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Magnetic fields in our Galaxy How much do we know JinLin Han National Astronomical Observatories Chinese Academy of Sciences Beijing, China [email protected] Thanks for cooperation with Dick Manchester (ATNF, Australia), G.J. Qiao (PKU, China),

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slide1
Magnetic fields in our GalaxyHow much do we know JinLin HanNational Astronomical ObservatoriesChinese Academy of SciencesBeijing, [email protected]

Thanks for cooperation with

Dick Manchester (ATNF, Australia), G.J. Qiao (PKU, China),

A.G. Lyne (Jodrell Bank, UK), K. Ferriere (Obs. Midi-Pyr. France)

slide2
Galactic B field: How much we want to know
  • B-Structure
  • Disk field: local
  • Disk: large structure?
  • Direction reversal in arm or interarm
  • Field in halo?
  • Field near GC?

Strength vs. scales

Spatial B-Energy Spectrum

B-Strength

1. Random vs. ordered ( <δB>2/B2 )

2. Local vs. large-scale B~ f(R)? B~ f(z)?

magnetic fields in our galaxy how much do we know
Magnetic fields in our GalaxyHow much do we know ?
  • Some background, reminding
  • Knowledge 10 years ago
  • Current knowledge
        • Central field & halo field
        • disk field
          • directions
          • strength
        • spatial magnetic energy spectrum
  • What we should know in future
observational tracers of magnetic fields
Observational tracers of magnetic fields
  • Polarization of starlight:perpendicular field in 2 or 3 kpc

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

  • Zeeman splitting: parallel field, in situ (masers, clouds)

△ ∝ B// ------ 30 masers

  • Polarization at infrared, mm:perpendicular field orientation // B⊥------ clouds & star formation regions
  • Synchrotron radiation:vertical field structures (added)

total intensity S ∝ B⊥2/7, p%∝ B⊥u2 / B⊥t2

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

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

starlight polarization local field arm
Starlight polarization:local field // arm
  • 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

zeeman effect b in molecular clouds
Zeeman Effect: B in molecular clouds

>30 people working for >30 years, get

<30 good measurements!

Difficult &Bad Luck!

Bourke et al. 2001, ApJ 554, 916

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

Reid & Silverstein 1990, ApJ 361, 483

Fish et al. 2003

41 clockwise

33 counterclockwise

Assume Bφ>> Br or Bz

ne: ISM: 1cm-3 ==> GMC: 103cm-3 ==> OH-maser: 107cm-3

observational tracers of magnetic fields1
Observational tracers of magnetic fields
  • Polarization of starlight: perpendicular field in 2 or 3 kpc

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

  • Zeeman splitting: parallel field, in situ (masers, clouds)

△ ∝ B// ------ 30 masers

  • Polarization at infrared, mm:perpendicular field orientation // B⊥ ------------- star formation regions
  • Synchrotron radiation:vertical field structures (added)

total intensity S ∝ B⊥2/7, p%∝ B⊥u2 / B⊥t2

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

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

polarization at mm sub mm infrared working toward measure b field of galactic scale
thermal emission (of dusts)

aligned by B-field in the clouds

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

Hildebrand et al. PASP 112, 1215

slide10
Synchrotron radiation:transverse B-structuresGlobal B-field structure from linearly polarized emission
  • Two Possible origin of polarization:
    • Large-scale magnetic field as vectors shown (convention)
    • Anisotropic random field compressed by large-scale density wave

How to distinguish?

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

MPIFR has a group working on this for 25 years!

No information of B-directions!

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

observational tracers of magnetic fields2
Observational tracers of magnetic fields
  • Polarization of starlight: perpendicular field in 2 or 3 kpc

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

  • Zeeman splitting: parallel field, in situ (masers, clouds)

△ ∝ B// ------ 30 masers

  • Polarization at infrared, mm : perpendicular field orientation // B⊥ ------------- star formation regions
  • Synchrotron radiation : vertical field structures (added)

total intensity S ∝ B⊥2/7, p% ∝ B⊥u2 / B⊥t2

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

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

pulsars best probes for large scale galactic b field
Pulsars: Best probes for Large-scale Galactic B-field

Pulsar distribution

  • Widely distributed in Galaxy
  • Distance from DM: 3-D B-field
  • Linearly polarized: RM easy to obs
  • No intrinsic RMs: Direct
why pulsars as probes for galactic b field
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
slide14

Just remind you …… Magnetic field should have complicated structure, but often is artificially said to have components of

Ordered:

irregular (random) + regular (uniform)

Line of sight:

perpendicular + parallel

Structural

toroidal(azimuthal) + poloidal (vertical)

Locations

Localized features &/or Global structure

our milky way galaxy what structure
Our Milky Way Galaxy: What structure?

Optical sky

  • how many spiral arms:
  • 2 or 4, or 3?
  • pitch angle of spiral:
  • 8o, 10o or 14o ?
  • Our Galaxy: we live near the edge
  • We do not know the structure of our own Galaxy
  • We want to work out (via radio view) how its magnetic field looks like, and where it originates from?

Radio sky

slide16
Knowledgeof

10 years ago ……

disk field 3 models which one
Galactic magnetic fields: 10 years agodisk field:* 3 models* which one?
  • Halo field:*no idea on halo field * Poloidal fieldsnear GC:Yessee nonthermal filaments

Concentric Rings Axi-symmetric Bi-Symmetric Spiral

Rings model spiral (ASS) (BSS)

axi symmetric spiral model by j p vallee
Axi-Symmetric Spiral modelby J.P. Vallee
  • Main Problem: fields go across the arms
  • Just one radius range for reversed fields
  • Not consistent with field reversals near

-- Perseus arm??

-- the Norma arm !!

?

?

BSS reversal

BSS reversal

ring model concentric rings of reversed fields
by R. Rand & S. Kulkarni (1989)R. Rand & A.Lyne(1994)Ring model:Concentric rings of reversed fields
  • Selection effect problem ??
  • Field lines go across the arms?
  • Inconsistent Formula for the BSS when modeling ??

It is the zero-order modelling only for azimuthal magnetic field !

There were not as many pulsar RMs as today….

bi symmetric spiral model
Bi-Symmetric Spiral Model

Proposed from RMs of

Extragalactic Radio Sources:

Simard-Normandin & Kronberg (1980)

Sofue & Fujimoto (1983)

Confirmed byPulsar RMs:

Han & Qiao (1994)

Indrani & Deshpande (1998)

Han, Manchester, Qiao (1999)

Han,Manchester, Lyne, Qiao(2002)

Supported bystarlight polarization

Heiles (1996)

The best match to all evidence

field reversals & pitch angle – 8°±2°

( the field stronger in interarm region ? ? )

slide21
Current knowledge ……
  • Central field & halo field
  • disk field:

directions & Strength

  • magnetic energy spectrum
poloidal toroidal fields near gc

Poloidal & Toroidal fields near GC

(from Novak et al. 2003)

Predicted B-direction

GC

Toroidal fields

(Novak et al. 2003, 2000)

permeated in the

central molecular zone

(400pc*50pc)

sub-mm obs of p%

toroidal field directions

determined by averaged

RMs of plumes or SNR!

Poloidal field

filaments Unique to GC

--- dipolar geometry!

(Morris 1994; Lang et al.1999)

150pc

magnetic fields in our galaxy near gc

Magnetic fields in our Galaxy: near 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)

slide24
Optical sky

Radio sky

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

RM distribution

Pulsars

  • The largest edge-on Galaxy in the sky
  • Pulsars and extragalactic radio sources as probes
to study halo field unique to our galaxy1
To study halo field:unique to our Galaxy

Extragalactic Radio Sources

RM distribution

away

from us

RM<0

RM>0

to us

  • The largest edge-on Galaxy in the sky
  • Pulsars and extragalactic radio sources as probes
anti symmetric rm sky a0 dynamo han et al 1997 a a322 98
Anti-symmetric RM sky: A0 dynamo(Han et al. 1997 A&A322, 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 identified on galactic scales

Bv

local vertical components from poloidal field
Local vertical components: frompoloidal field?

North Galactic Pole

Unique measurement

of Vertical B-component

Bv=0.2~0.3G

pointing from SGP to NGP

(Effect of the NPS discounted already!)

South Galactic Pole

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

magnetic field configurations for basic dynamos
Magnetic field configurations for basic dynamos

A0

M31:only 21 polarized bright

background sources available !!

Han, Beck, Berkhuijsen (1998):

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

S0

S1

slide30
Current knowledge ……
  • Central field & halo field
  • disk field:
      • directions
      • Strength
  • magnetic energy spectrum
bi symmetric spiral model1
Bi-Symmetric Spiral Model

Proposed from RMs of

Extragalactic Radio Sources:

Simard-Normandin & Kronberg (1980)

Sofue & Fujimoto (1983)

Confirmed byPulsar RMs:

Han & Qiao (1994)

Indrani & Deshpande (1998)

Han, Manchester, Qiao (1999)

Han,Manchester, Lyne, Qiao(2002)

Supported bystarlight polarization

Heiles (1996)

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

possible field directions

Field directionsnewly determined

??

Coherent B-fielddirections>5 kpc along Norma arm

Another reversed field in large-scale?

Han et al. 2002, ApJ 570, L17

large scale magnetic field in the galactic disk
Large-scale magnetic field in the Galactic disk

The largest coherent field structrue detected in the Universe!

slide34
Synchrotron radiation:transverse B-structuresGlobal B-field structure from linearly polarized emission
  • Two possible reasons for polarization ofSynchrotron radiation ofExternal Galaxies:
    • Large-scale magnetic field as vectors shown (conventionally)
    • Anisotropic random field compressed by large-scale density wave

Polarized radio mission origins from

large-scale uniform field?

Based on pulsar data in our galaxy =RM/DM:

YES!

But maybe partially

Han et al. 2002 ApJ 570, L17

How to distinguish?

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

MPIFR has a group working on this for 25 years!

No information of B-directions!

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

slide35
Current knowledge ……
  • Central field & halo field
  • disk field:
      • directions
      • Strength
  • magnetic energy spectrum
slide36
Current knowledge ……
  • Central field & halo field
  • disk field:

directions & Strength

  • magnetic energy spectrum
slide37
Why our Galaxy has magnetic field?Probably Dynamo!How dynamo works? Alpha-Omega effect.

Dynamo Really works?

Computer Simulations….

slide38
Many Simulations of dynamos

---- check spacial B-energy spectrum & its evolution

e.g. Magnetic energy distribution on different spatial scales (k=1/λ)

  • No real measurements
  • to check whether
  • dynamo works or not!
  • Many papers by
  • N.E. L. Haugen, A. Brandenburg, W. Dobler, …..
  • A. Schekochihin, S.C. Cowley, S. Taylor, J. Moron, …..
  • E. Blackman, J. Maron …..
  • Others …..

Far away from telling anything about a real galaxy ……

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

slide40
Kolmogorov

over 12 orders in scale?

Spatial fluctuation spectrum for electron density

“The Big Powerlaw in the Sky”

(Armstrong, Rickett & Spangler 1995)

10 pc

B-field & electrons coupling? If so,

B-energy spectrum?

1000 km

slide41
Minter & Spangler 1996

Spacial energy spectrum of BPreviously only available information from RM structure function

λ< ~4pc: consistent to Kolmogorov 3D

80>λ> ~4pc: turbulence in 2D?

pulsar rm distribution in galactic plane red new measurements by parkes 64m telescope
Pulsar RM distribution in Galactic planered: new measurements by Parkes 64m telescope
spatial magnetic energy spectrum of our galaxy han et al 2004 apj 610 820
Spatial magnetic energy spectrum of our Galaxy(Han et al. 2004, ApJ 610, 820)

By pulsar

RM/DM

Email from A. Minter

Minter & Spangler 1996

conclusive remarks more data needed best we can say up to now
Conclusive Remarks More data needed--Best we can say up to now
  • Spatial Energy spectrum
  • Radial

Dependence

(unpublished)

  • Halo field
  • Disk field
slide46
Halo field: structure and field strength
  • What difference:
  • fields in arm and interarm
  • Structure in large regions: More pulsars?
  • Structure in more details
  • Field in intergalactic space

Current doing & future ……

rms of egrs for the halo b field
RMs of EGRs for the halo B-field

Only about 1000 RMs available in literature upto now...

We are using Effelsberg -100m telescope to make a RM survey of 1700 sources, enlarge the cover density by a factor of three in most sky area……

galactic plane polarization survey at 6cm
Galactic plane polarization survey at 6cm
  • Cooperation with
  • MPIfR
  • System hardware
  • & software
  • almost ok
  • No data at this
  • frequency
  • Less affacted by
  • foreground RM
  • Useful for CMB
  • polarization

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

MPIfR 6cm receiver

总强度图

偏振强度图

Urumqi 25m telescope

field reversals exterior to the perseus arm it is fine
Field reversals exterior to the Perseus arm -- it is fine!

Brown et al. 2003, ApJ 593, L29

Mitra et al. 2003, A&A 398,993

Han et al. 1999

Evidence at 150

Evidence for two reversals at l~70 is hard!

Han et al. 1999

Lyne & Smith 1989

field reversals exterior to the perseus arm
Field reversals exterior to the Perseus arm

Will be checked by

RMs of ALFA PSRs

(to be discovered!)

Han et al. 1999

Weisberg et al. 2004

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