever

sal in arm or interarm

4. Field in halo?5. Field near GC?

Strength vs. Strength vs. scalesscales

Spatial B-Spatial B-Energy Energy

SpectrumSpectrum

Magnetic fields in our Magnetic fields in our

GalaxyGalaxyHow much do we know ?How much do we know ?

• Some background, remindingSome background, reminding

• Knowledge 10 years agoKnowledge 10 years ago

• Current knowledgeCurrent knowledge– Central field & halo field Central field & halo field

– disk fielddisk field

» directions directions

» strengthstrength

– spatial magnetic energy spectrumspatial magnetic energy spectrum

• What we should know in futureWhat we should know in future

Observational tracers of magnetic fieldsObservational tracers of magnetic fields• Polarization of starlight: perpendicular field in 2 or 3 kpcperpendicular field in 2 or 3 kpc

orientation // B⊥ ------------- 9000 stars

• Zeeman splitting: parallel field, parallel field, in situin situ (masers, clouds) (masers, clouds) △ ∝ B// ------ 30 masers

• Polarization at infrared, mm: perpendicular field perpendicular field orientation // B⊥ ------ clouds & star formation regions

• Synchrotron radiation: vertical field structures (added)vertical field structures (added)

total intensity S ∝ B⊥2/7, p% B∝ ⊥u

2 / B t⊥2

• Faraday rotation: parallel field, integrated (the halo & disk)parallel field, integrated (the halo & disk)

RM ∝ ∫ ne B// ds ------ 500 pulsars + >1000 EGSes

• 9000 stars have polarization measured• mostly nearby (1~2kpc)• polarization percentage increases with distance

Zweibel & Heiles 1997, Nature 385,131 Berdyugin & Teerikorpi 2001, A&A 368,635

Starlight polarization:Starlight polarization: local field // armlocal field // arm

Zeeman Effect: Zeeman Effect: B in molecular cloudsB in molecular clouds

Bourke et al. 2001, ApJ 554, 916

>30 people working for >30 people working for >30 years, get>30 years, get

<30 good measurements!<30 good measurements!

Difficult &Difficult & Bad Luck!Bad Luck!

Maser B-fields: Nothing to do with large-scale B-field?!Nothing to do with large-scale B-field?!

ne: ISM: 1cm-3 ==> GMC: 103cm-3

==> OH-maser: 107cm-3

Fish et al. 2003

Reid & Silverstein 1990, ApJ 361, 483

41 clockwise33 counterclockwiseAssume Bφ>> Br or Bz



• Zeeman splitting: parallel field, in situ (masers, clouds)parallel field, in situ (masers, clouds) △ ∝ B// ------ 30 masers

• Polarization at infrared, mm: perpendicular field perpendicular field orientation // B⊥ ------------- star formation regions

• Synchrotron radiation: vertical field structures (added)vertical field structures (added)


2 / B t⊥2



Polarization at mm, sub-mm, infraredWorking toward measure B-field of galactic scale

• thermal emission (of dusts)• aligned by B-field in the clouds

Hildebrand et al. PASP 112, 1215

Synchrotron radiation: transverse B-structurestransverse B-structuresGlobal B-field structure from linearly polarized emission

How to distinguish?How to distinguish?

RM maps helps on directions of RM maps helps on directions of (disk &) halo field!(disk &) halo field!

MPIFR has a group working on MPIFR has a group working on this for 25 years!this for 25 years!

No information of B-directions!

Han et al. 1999, A&A 384, 405

Two Possible origin of polarization:

• Large-scale magnetic field as vectors shown (convention)Large-scale magnetic field as vectors shown (convention)• Anisotropic random field compressed by large-scale density waveAnisotropic random field compressed by large-scale density wave



• Zeeman splitting: parallel field, in situ (masers, clouds)parallel field, in situ (masers, clouds) △ ∝ B// ------ 30 masers

• Polarization at infrared, mm : perpendicular field perpendicular field orientation // B⊥ ------------- star formation regions

• Synchrotron radiation : vertical field structures (added)vertical field structures (added)


2 / B t⊥2



Pulsars: Best probes for Large-scale Galactic B-field

• Widely distributed in GalaxyWidely distributed in Galaxy• Distance from DM: Distance from DM: 3-D3-D B-field B-field• Linearly polarized: RM easy to obsLinearly polarized: RM easy to obs• No intrinsic RMs: No intrinsic RMs: Direct Direct 

Pulsar distribution

Why? Pulsars as probes for Galactic B-field• Polarized. Widely spread in our Galaxy. Faraday rotation:

• Distances estimated from pulse dispersion:

» <===<=== the delay tells DM»

» the rotation of position» angles tells RM value ===>===>

» Average field strength is

0

Dist

eDM n dl

sec103.83

3

MHz

DMt

22

21

21

PAPA

RM

GDM

RMB 232.1＝

Dist

e

PSR

Sunobs

ee

dlnBdll

lBlncm

eRM

0||2

||42

3

820.0])(

)[()(2

• Ordered:Ordered: irregular (random) + regular (uniform)

• Line of sight:Line of sight: perpendicular + parallel

• StructuralStructuraltoroidal (azimuthal) + poloidal (vertical)

• LocationsLocationsLocalized features &/or Global structure

Just remind you ……

Magnetic field should have complicated structure, but often is

artificially said to have components of

Our Milky Way Galaxy: What structure? Our Milky Way Galaxy: What structure?

• Our Galaxy: we live near the we live near the

edgeedge

• We do not knowdo not know the structure

of our own Galaxy

• We want to work out (via We want to work out (via

radio view)radio view) how its magnetic

field looks like, and where it

originates from?

Optical sky

Radio sky

• how many spiral arms: 2 or 4, or 3?• pitch angle of spiral: 8o, 10o or 14o ?

KnowledgeKnowledge ofof

10 years ago ……10 years ago ……

• disk field:disk field:

* 3 models * which one?which one?

Concentric Rings Axi-symmetric Bi-Symmetric Spiral Concentric Rings Axi-symmetric Bi-Symmetric Spiral

Rings model spiral (ASS) (BSS)Rings model spiral (ASS) (BSS)

Galactic magnetic fields: 10 years Galactic magnetic fields: 10 years agoago

• Halo field:Halo field:

* no idea on halo fieldno idea on halo field * Poloidal fieldsPoloidal fields near GC:near GC: YesYes

see see nonthermal filamentsnonthermal filaments

Axi-Symmetric Spiral model by J.P. Vallee• Main Problem: fields go across the arms• Just one Just one radius radius range for reversedrange for reversed fields fields• Not consistent with field reversals near -- Perseus arm???? -- the Norma arm !!the Norma arm !!

BSS reversalBSS reversalBSS reversal

?

?

Ring model: Concentric rings of reversed fields• Selection effect problem ??Selection effect problem ??• Field lines go across the arms?Field lines go across the arms?• Inconsistent Formula for the BSS when Inconsistent Formula for the BSS when

modeling ??modeling ??It is the zero-order modelling only fIt is the zero-order modelling only f

or azimuthal magnetic field !or azimuthal magnetic field !

by R. Rand & S. Kulkarni (1989)

R. Rand & A.Lyne(1994)

There were not as many pulsar RMs as today….There were not as many pulsar RMs as today….

Bi-Symmetric Spiral ModelBi-Symmetric Spiral ModelProposed from RMs of

Extragalactic Radio Sources:

Simard-Normandin & Kronberg (1980)

Sofue & Fujimoto (1983)

Confirmed by Pulsar RMs: Han & Qiao (1994) Indrani & Deshpande (1998) Han, Manchester, Qiao (1999) Han,Manchester, Lyne, Qiao(2002)

Supported by starlight polarization

Heiles (1996)

The best match to all evidenceThe best match to all evidence field reversals & pitch angle – 8°±2°( the field stronger in interarm region ? ? ? ? )

Current knowledge ……Current knowledge ……•Central field & halo field Central field & halo field •disk field: disk field: directions & Strengthdirections & Strength•magnetic energy spectrummagnetic energy spectrum

Poloidal & Toroidal fields near GCPoloidal & Toroidal fields near GCToroidal fieldsToroidal fields

(Novak et al. 2003, 2000)• permeated in the central molecular zonecentral molecular zone (400pc*50pc)(400pc*50pc)

• sub-mm obs of p%• toroidal field directions determined by averaged RMs of plumes or SNR!

Poloidal fieldPoloidal field filaments Unique to GC --- dipolar geometry! (Morris 1994; Lang et al.1999)

(from Novak et al. 2003)

150pc

GC

Predicted B-direction

Magnetic fields in our Galaxy: near GCnear GC

Spiral arms & B- fields continue near GC?

– Yes in NGC 2997 (Han et al. 1999)–- How strong?

Poloidal fields– reason for jets?– dipole field?– related to vertical-B?– how strong?

(from B.D.C. Chandran 2000)

Optical sky

Radio sky

To study halo field: unique to our Galaxyunique to our Galaxy

• The largest edge-on Galaxy in the sky

• Pulsars and extragalactic radio sources as probes

PulsarsRM distribution

• The largest edge-on Galaxy in the sky

• Pulsars and extragalactic radio sources as probes

Extragalactic Radio SourcesExtragalactic Radio Sources RM distribution

To study halo field: unique to our Galaxyunique to our Galaxy

 away from usRM<0

RM>0 to us

Anti-symmetric RM sky: Anti-symmetric RM sky: A0 dynamoA0 dynamo (Han et al. 1997 A&A322, (Han et al. 1997 A&A322, 98)98)

Evidence for global scale

• High anti-symmetry to the

Galactic coordinates

• Only in inner Galaxy

• nearby pulsars show it at

higher latitudes

Implications

• Consistent with field

configuration of A0

dynamo

• The first dynamo mode The first dynamo mode

identified on galactic identified on galactic

scalesscales

Bv

Unique measurement

of Vertical B-component

Bv ＝ 0.

2 ～ 0.3G pointing from SGP to NGP (Effect of the NPS discounted already!)

Local vertical components: fromLocal vertical components: from poloidal field?poloidal field?

South Galactic Pole

North Galactic Pole

(see Han & Qiao 1994; Han et al. 1999)

Magnetic field configurations for basic dynamos

M31: only 21 polarized brightbackground sources available !!

Han, Beck, Berkhuijsen (1998):

An even mode (S0) dynamo may operate in M31 !

A0

S0

S1

Current knowledge ……Current knowledge ……•Central field & halo field Central field & halo field •disk field: disk field:

»directions directions »StrengthStrength

•magnetic energy spectrummagnetic energy spectrum

Bi-Symmetric Spiral ModelBi-Symmetric Spiral ModelProposed from RMs of

Extragalactic Radio Sources:

Simard-Normandin & Kronberg (1980)

Sofue & Fujimoto (1983)

Confirmed by Pulsar RMs: Han & Qiao (1994) Indrani & Deshpande (1998) Han, Manchester, Qiao (1999) Han,Manchester, Lyne, Qiao(2002)

Supported by starlight polarization

Heiles (1996)

The best match to all evidenceThe best match to all evidence field reversals & pitch angle – 8°±2°( the field stronger in interarm region ? ? ? ? )

CCW B-field along the Norma arm: from New Pulsar RMs

possible field directions

Field directions newly

determined

Han et al. 2002, ApJ 570, L17Han et al. 2002, ApJ 570, L17

Coherent B-fieldB-field directionsdirections >5 kpc along Norma ar

m

Another reversed field in large-sclarge-sc

aleale?

????

Large-scale magnetic field in the Galactic disk

The largest The largest coherentcoherent field structrue detected in the Universe! field structrue detected in the Universe!

How to distinguish?How to distinguish?

RM maps helps on directions of RM maps helps on directions of (disk &) halo field!(disk &) halo field!

MPIFR has a group working on MPIFR has a group working on this for 25 years!this for 25 years!

No information of B-directions!

Synchrotron radiation: transverse B-structurestransverse B-structuresGlobal B-field structure from linearly polarized emission

Han et al. 1999, A&A 384, 405

Two possible reasons for polarization of Synchrotron radiation ofSynchrotron radiation of External Galaxies:External Galaxies:

• Large-scale magnetic field as vectors shown (conventionally)Large-scale magnetic field as vectors shown (conventionally)• Anisotropic random fieldAnisotropic random field compressed by large-scale density wave compressed by large-scale density wave

Polarized radio mission origins fromPolarized radio mission origins from

large-scale uniform field?large-scale uniform field?

Based on pulsar data in oBased on pulsar data in o

ur galaxy =RM/DM:ur galaxy =RM/DM:

YES!YES! But maybeBut maybe partially partially

Han et al. 2002 ApJ 570, L17Han et al. 2002 ApJ 570, L17

Current knowledge ……Current knowledge ……•Central field & halo field Central field & halo field •disk field: disk field:

»directionsdirections »StrengthStrength

•magnetic energy spectrummagnetic energy spectrum

Current knowledge ……Current knowledge ……•Central field & halo field Central field & halo field •disk field: disk field: directions & Strengthdirections & Strength•magnetic energy spectrummagnetic energy spectrum

Why our Galaxy has magnetic field?Why our Galaxy has magnetic field?

Probably Dynamo!Probably Dynamo!

How dynamo works? How dynamo works?

Alpha-Omega effect.Alpha-Omega effect.

Dynamo Really works?Dynamo Really works?

Computer SimulationsComputer Simulations….….

Many Simulations of dynamosMany Simulations of dynamos---- ---- check spacial B-energy spectrum & its evolution check spacial B-energy spectrum & its evolution e.g. Magnetic energy distribution on different spatial scalese.g. Magnetic energy distribution on different spatial scales (k=1/λ) (k=1/λ)

Many papers by Many papers by • N.E. L. Haugen, A. Brandenburg, W. Dobler, …..N.E. L. Haugen, A. Brandenburg, W. Dobler, …..• A. Schekochihin, S.C. Cowley, S. Taylor, J. Moron, ….. A. Schekochihin, S.C. Cowley, S. Taylor, J. Moron, ….. • E. Blackman, J. Maron …..E. Blackman, J. Maron …..• Others …..Others …..

No real measurements No real measurements

to check whether to check whether

dynamo works or not! dynamo works or not!

Far away from telling anything about a real galaxy ……Far away from telling anything about a real galaxy ……

Don’t know much about the large-scale magnetic Don’t know much about the large-scale magnetic field ...field ...

What spatial magnetic energy spectrum What spatial magnetic energy spectrum

does does our Galaxy have?our Galaxy have?

Spatial fluctuation spectrum for electron densitySpatial fluctuation spectrum for electron density

““The Big Powerlaw in the SkyThe Big Powerlaw in the Sky””

B-field & electrons B-field & electrons

coupling? If so, coupling? If so,

B-energy spectrum?B-energy spectrum?

KolmogorovKolmogorovover 12 orderover 12 orders in scale?s in scale?

10 pc

1000 km

(Armstrong, Rickett & Spangler 1995)

Minter & Spangler 1996Minter & Spangler 1996

Spacial energy spectrum of B PreviouslyPreviously only only available information from available information from RM structure function RM structure function

λ< ~4pc: consistent to Kolmogorov 3D 80>λ> ~4pc: turbulence in 2D?

Pulsar RM distribution in Galactic planered: new measurements by Parkes 64m telescope

PSR

Sun e dllBlnRM )()( ||

Spatial magnetic energy spectrum of our GalaxySpatial magnetic energy spectrum of our Galaxy (Han et al. 2004, ApJ 610, 820)(Han et al. 2004, ApJ 610, 820)

Minter & Spangler Minter & Spangler 19961996

By pulsar By pulsar

RM/DMRM/DM

Email from A. MinterEmail from A. Minter

Conclusive Remarks Conclusive Remarks More data neededMore data needed ---- Best we can say Best we can say up to nowup to now • Halo field

• Disk field

• Spatial Energy spectrum

• Radial Radial DependenceDependence(unpublished)(unpublished)

If I have time, I tell you more aboutwhat we are doing ……

Thanks for your attention.Thanks for your attention.

Current doing & future ……Current doing & future ……

• Halo field: structure and field strengthHalo field: structure and field strength

• What difference: What difference:

fields in arm and interarmfields in arm and interarm

• Structure in large regions: More pulsars?Structure in large regions: More pulsars?

• Structure in more detailsStructure in more details

• Field in intergalactic spaceField in intergalactic space

RMs of EGRs for the halo B-field RMs of EGRs for the halo B-field Only about 1000 ROnly about 1000 RMs available in literMs available in literature upto now...ature upto now...

We are using EffelWe are using Effelsberg -100m telescsberg -100m telescope to make a RM ope to make a RM survey of 1700 sousurvey of 1700 sources, enlarge the crces, enlarge the cover density by a fover density by a factor of three in moactor of three in most sky area……st sky area……

Difference for RMs of PSRs & EGResDifference for RMs of PSRs & EGRes

Galactic plane polarization survey at 6cmGalactic plane polarization survey at 6cm

Urumqi 25m telescope

MPIfR 6cm receiver

Just messioned last week…. Will finish in 3 years ...Just messioned last week…. Will finish in 3 years ...

• Cooperation withCooperation with MPIfRMPIfR• System hardwareSystem hardware & software& software almost okalmost ok• No data at this No data at this frequencyfrequency• Less affacted by Less affacted by foreground RMforeground RM• Useful for CMB Useful for CMB polarizationpolarization

总强度图总强度图偏振强度图偏振强度图

Field reversals exterior to the PField reversals exterior to the Perseus arm -- it is fine!erseus arm -- it is fine!Brown et al. 2003, ApJ 593, L29Mitra et al. 2003, A&A 398,993

Han et al. 1999

Han et al. 1999Lyne & Smith 1989

Evidence at 150<l<100 is very weak, butEvidence at 150<l<100 is very weak, butEvidence for Evidence for two reversalstwo reversals at l~70 is hard! at l~70 is hard!

Field reversals exterior to the Perseus armField reversals exterior to the Perseus arm

Han et al. 1999

Weisberg et al. 2004

Will be checked byWill be checked by

RMs of RMs of ALFA PSRsALFA PSRs

(to be discovered!)(to be discovered!)

Field strength & B-energy vs. Distance/scalesField strength & B-energy vs. Distance/scales(Han et al. 2004, ApJ 610, 820)(Han et al. 2004, ApJ 610, 820)

thanks for cooperation with dick manchester (atnf, australia), g.j. qiao (pku, china),

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