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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 The Magnetic Milky Way Galaxies are magnetic! (see review talk by Rainer Beck)

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magnetic fields turbulence in the milky way

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

slide2

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)

mapping magnetic fields
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)

the international galactic plane survey
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
norma field total intensity

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)

slide6

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)

slide7

Norma Field: Faraday Rotation

Rotation measures from Southern Galactic Plane Survey (Gaensler et al 2001)

slide8

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)

slide9

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?

slide10

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

slide11

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)

slide12

Foreground Depolarization

  • Fluctuations on scales < 1 pc also seen in LMC (see poster)
slide13

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