Magnetic Fields & Turbulence in The Milky Way

1. Magnetic Fields & Turbulence in The Milky Way Bryan M. Gaensler Harvard-Smithsonian CfA + Marijke Haverkorn, Jo-Anne Brown, Natalya Bizunok, Naomi McClure-Griffiths, John Dickey, Anne Green

2. The Magnetic Milky Way • Galaxies are magnetic! (see review talk by Rainer Beck) • Milky Way magnetism similarly illuminated by • synchrotron emission from ultrarelativistic electrons • Two components: • - diffuse emission (cosmic ray component of ISM) • - discrete synchrotron sources (supernova remnants) Radio polarization of M51 (Beck et al 1987) Parkes 2.4 GHz survey (Duncan et al 1995)

3. Mapping Magnetic Fields } • Optical dust polarization • Synchrotron emission / polarization B┴ (orientation, but not direction) • Infrared dust polarization • Zeeman splitting B║ (but long integrations, high ρ) • Faraday rotation is powerful probe of B║ • - no extinction • - "absorption" experiment • - provides direction of B • - weighted by ne PSR B1154-62 (Gaensler et al 1998)

4. The International Galactic Plane Survey Dominion Radio Astrophysical Observatory (CGPS) Australia Telescope (SGPS) Very Large Array (VGPS) • Main goal of IGPS is to image H I over –107o < l < 180o at 1'-2' resolution • … but record full 1.4 GHz polarization also (except for 20o < l < 63o) • → unprecedented probe of polarization and Faraday rotation in Milky Way

5. Norma Field: Linear Polarization Norma Field: Total Intensity Extragalactic sources H II regions Diffuse emission SNRs (Q2 + U2)1/2 from Southern Galactic Plane Survey (Gaensler et al 2001) Stokes I from Southern Galactic Plane Survey (Gaensler et al 2001)

6. Norma Field: Linear Polarization Extragalactic sources Diffuse emission Point sources H II regions Diffuse emission SNRs Voids (Q2 + U2)1/2 from Southern Galactic Plane Survey (Gaensler et al 2001)

7. Norma Field: Faraday Rotation Rotation measures from Southern Galactic Plane Survey (Gaensler et al 2001)

8. Polarized Point Sources • Background radio galaxies & AGN → plumb line through entire Galaxy • RMobserved = RMintrinsic + RMIGM + RMMilky Way  RMMilky Way • Combined with model for ne (Cordes & Lazio 2002), can directly yield B║ IGPS RMs (Brown et al 2004)

9. Magnetic Geometry of the Galaxy • Clear signatures of large-scale coherent structure Crux Norma Carina Binned SGPS RMs (Brown et al 2004) • Rough modulation in RM as function of l • - increase in ne in spiral arms, but no obvious field reversals across arms • l > 305o : RM < 0 • l < 305o : RM > 0 • Detailed modeling underway } field reversal inside Carina spiral arm?

10. Diffuse Polarized Emission SGPS polarization (Haverkorn et al 2004) • No correlation between total & polarized intensity • Smooth polarized background • - invisible in total intensity • - foreground gas produces structure in Q & U • - three-dimensional probe of warm ionized medium } see talk by Haverkorn

11. Voids in Polarization • Voids match regions of Hα emission • → beam depolarization by turbulent ionized gas • H II region RCW 94 • - mild depol. in interior implies • Brandom ~ 1.2 μG, with • outer scale δl ~ 0.2 pc • - strong depol. around rim • due to RM gradient in PDR • region w. adjacent molecular cloud • High RMs around edges • of voids: swept up fields? • Presence/absence of voids gives • distance to polarized background • - in this case d ~ 3.5 kpc, • i.e. Crux spiral arm H II region RCW 94 Hα emission (SHASSA; Gaustad et al 2001) SGPS polarized intensity (Gaensler et al 2001)

12. Foreground Depolarization • Fluctuations on scales < 1 pc also seen in LMC (see poster)

13. Summary • Approx. 800 new extragalactic rotation measures, all in Galactic plane • - no magnetic field reversals in outer Galaxy • - at least one field reversal inside Sagittarius-Carina spiral arm • Complicated regions of diffuse linear polarization • - Faraday rotation from otherwise invisible magnetized structures • - probe of structure & turbulence in warm ionized ISM • Prominent voids in polarization • - depolarization by small-scale RM fluctuations in SNRs / H II regions • - yields strength & scale of random component of magnetic field • The future: • - complete analysis of IGPS polarization & release to community • - similar observations of Magellanic Clouds • - exploration of full Magnetic Universe with SKA • → Transformational view of magnetic fields & ionized gas! } see posters