Magnetic fields in our Galaxy
Download
1 / 52

Thanks for cooperation with Dick Manchester (ATNF, Australia), G.J. Qiao (PKU, China), - PowerPoint PPT Presentation


  • 91 Views
  • Uploaded on

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),

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Thanks for cooperation with Dick Manchester (ATNF, Australia), G.J. Qiao (PKU, China), ' - drew


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

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)


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


    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 <B>


    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


    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


    Knowledgeof

    10 years ago ……


    Disk field 3 models which one

    Galactic magnetic fields: 10 years ago

    disk 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 ? ? )


    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)


    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

    <B> away

    from us

    RM<0

    RM>0

    <B> 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


    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!


    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 <B>=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


    Current knowledge ……

    • Central field & halo field

    • disk field:

      • directions

      • Strength

  • magnetic energy spectrum


  • Current knowledge ……

    • Central field & halo field

    • disk field:

      directions & Strength

    • magnetic energy spectrum


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

    Dynamo Really works?

    Computer Simulations….


    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 ...



    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


    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


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

    Thanks for your attention.


    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<l<100 is very weak, but

    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


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


    ad