T'G'Arshakian MPI fr Radioastronomie Bonn

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T.G.Arshakian MPI für Radioastronomie (Bonn)
Exploring the weak magnetic fields with LOFAR
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Outline

• Advantages of low frequency radio astronomy
• Observations of regular magnetic fields
• Faraday rotation is a powerful tool to detect
  weak magnetic fields
• What can LOFAR observe?

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Advantages of low-frequency (LF) radio astronomy

• LF emission is purely nonthermal in nearby
  galaxies, IGM and ISM
• Radio synchrotron emission is a measure of the
  strength of the total magnetic field (Btot)
• Allows detailed studies for few dozens of
  nearby galaxies .
• Linear polarization degree of ordering of the
  magnetic field
• Fully ordered field can polarize the signal up
  to 75.
• Small Rotation Measures (RM ? ne B dr) can be
  measured (?RM ?-2) ? weak magnetic fields.
• RM Synthesis (Brentjens de Bruyn 2005)
  separate RM components from regions along the LOS
  (from multichannel spectro-polarimetry).

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Observing weak magnetic fields

• Synchrotron intensity I B?1? ?-?
• ? 10x smaller B gives same intensity when
  observing at 100x smaller frequency ? (for ?1)
• Synchrotron lifetime
• ?50 MHz, B?10µG Ee0.6 GeV ? tsyn 1.5 108
  yr
• ?50 MHz, B? 3µG Ee1.8 GeV ? tsyn 9
  108 yr
• Inverse Compton loss from CMB dominates for
  weaker fields (lt3µG for nearby objects)

? Observing at low frequencies traces old,
low-energy cosmic-ray electrons in weak magnetic
fields ? CR electrons may travel to large
distances in weak magnetic fields
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Regular magnetic fields in the disk

• M 51
• VLAEff 6cm
• total intensity
• B-vectors
• (Fletcher Beck)

Weak magnetic fields may exist in the outer
disk regions.
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Regular fields in the halo

• NGC5775
• 6cm total
• polarized
• intensity
• (Tüllmann et al.
• 2000)

X-shaped halo field Vertical field
components increasing with increasing height
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NGC253 6cm polarized intensity (PhD Heesen 2007)
VLA Effelsberg
NGC253 6cm total polarized intensity (PhD
Heesen2007)
Weak magnetic fields may exist in the outer
halo regions.
X-shaped halo field
Disk halo field
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NGC253 6cm polarized intensity (PhD Heesen 2007)
VLA Effelsberg
NGC253 6cm total polarized intensity (PhD
Heesen2007)
X-shaped halo field
Disk halo field
A presence of regular magnetic fields in the
disks and halos makes Faraday rotation a perfect
tool to study the weak magnetic field structure
in spiral galaxies
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• Radio halos
• and their rotation measures
• are best observed
• at low frequencies (LOFAR)

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Components of Faraday rotation
RM RMIGM RMcl RMgal RMMW RMion
lt1 10000 1000 1000 10
rad m-2
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LOFAR RM Survey (120-240 MHz)

• LOFAR can measure very low Faraday rotation
  measures (below
• 1 rad m-2) and hence very weak magnetic fields
• Face on galaxies (outer disk) RMlt10 rad m-2
• Galaxy halos, cluster halos, relics,
  intergalactic filaments
• ne10-3 cm-3, B?? 1 µG, L1 kpc RM1 rad
  m-2
• ne10-2 cm-3, B?? 1 µG, L100 pc RM1 rad
  m-2
• Intergalactic magnetic fields
• ne10-3 cm-3, B?? 0.1 µG, L1 kpc RM0.1
  rad m-2

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RM mapping of nearby galaxies LMC and SMC
200 RMs behind LMC
Gaensler et al. 2005
Few RMs behind SMC
Mao et al. 2008
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RM mapping of galaxies clusters M 31 and
Abell 514
RMs of 21 polarized at 1.4GHz sources shining
through M31 (Han et al. 1998)
5 RMs through Abell 514 (Govoni et al. 2001)
RMs through 30 clusters (Johnston-Hollitt 2003)
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RM mapping of nearby galaxies with the SKA
SKA RM survey will detect many polarized sources
behind nearby galaxies thus allowing the RM
mapping of the foreground galaxy and
reconstruction of its magnetic field structures.
RM mapping of the foreground galaxy M31 at 1.4 GHz
SKA RM survey(simulation by Bryan Gaensler)
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Number counts of polarized background sources at
1.4 GHz
Number counts per 1 deg2 (dotted line Taylor
et al. 2007) 1. observed number counts (P gt
0.5 mJy) 2. extrapolated to 0.01 mJy (P lt 0.5
mJy)
With a SKA sensitivity of 0.05 µJy (T100 h)
50000 polarized sources behind M 31, tens to
hundreds sources behind a galaxy at a distance
from 10 Mpc to 100 Mpc (z lt 0.025).
With a sensitivity of 0.01 mJy (T1m) about 1000
polarized sources will be detected towards
nearest spiral galaxy M 31.
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RM patterns and perspectives for the SKA
RM patterns of galaxies with different magnetic
field configurations
Recognition of simple structures of regular
magnetic fields can be reliably performed from a
limited sample of gt 20 RM measurements (Stepanov
et al. 2008)
SKA perspectives 600 spiral galaxies (lt10 Mpc,
?p0.2 µJy) can be recognized within T 15 min
SKA observation time at 1.4 GHz 60.000
galaxies (?100 Mpc, ?p0.015 µJy) with T 100 h.
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RM mapping with LOFAR
LOFAR (LWA, ASKAP and SKA-AA) can detect
smaller RM values and recognize weak galactic and
intergalactic magnetic fields if background
sources are still polarized at low frequencies
(lt350 MHz) ??? Lack of polarization surveys ?
GMRT test observations of polarization signal
from highly polarized pulsars - Dec 13 (Arshakian
et al., MPIfRNCRA)
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Number counts of pol. sources at 350 MHz
350 MHz data from Haverkorn 2003, Schnitzeler 2008
Array Source/deg2
DFA-1h
1 (1818) IFA-1h
4 (181814) IFA-10h
10 (181814) IFA-100h
25 (181814)
1400 MHz
350 MHz
Strong depolarization at 350 MHz and lower
sensitivity of LOFAR ? low number density of
polarized sources at 350 MHz
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The radio galaxy B183462

• Schoenmakers et al (2000)
• z 0.54, 1.2 Mpc size !

all 4 lobes polarized RM 56 - 60 rad/m2
(0.6-8.4 GHz)
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Depolarization in lobes of B183462
WSRT project on low freq polarization at 150 MHz
and 350 MHz in giants (Tigran Arshakian et al.)

• 350 MHz
• Inner lobes depolarize at 350 MHz.
• Outer lobes still at 13 polarization.
• 150 MHz (preliminary results)
• Outer lobes depolarize lt 5
• very little internal gas in outer lobes but
  worries about beam depolarization ? LOFAR
• What is origin of RM difference of 3 rad/m2
  between outer lobes ?
• Either due to
• - our Galaxy (gradients ?)
• - cocoons of lobes (rather high ne, B needed)
• - variations in true IGM on scales of 1 Mpc
  ?
• RM 3 rad/m2 10-6 x 0.1 ?G x 30 Mpc
• (filaments of cosmic web ?)

de Bruyn
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Polarization of sources at 150 MHz

• Fractional polarization of point sources is lt 1
  - GMRT EoR effort (Pen et al. 2008
  arXiv0807.1056) beam and/or depth
  depolarization.
• Double-double radio galaxy 183224 WSRT
  observations
• Fractional polarization of outer lobes at 150
  MHz is smaller (gt3 times) than at 350 MHz.

Depolarization is much stronger at 150 MHz ?
number density of polarized sources should be
very low
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Coma cluster and filaments
Area50 deg2 10h NBG500
Area3 deg2 10h NBG30
Coma cluster and around Arecibo DRAO 408 MHz
(73cm) (Kronberg et al. 2007)
Diffuse emission filament (?)
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RM mapping with LOFAR

IFA-10h (181814) gives 10 pol. sources per sq.
degree at HB

• Is possible only for nearby sources with large
  angular sizes covering the sky area of several
  sq. degrees
• LOFAR HB frequencies are preferable (DP is
  lower)
• Recognition of magnetic field structures is
  possible with NBGgt20 polarized sources
• Galaxies ( 3 deg2 NBG30) M31
• Clusters (50 deg2 NBG500) Coma
  cluster
• Filaments(?) (3 deg2 NBG30 ) - in the
  Coma cluster

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Summary

• RM measurements (120 MHz and 240 MHz) provide a
  powerful tool to measure weak magnetic fields.
• Recognition and detection of weak magnetic field
  structures is possible for nearby objects with
  large angular sizes distant objects (or small
  angular sizes) require a lot of observational
  time.
• For objects with small angular sizes diffuse
  polarized emission has to be detected to measure
  RM.