Large-Scale Structure of the Galactic Magnetic Field Marijke Haverkorn & Jo-Anne Brown NRAO/UC-Berkeley University of Calgary
Outline On the Large-Scale Structure of the GMF... Why do we care? What do we know? - How do we know it? What will we know? What can CMBPol contribute?
Why do we care? CMB Foregrounds are caused by Galactic magnetic field: Polarized Galactic synchrotron emission Dust polarization Both synchrotron and dust foreground components are stronger than the CMB B-mode! Galactic science origin of galactic magnetism: primordial and/or dynamo? magnetic field is dynamically important propagation and acceleration of cosmic rays
What do we know?
What do we know? First ask: How do we know it?
Magnetic Field Measurement Techniques Faraday Rotation of polarized radiation Zeeman splitting Polarization of infrared emission from dust grains Polarization of starlight by dust grains Synchrotron radiation intensity Polarization of synchrotron radiation
Magnetic Field Measurement Techniques Faraday Rotation of polarized radiation Zeeman splitting Polarization of infrared emission from dust grains Polarization of starlight by dust grains Synchrotron radiation intensity Polarization of synchrotron radiation
Faraday Rotation τ = τ 0 + λ 2 RM
Sources of linear polarization: Pulsars, External Galaxies ( point sources) Galactic synchrotron: see Maik Wolleben s talk
Compact Polarized Sources Pulsars: ~600 EGS: ~2000 Positive RM Negative RM
Compact Polarized Sources Pulsars: ~400 EGS: ~1000 Positive RM Negative RM
returning to... What do we know?
Galactic Magnetic Field Basics Two components: large, small Aligned with the disk Follows the spiral arms (?) Local field is clockwise Sagittarius field is counter-cw Determining the existence and location of additional reversals will help us understand the origin and evolution of the field - and possibly of the Galaxy itself!
And that is where the agreement ends
Possible symmetries Axisymmetric Mixed spiral symmetry Bisymmetric
Vertical Symmetry and Reconnection Quadrupolar (even symmetry) Dipolar (odd symmetry)
Recent Model Suggestions Axisymmetric and even (e.g. Page et al 2007, WMAP modeling) Axisymmetric and odd (e.g. Stanev 1997) Bisymmetric and even (Harari et al 1999, Prouza & Smída 2003) Bisymmetric and odd (Tinyakov & Tkachev 2002) Page et al (2007), from Sun et al (2008)
Recent Model Suggestions Axisymmetric and even Axisymmetric and odd Bisymmetric and even Bisymmetric and odd Reversal in every arm (Weisberg et al 2004)
Recent Model Suggestions Axisymmetric and even Axisymmetric and odd Bisymmetric and even Bisymmetric and odd Reversal in every arm Reversal in arms/interarms (Han et al 2007)
Recent Model Suggestions Axisymmetric and even Axisymmetric and odd Bisymmetric and even Bisymmetric and odd Reversal in every arm Reversal in arms/interarms One reversal in inner Galaxy (Brown et al 2007)
Recent Model Suggestions Axisymmetric and even Axisymmetric and odd Bisymmetric and even Bisymmetric and odd Reversal in every arm Reversal in arms/interarms One reversal in inner Galaxy Ring-shaped field (Vallée 2005)
Some models explain part of the GMF Brown Brown et al 2007: reversal between Carina and Crux arm Weisberg et al 2004: reversal between Sagittarius and Scutum arm
but none fit all the observational data! Sun et al (2007), Men et al (2008), Noutsos et al (2008) Sun et al (2008) RM [rad m -2 ] longitude [deg]
Typical χ 2 s are MUCH too high: Noutsos et al 2008
Why don t the current models fit the data? 1. Random field component is not completely averaged out = +
Why don t the current models fit the data? 1. Random field component is not completely averaged out 2. Magnetic pitch angle is not constant Beck 2007
Why don t the current models fit the data? 1. Random field component is not completely averaged out 2. Magnetic pitch angle is not constant 3. Mixed dynamo modes NGC 4254 - Chyzy 2008
What will we know?
halo and disk-halo connection filling in of Galactic plane
Galactic caps: vertical component of field (Mao, Gaensler et al) Northern sky survey: LOFAR all Southern sky survey: ASKAP, MWA
WMAP sensitivity is too low to sample Galactic magnetic field in the halo 23 GHz Planck and/or CMBPol needed
How can CMBPol contribute?
Sensitivity? -- probing the magnetic halo. HOWEVER: Planck Resolution? -- less depolarization: depends on halo scale size
Outline/Summary On the Large-Scale Structure of the GMF... What do we know? at least 1 reversal, concentrated along disk, many different scale sizes - How do we know it? primarily Radio Polarization & Faraday Rotation What will we know? - actual number of large-scale reversals - magnitude, disk and vertical symmetries How can CMBPol contribute? Sensitivity: magnetic field in Galactic halo Depolarization: small-scale structure in halo