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Galactic Magnetic Field Research with LOFAR

Galactic Magnetic Field Research with LOFAR. Wolfgang Reich Max-Planck-Institut für Radioastronomie Bonn, Germany. The Galactic magnetic field. What we want to know : - global field structure: disk + halo - regular/random component f(r) - field strength f(r) - field reversals

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Galactic Magnetic Field Research with LOFAR

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  1. Galactic Magnetic Field Research with LOFAR Wolfgang Reich Max-Planck-Institut für Radioastronomie Bonn, Germany

  2. The Galactic magnetic field • What we want to know : • - global field structure: disk + halo • - regular/random component f(r) • - field strength f(r) • - field reversals • - local peculiarities • What to do: • - measurements • - modelling • - what can LOFAR contribute ?

  3. The Galactic magnetic field • Observational methods (local results): • Starlight polarization: perpendicular field  3 kpc • Zeeman splitting: parallel field local e.g. masers, clouds • Polarized dust: perpendicular field star forming regions

  4. The Galactic 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)

  5. Total intensity all-sky surveys Longair (2004)

  6. Polarized intensity all-sky surveys 1.4 GHz DRAO (Wolleben et al., 2006) + Villa Elisa (Testori et al., 2008) 22.8 GHz WMAP (Page et al. 2007) depolarization Low percentage polarization outside local features.

  7. RMs from Extragalactic SourcesCurrently available data (compiled by JinLin Han) Brown et al. 2007 Brown et al. 2003 Han et al. 1997

  8. B-field Galactic components RM, () Synchrotron Emission I () + PI () CR thermal ne NE2001

  9. Models should agree with all observations Radio observational constrains on Galactic 3D-emission modelsSun X.H., Reich, W., Waelkens, A., Enßlin, T.A. 2008, A&A, 477, 573+ some recent progressSimulations based on the “Hammurabi” code:Waelkens, A., Jaffe, T., Reinecke, R., Kitaura, F., Enßlin T.A., 2008, A&A, submitted (astro-ph 0807.2262)

  10. 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)

  11. The method applied

  12. Galactic thermal electron distributionNE2001 (Cordes & Lazio, 2002) WMAP • 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) +fe NE2001 thermal component: WMAP NE2001 +fe NE2001

  13. RM data of EGS Han et al. 1997 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 !

  14. Galactic 3D modeling: the regular magnetic disk field ASS+RING ASS+ARM BSS BSS ASS radial and height dependence local regular field: 2G regular center field: 2G scale height: 1 kpc

  15. Galactic 3D modeling: regular magnetic halo field B-disk - B-Halo Moss & Sokoloff, 2008, AA, 487,197: galactic dynamo theory is unable to accout for this B-field configuration radial and height dependence: |z|<1.5 kpc |z|>1.5 kpc (not sensitive) strength at solar radius: 7 G at z = 1.5kpc B-disk + B-Halo RM-Observations don’t agree with BSS+Halo model CGPS RMs: Brown et al.

  16. Disk field: ASS + one reversal

  17. Galactic 3D modeling: random fields, CR electrons and local excess of synchrotron emission Random fields: Gaussian, homogeneous (3 G); high-resolution sim. (Kolmogorov) Local excess of synchrotron emission: • Observational evidence • isotropic high latitude (>30°) emission • enhanced local CR electrons OR random fields CR electrons:power law • spectral index of –3 (high)/ -2(low) • normalization factor: • truncation at 1 kpc Fleishman & Tokarev (1995)

  18. Galactic 3D modeling: fit of 22.8 GHz (PI) observations PI N-S asymmetrie too large ASS field consistent with PI asymmetry in the plane

  19. Galactic 3D modeling: depolarization at 1.4 GHz • 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=fe1.5 for fe=0.05,fnb = 0.01 RM=RM0+RMr/fnb0.5 fan region original NPS Loop I Large RM scatter fnb=0.01

  20. CGPS RM-data (Brown et al., 2001) overlaid on the Effelsberg 11cm total intensity survey (Fürst et al., 1990) W1 Large RM Scatter lb=119°,2.5°: map size 8°x5° Mean RM ~ -150 rad m-2

  21. 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 !!

  22. All-sky simulations at 15‘ angular resolution: diffuse Galactic emission to be seen by LOFAR synchrotron spectral index  = 2.5 10 MHz 30 MHz 50 MHz 70 MHz Galactic plane: 0° < L < 90°, -20° < B < 20°

  23. 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

  24. Problem: Cloud Distance ? RM - Synthesis OFF A B C FS +5 ON C B A LOFAR will detect small RMs from small clouds

  25. High resolution 151 MHz simulations (Sun & Reich) Field size 6°x6° resolution 7.2” centre (l,b) 190°, 48° I 160..100K PI 20..0K Random B-field spectrum with Kolmogorov-like power law PC 4.6+/- 2.3% I 160..100K RM +/-70 rad/m2 mean -8 +/-30 Same area with different distribution of random B-field

  26. Thank you !

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