Magnetic fields in our GalaxyMagnetic fields in our GalaxyJinLin HanJinLin Han

National Astronomical ObservatoriesNational Astronomical ObservatoriesChinese Academy of SciencesChinese Academy of Sciences

Beijing, ChinaBeijing, Chinahjl@bao.ac.cnhjl@bao.ac.cn

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

Magnetic fields in our GalaxyMagnetic fields in our GalaxyWhat RMs of pulsars & EGRes can tell us?What RMs of pulsars & EGRes can tell us?

• 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

We advanced a lot of knowledge on magnetic fields of We advanced a lot of knowledge on magnetic fields of our Galaxy in last 10 years! --- More coming.our Galaxy in last 10 years! --- More coming.

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⊥ ------------- 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 ------ 550 pulsars + >2000 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

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, 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)

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

Magnetic field Magnetic field around GCaround GC

Comparison of magnetic fields Comparison of magnetic fields of nearby Galaxies to our ownsof nearby Galaxies to our owns

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

Poloidal & Toroidal fields near GCPoloidal & Toroidal fields near GCLarge-scale Large-scale ToroToroidal fieldsidal fieldspermeated in thecentral molecular zonecentral molecular zone (170pc*30pc)(170pc*30pc)• 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!? (Yusef-Zadeh et al., 1984;1997 Morris 1994; Lang et al.1999)

(after Novak et al. 2003)

150pc

GC

Predicted B-direction

Magnetic fields in our Galaxy: near GCnear GCSpiral arms to centre:

continue near GC? – Yes in NGC 2997 (Han et al. 1999)–- How strong? Do not know!

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

(from B.D.C. Chandran 2000)

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

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

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, 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)

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 ------ 550 pulsars + >2000 EGSes

Pulsars: Uique 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 <B>Direct <B>

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

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 ? ? ? ? )

Galactic B-Field ： 10 years ago

Sun Galactic center

Disk field:A few kpc!

Center:Poloidal field

3 models: which ？

Halo field ：no idea ！

30kpc

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)

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

<B> away from usRM<0

RM>0<B> 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

Unpublished databaseUnpublished database ……EGRes ……EGRes

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

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……

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

»directions directions »StrengthStrength

•magnetic energy spectrummagnetic energy spectrum

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

(Han et al. 2005, to be submitted)(Han et al. 2005, to be submitted)

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

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?

????

Pulsar RM distribution in Galactic plane

red: new measurements by Parkes 64m telescope (Han et al. 2005, to be submitted)(Han et al. 2005, to be submitted)

Large-scale magnetic field in the Galactic disk (Han et al. 2005, to be submitted)(Han et al. 2005, to be submitted)

• alwaysalways counter counterclockwise clockwise in arm region!in arm region!• clockwiseclockwise in in interarm regioninterarm region ? ?• Different from previous models ! Tight BSSDifferent from previous models ! Tight BSS ？？• More data still needed!More data still needed!

Weisberg et al. 2004:

large-scale magnetic fields

lies in arm region?!

Yes!

Large-scale magnetic field in the Galactic disk (Han et al. 2005, to be submitted)(Han et al. 2005, to be submitted)

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

»directionsdirections »StrengthStrength

•magnetic energy spectrummagnetic energy spectrum

Radial dependence of regular field strengthRadial dependence of regular field strength

(Han et al. 2005, to be submitted)(Han et al. 2005, to be submitted)

GDM

RMB

232.1＝

Radial dependence of regular field strengthRadial dependence of regular field strength

(Han et al. 2005, to be submitted)(Han et al. 2005, to be submitted)

kpc5.10.7

G2.06.1

])(

exp[

0

0regular

B

B

R

BR

RRB(R)B

＝

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

Spatial 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

We now can tell about ---- We now can tell about ---- Global structure of Galactic magnetic fieldGlobal structure of Galactic magnetic field

• Halo field structure

• Disk field structure

Galactic B-Field ： 10 years ago

Sun Galactic center

Disk field:A few kpc!

Center:Poloidal field

3 models: which ？

Halo field ：no idea ！

30kpc

• Spatial Energy spectrum

• Radial dependenceRadial dependence

Conclusive Remarks: Conclusive Remarks: More data neededMore data needed ---- Pulsars are unique powerful probesPulsars are unique powerful probes • Halo field

• Disk field

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

Thanks for your attention.Thanks for your attention.

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!

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

Rainer Beck: Rainer Beck: 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)