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

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

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

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  1. Magnetic fields in our GalaxyHow much do we know JinLin HanNational Astronomical ObservatoriesChinese Academy of SciencesBeijing, Chinahjl@bao.ac.cn 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)

  2. 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)?

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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>

  13. 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

  14. 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

  15. 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

  16. Knowledgeof 10 years ago ……

  17. 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)

  18. 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

  19. 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….

  20. 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 ? ? )

  21. Current knowledge …… • Central field & halo field • disk field: directions & Strength • magnetic energy spectrum

  22. 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

  23. 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)

  24. Optical sky Radio sky

  25. 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

  26. 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

  27. 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

  28. 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)

  29. 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

  30. Current knowledge …… • Central field & halo field • disk field: • directions • Strength • magnetic energy spectrum

  31. 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 ? ? )

  32. 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

  33. Large-scale magnetic field in the Galactic disk The largest coherent field structrue detected in the Universe!

  34. 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

  35. Current knowledge …… • Central field & halo field • disk field: • directions • Strength • magnetic energy spectrum

  36. Current knowledge …… • Central field & halo field • disk field: directions & Strength • magnetic energy spectrum

  37. Why our Galaxy has magnetic field?Probably Dynamo!How dynamo works? Alpha-Omega effect. Dynamo Really works? Computer Simulations….

  38. 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 ...

  39. What spatial magnetic energy spectrum does our Galaxy have?

  40. 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

  41. 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?

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

  43. 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

  44. Conclusive Remarks More data needed--Best we can say up to now • Spatial Energy spectrum • Radial Dependence (unpublished) • Halo field • Disk field

  45. If I have time, I tell you more aboutwhat we are doing …… Thanks for your attention.

  46. 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 ……

  47. 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……

  48. Difference for RMs of PSRs & EGRes

  49. 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

  50. 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

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