aspen2005han

Magnetic fields in our GalaxyHow much do we know JinLin HanNational Astronomical ObservatoriesChinese Academy of Sciences Beijing, Chinahjl@bao.ac.cn: 

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

Slide2: 

Galactic B field: How much we want to know B-Strength 1. Random vs. ordered ( <δB>2/B2 ) 2. Local vs. large-scale B~ f(R)? B~ f(z)? 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

Magnetic fields in our GalaxyHow much do we know ?: 

Magnetic fields in our Galaxy How 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: 

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: local field // arm

Zeeman Effect: B in molecular clouds: 

Zeeman Effect: B in molecular clouds Bourke et al. 2001, ApJ 554, 916 >30 people working for >30 years, get <30 good measurements! Difficult & Bad Luck!

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

Maser B-fields: 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 clockwise 33 counterclockwise Assume Bφ>> Br or Bz

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⊥ ------------- 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, infraredWorking toward measure B-field of galactic scale: 

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 Hildebrand et al. PASP 112, 1215

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

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

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⊥ ------------- 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 Widely distributed in Galaxy Distance from DM: 3-D B-field Linearly polarized: RM easy to obs No intrinsic RMs: Direct <B> Pulsar distribution

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

Slide16: 

Knowledge of 10 years ago ……

disk field: * 3 models * which one?: 

disk field: * 3 models * which one? Concentric Rings Axi-symmetric Bi-Symmetric Spiral Rings model spiral (ASS) (BSS) Galactic magnetic fields: 10 years ago Halo field: * no idea on halo field * Poloidal fields near GC: Yes see nonthermal filaments

Axi-Symmetric Spiral model by J.P. Vallee: 

Axi-Symmetric Spiral model by 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: 

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 ! by R. Rand & S. Kulkarni (1989) R. Rand & A.Lyne(1994) 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 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 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 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) (from Novak et al. 2003) 150pc GC Predicted B-direction

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 The largest edge-on Galaxy in the sky Pulsars and extragalactic radio sources as probes Pulsars RM distribution

To study halo field: unique to our Galaxy: 

The largest edge-on Galaxy in the sky Pulsars and extragalactic radio sources as probes Extragalactic Radio Sources RM distribution To study halo field: unique to our Galaxy <B> away from us RM<0 RM>0 <B> to us

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?: 

Unique measurement of Vertical B-component Bv＝0.2～0.3G pointing from SGP to NGP (Effect of the NPS discounted already!) Local vertical components: from poloidal field? South Galactic Pole North Galactic Pole (see Han & Qiao 1994; Han et al. 1999)

Magnetic field configurations for basic dynamos: 

Magnetic field configurations for basic dynamos M31: only 21 polarized bright background sources available !! Han, Beck, Berkhuijsen (1998): An even mode (S0) dynamo may operate in M31 ! A0 S0 S1

Slide30: 

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

Bi-Symmetric Spiral Model: 

Bi-Symmetric Spiral Model Proposed 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 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 directions newly determined Han et al. 2002, ApJ 570, L17 Coherent B-field directions >5 kpc along Norma arm Another reversed field in large-scale? ??

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: 

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! Synchrotron radiation: transverse B-structures Global B-field structure from linearly polarized emission Han et al. 1999, A&A 384, 405 Two possible reasons for polarization of Synchrotron radiation of External 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

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) 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 ….. No real measurements to check whether dynamo works or not! Far away from telling anything about a real galaxy …… Don’t know much about the large-scale magnetic field ...

What spatial magnetic energy spectrum does our Galaxy have?: 

What spatial magnetic energy spectrum does our Galaxy have?

Slide40: 

Spatial fluctuation spectrum for electron density “The Big Powerlaw in the Sky” B-field & electrons coupling? If so, B-energy spectrum? 10 pc 1000 km (Armstrong, Rickett & Spangler 1995)

Slide41: 

Spacial energy spectrum of B Previously only available information from 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 : 

Pulsar RM distribution in Galactic plane red: 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) Minter & Spangler 1996 By pulsar RM/DM Email from A. Minter

Conclusive Remarks More data needed -- Best we can say up to now : 

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

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

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

Slide46: 

Current doing & future …… 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

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……

Difference for RMs of PSRs & EGRes: 

Difference for RMs of PSRs & EGRes

Galactic plane polarization survey at 6cm: 

Galactic plane polarization survey at 6cm Urumqi 25m telescope MPIfR 6cm receiver Just messioned last week…. Will finish in 3 years ... Cooperation with MPIfR System hardware & software almost ok No data at this frequency Less affacted by foreground RM Useful for CMB polarization 总强度图 偏振强度图

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 Han et al. 1999 Lyne & Smith 1989 Evidence at 150<l<100 is very weak, but Evidence for two reversals at l~70 is hard!

Field reversals exterior to the Perseus arm: 

Field reversals exterior to the Perseus arm Han et al. 1999 Weisberg et al. 2004 Will be checked by RMs of ALFA PSRs (to be discovered!)

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)