Magnetic Fields in External Galaxies and the Deep Polarization Sky

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Magnetic Fields in External Galaxiesand theDeep
Polarization Sky

• Russ Taylor
• Jeroen Stil Julie Grant
• University of Calgary

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Outline

• Deep Polarized Sky
• Cosmic Evolution of Magnetic Fields
• Magnetic fields in galaxies
• Cosmic web
• Observational Opportunities and Strategies

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Deep Sky Polarimetry
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Predicted Differential Source Counts
Extrapolation from p (p/S) distribution of NVSS
sources with S gt 80 mJy
Total Intensity
Polarized Intensity
(Beck and Gaensler 2004) in Science with the SKA
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Radio Source Counts
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The ELAIS N1 ISM Hole
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First 10 fieds (25) of ELAIS N1 Field
1440 hours 4 sq degrees RMS 80 µJy
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Maximum Likelihood Fit in log I - log P

• Evidence for a higher degree of polarization for
  faint sources
• No binning of data or simulations
• Best fit
• Median P 4.8

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ELAIS N1 Polarized Sources

• Median P 4.8 for 5 lt S lt 40 mJy
• Compare to P1.8 for S gt 100 mJy

Beck Gaensler (2004) distribution
Taylor et al., 2007, ApJ, 666, 201.
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Number Counts of Faint Polarized Sources
Stokes I
Polarized
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Change in P distribution with flux density from
NVSS
Stil and Taylor 2007, in prep.
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GALFACTS
CGPS
SGPS
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Radio Source Counts
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Mapping in the RM Sky

• Smoothing interpolation (Johnston-Hollitt et
  al. 2002)
• Decomposition into spherical harmonics (Dineen
  Coles 2005)
• Bayesian convolution, Markov chain analysis
  (Short, Higdon Kronberg 2007)

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GALFACTS gt10 Rotation Measures (10 per square
degree)
Based on 10 Rotation Measures
Johnston-Hollitt et al. (2002)
Short, Hidgon and Kronberg (2005)
Dineen Coles (2005)
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Compact Extragalactic Sources in GALFACTS

• Measure polarized properties of complete sample
  of 4x105 sources down to p 0.5 mJy _at_ 1.4 GHz
• Although still dominated by AGN will contain
  significant numbers of nearby galaxies - a first
  step
• P distribution as function of
• Spectral index
• Galaxy type
• Rotation measure, if not dominated by Galactic
  foreground at high latitude
• Latitude

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1.4 GHz Flux Density versus Redshift
Filled diamonds - star forming galaxies Open
squares - photometric redshifts
Barger et al. 2007, ApJ 654, 764.
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Simple Model of Global Magnetic Field Geometry
(m0)
i 0
i 60º
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Measured Integrated Fractional Polarization of a
Sample of Nearby Disk Galaxies
Lines are predictions based on m0 model
(a) Without Faraday Depolarization
(b) with Faraday Depolarization
Stil, Taylor, Beck Krause, 2007, in prep.
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Measured Integrated Polarization Position Angle
of a Sample of Nearby Disk Galaxies

• high inclination disk
• X low inclination disks

Stil, Taylor, Beck Krause, 2007, in prep.
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Integrated polarized radiation from nearby disk
galaxies

• Has significant range of fractional polarization
• up to 18 at 1.4 GHz
• Correlates with global properties (inclination
  and PA) of the disk
• Coherent global fields exist
• Depolarization is important at 1.4 GHz
• Depends on ratio of random (turbulent) to uniform
  (global scale) field strength

Integrated studies of high z galaxies can provide
information about magnetic field strength, its
global properties and the relative strength of
turbulent and uniform components
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Key observations

• Some strategic issues
• SKA development timeline
• 2012 - 1 SKA _at_ mid range (300 MHz to 3 GHz)
  with FOV expansion technology
• 2014 phase I SKA 10 SKA _at_ mid range
• 2020 full SKA
• Arecibo is 3-5 SKA
• SKA pathfinder at gt 3 GHz (3 - 8 GHz?)
• 2012 - 2020

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Key observations Arecibo Deep Polarimetric Survey

• gt 1 GHz bandwidth in 5-8 GHz window
• Several thousand channels for accurate RM
  measurements
• detection limit at least 30 microJy over large
  area
• Angular resolution lt 1
• Stokes I confusion limit
• More accurate positions for ID
• Stokes parameters and RM for gt 106 sources s
• Sample deep into the star forming galaxy
  population out to z gt 1
• Measure magnetic properties of star forming
  galaxies with cosmic time from integrated
  polarized light
• Note at gt 5GHz minimal depolarization for
  comparison to lower frequency surveys with 1 SKA
  pathfinders
• High density 3-dimensional RM grid (RM vs
  position and z) for magnetic ISM (magnetic cosmic
  web)

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Key observations Challenges

• FOV expansion technology at gt5 GHz
• Focal plane feed array
• Phased array feed
• Bandwidth gt 1 GHz

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Key observations Challenges

• High Dynamic Range Continuum Imaging with
  off-axis feed arrays

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Wave-front Aberration Analysis
Observed Beam
Aperture Illumination
Aperture Phase
Model Beam
Sukhpreet Guram University of Calgary, M.Sc.
Thesis
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Multi-beam Clean Deconvolution
GALFACTS Pilot
NVSS
Sukhpreet Guram University of Calgary, M.Sc.
Thesis
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Key observations Challenges

• Data processing and Management

GALFACTS Processing Centre
NAIC - University Consortium Partnership
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Key observations Challenges

• FOV expansion technology at gt5 GHz
• High dynamic range imaging with off-axis feed
  arrays
• Data processing and management

THESE ARE SKA PATHFINDING CHALLENGES
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Key observations Challenges

• FOV expansion technology at gt5 GHz
• Focal plane feed array
• Phased array feed
• Bandwidth gt 1 GHz

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THE END
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Beyond the Polarization Horizon
Jeroen Stil Department of Physics and
Astronomy The University of Calgary
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Sky Density of Polarized Sources in the NVSS
Ha (Finkbeiner 2003)
Polarized Intensity (Wolleben et al. 2006)
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Conclusions

• Depolarisation of extragalactic sources in the
  NVSS reveals unknown structures in the local
  magneto-ionic medium
• We produce a fully-sampled rotation measure
  amplitude image of the sky, if bandwidth
  depolarisation dominates everywhere
• For more information Stil Taylor (2007) ApJL
  in press. arXiv0705.2741

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Galactic Disk Continuum Emission