Galactic MagneticGalactic Magnetic Field Field Research with LOFARResearch with LOFAR

Wolfgang Reich

Max-Planck-Institut für Radioastronomie Bonn, Germany

The The GalacticGalactic magnetic field magnetic field

What we want to know :What we want to know :- global field structure: disk + halo - global field structure: disk + halo - regular/random component f(r)- regular/random component f(r)- field strength f(r) - field strength f(r) - field reversals - field reversals - local peculiarities- local peculiarities

What to do:What to do:- measurements - measurements - modelling- modelling- what can LOFAR contribute ?- what can LOFAR contribute ?

The The GalacticGalactic magnetic field magnetic field

Observational methods (local results):

Starlight polarization: perpendicular field

3 kpcZeeman splitting: parallel field

local e.g. masers, cloudsPolarized dust: perpendicular field

star forming regions

The The GalacticGalactic magnetic field magnetic field

Observational methods (global results):

Synchrotron emission I: perpendicular field

Synchrotron emission PI: perpendicular /

regular component

Rotation measures (PSR, EGS): parallel field

Needs: cosmic ray density/spectrum f(r,z) thermal electron density and filling factor f(r,z)

Total intensity all-sky surveysTotal intensity all-sky surveys

Longair (2004)

Polarized intensity all-sky surveysPolarized intensity all-sky surveys

depolarization

1.4 GHz DRAO (Wolleben et al., 2006) 1.4 GHz DRAO (Wolleben et al., 2006) + Villa Elisa (Testori et al., 2008)+ Villa Elisa (Testori et al., 2008)

22.8 GHz WMAP (Page et al. 2007)22.8 GHz WMAP (Page et al. 2007)

Low percentage polarization outside local features.

RMs from Extragalactic SourcesRMs from Extragalactic Sources Currently available data (compiled by JinLin Currently available data (compiled by JinLin

Han)Han)

Brown et al. 2007Brown et

al. 2003

Han et al. 1997

CR thermal ne

B-field

Synchrotron Emission I () + PI ()

NE2001

RM, ()

Galactic components

Models should agree with all observations

Radio observational constrains on Galactic 3D-emission models

Sun X.H., Reich, W., Waelkens, A., Enßlin, T.A. 2008, A&A, 477, 573 + some recent progress

Simulations based on the “Hammurabi” code:Waelkens, A., Jaffe, T., Reinecke, R., Kitaura, F., Enßlin T.A.,

2008, A&A, submitted (astro-ph 0807.2262)

Galactic 3D models

• Various 3D models available for:• thermal electron distribution -- PSR DMs (NE2001)• magnetic field structure -- RMs of pulsars / EGSs• CR electrons -- propagation of CR

• New 3D model in agreement with all-sky observations:• optically thin free-free emission from WMAP • low-frequency thermal absorption• 22, 45, 408, 1420 MHz I maps• 22.8 GHz PI map (= intrinsic)• highly depolarized 1.4 GHz PI map• RMs of EGS (PSR RMs not yet included)

The method applied

Galactic thermal electron distributionNE2001 (Cordes & Lazio, 2002)

• NE2001 does not reproduce low frequency absorption

• diffuse thermal emission is clumpy

• in the plane: HII regions + small filling factor fe (z) (Berkhuijsen et al., 2006)

thermal component: WMAP NE2001

NE2001

+fe

WMAP

NE2001

+fe

RM data of EGS High latitude RMs

interpolated RM map includes new Effelsberg L-band RM survey (~1500 sources : Han, Reich et al. in prep.)

RMs asymmetric to the plane and the centre towards the inner Galaxy. Not local (Han et al. 1999).

RMs along the Galactic plane

EGS in CGPS (Brown et al. 2003 ) EGS in SGPS (Brown et al. 2007)

Large RM fluctuations !

Han et al. 1997

radial and height dependence

Galactic 3D modeling: the regular magnetic disk fieldASS+RING ASS+ARM BSS

local regular field: 2Gregular center field: 2Gscale height: 1 kpc

ASS BSS

radial and height dependence:

|z|<1.5 kpc

|z|>1.5 kpc (not sensitive)

strength at solar radius: 7 G at z = 1.5kpc

Galactic 3D modeling: regular magnetic halo field

RM-Observations don’t agree with BSS+Halo model

CGPS RMs: Brown et al.

B-disk - B-Halo

B-disk + B-Halo

Moss & Sokoloff, 2008, AA, 487,197: galactic dynamo theory is unable to accout for this B-field configuration

Disk field: ASS + one reversal

Galactic 3D modeling: random fields, CR electrons and local excess of synchrotron emission

CR electrons: power law spectral index of –3 (high)/ -2(low) normalization factor:

truncation at 1 kpc

Local excess of synchrotron emission:

Observational evidence

isotropic high latitude (>30°) emission

enhanced local CR electrons OR random fields

Random fields: Gaussian, homogeneous (3 G); high-resolution sim. (Kolmogorov)

Fleishman & Tokarev (1995)

Galactic 3D modeling: fit of 22.8 GHz (PI) observations

ASS field consistent with PI asymmetry in the plane

PI N-S asymmetrie too large

Galactic 3D modeling: depolarization at 1.4 GHz

fan region

Loop I

NPS

problem: modeled depolarization insufficient !!

proposed solution fnb = fefc

fe: filling factor of ne fc: coupling factor between ne and b

let b ~ n0.5, fc~fe0.5, fnb=fe

1.5

for fe=0.05, fnb = 0.01

RM=RM0+RMr/fnb0.5

original

fnb=0.01

Large RM scatter

CGPS RM-data (Brown et al., 2001) overlaid on the

Effelsberg 11cm total intensity survey (Fürst et al., 1990)

W1

Mean RM ~ -150 rad m-2

lb=119°,2.5°: map size 8°x5°

Large RM Scatter

Implications in turn for NE2001:

• NE2001 needs modification by including filling factor and scale height of thermal electrons

Sun et al. (2008) suggest:

Scale height increase from ~1 kpc to ~2 kpc

Halo-field will decrease to 2 G

avoids unphysical truncation of CR at z = 1 kpc

Gaensler et al., 2008, astro/ph 0808.2550 – reanalysis of scale height gives ~1.8 kpc !!

All-sky simulations at 15‘ angular resolution: diffuse Galactic emission to

be seen by LOFAR

synchrotron spectral index = 2.5

Galactic plane: 0° < L < 90°, -20° < B < 20°

10 MHz

50 MHz

30 MHz

70 MHz

Expected LOFAR input for 3D-modelling

• synchrotron spectral index variations

• thermal scale height

• local synchrotron emissivity in 3D by optically thick HII-regions

• Ne – B relation for small clumps

• high resolution Faraday screen mapping with high RM resolution

Problem: Cloud Distance ?Problem: Cloud Distance ? RM - Synthesis RM - Synthesis

A B C

FS +5

BA C

LOFAR will detect small RMs from small clouds

OFF

ON

High resolution 151 MHz simulations (Sun & Reich)High resolution 151 MHz simulations (Sun & Reich)

I 160..100K

Same area with different distribution of random B-field

I 160..100K

Random B-field spectrum with Kolmogorov-like power law

PI 20..0K

RM +/-70 rad/m2 mean -8 +/-30

PC 4.6+/- 2.3%

Field size 6°x6° resolution 7.2” centre (l,b) 190°, 48°

Thank you !Thank you !