Observations of Magnetic Fields in Regular and Irregular Clusters

1
Observations of Magnetic Fields in Regular and
Irregular Clusters

• Federica Govoni
• INAF Osservatorio Astronomico di Cagliari ,
  Italy

Collaborators Matteo Murgia INAF -
Istituto di Radioastronomia Cagliari,
Italy Luigina Feretti INAF - Istituto
di Radioastronomia Bologna, Italy Gabriele
Giovannini Dip. di Astronomia, Univ. di Bologna,
Italy Emanuela Orru Dip. di Fisica,
Univ. di Cagliari, Italy
2
A2029
COMA
Regular Cluster
Irregular Cluster
3
Summary of the Talk

• DIFFUSE SYNCHROTRON RADIO EMISSIONS

Halos, Relics (Irregular Clusters) Mini-Halos
(Regular Clusters)

• ROTATION MEASURE OF RADIO GALAXIES IN CLUSTERS

CLUSTER MAGNETIC FIELDS
STRENGTH
POWER SPECTRUM
GAS DENSITY RELATION
4
Diffuse Synchrotron Emission in Irregular
Clusters HALOS and RELICS
X-RAY RADIO Govoni et al. (2004)
A665
A754
A520
Bacchi et al. (2003)
Giovannini Feretti (2000)
Govoni et al. (2001)
A2163
A1914
A2256 Clarke TALK Eilek et al. POSTER Delain
Rudnick POSTER A2744 ... Orru et al.
POSTER A209 ... Giovannini et al. POSTER
Feretti et al. (2001)
Bacchi et al. (2003)
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Diffuse Synchrotron Emission in Regular Clusters
MINI-HALOS
NEW MINI-HALO DETECTIONS Govoni et al., in
prep. VLA observations at 1400 MHz Beam 45
A2029
A1835
Sign of a diffuse, extended emission in A2029 (
VLA observation, 74MHz) Markovic et al.
(2003) Eilek et al. POSTER
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Diffuse Synchrotron Emission in Regular Clusters
MINI-HALOS
PKS0745-191 Baum ODea (1991) VIRGO
Owen et al. (2000) A2390
Bacchi et al. (2003) A2626 Rizza
et al. (2000), Gitti
et al. (2003)
Brentjens de Bruyn TALK
PERSEUS CLUSTER
Fabian et al. (2000)
Burns et al. (1992), Sijbring (1993)
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Diffuse Synchrotron Emission
SYNCHROTRON EMISSION ? MAGNETIC FIELD STRENGTH
Magnetic field strength averaged over the entire
source volume CLASSICAL EQUIPARTITION ASSUMPTION

• ? Magnetic field filling factor

• k ratio of the energy in relativistic protons to
  that in electrons

• VVolume

• LSynchrotron Luminosity

HALOS Beq 0.1-1µG e.g. Giovannini
Feretti (2002)
MINI-HALOS Beq 0.5µG (A2029, A1835)
RELICS Beq 0.5-2µG e.g. Ensslin et al.
(1998)
Magnetic field values consistent with those
suggested from the few detection of Inverse
Compton hard X-ray emission in clusters with
radio halos e.g. Rephaeli Gruber (2002), Fusco
Femiano et al. (2004)
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Rotation Measure of Radio Galaxies in Clusters
NGC326 Murgia et al. (2001)
Kronberg , Physics Today (2002)
20 cm
6 cm
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Rotation Measure of Radio Galaxies in Clusters
NGC326 Murgia et al., in prep.
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Rotation Measures in Regular Clusters
Hydra A Taylor Perley (1993)
Lane et al. (2004)
-12000 Rad/m/m
5000 Rad/m/m
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Rotation Measures in Irregular Clusters
A400 (3C75) Eilek Owen (2002)
-170 Rad/m/m
170 Rad/m/m
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Rotation Measure of Radio Galaxies in Clusters
A2255
RM images obtained by using the PACERMAN
(Polarization Angle CorrEcting Rotation Measure
Analysis) Algorithm Dolag et al. (2005)
A119 Feretti et al. (1999) 3 sources
3C129 Taylor et al. (2001) 2 sources
A514 Govoni et al. (2001) 5 sources
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Statistical Studies of Rotation Measures
THE ROTATION MEASURE OF RADIO GALAXIES IS
REPRESENTATIVE OF THE CLUSTER MAGNETIC FIELD
SAMPLE OF IRREGULAR CLUSTERS Clarke (2004)

• EMBEDDED GALAXIES
• BACKGROUND GALAXIES
• CONTROL SAMPLE

Lawler Dennison (1982) Vallee et al.
(1986) Kim et al. (1991) Clarke et al.
(2001) Johnston-Hollitt Ekers (2004)
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The Case of A2065
THE ROTATION MEASURE OF RADIO GALAXIES IS
REPRESENTATIVE OF THE CLUSTER MAGNETIC FIELD
z0.073
In collaboration with K. Dolag RM of radio
sources in hot galaxy clusters
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Results from Rotation Measures
Lawler Dennison (1982) Tribble (1991)
Felten (1996) Sokoloff et al. (1998)
APPROXIMATION Magnetic field tangled on a
single scale ?c. Screen made by cells of
uniform size, electron density, and magnetic
field strength, with a field orientation at
random angles in each cell.
ANALYTICAL APPROXIMATION
ltRMgt-91 Rad/m2 sRM78 Rad/m2
RM Structures of 5-10 kpc
Magnetic fields in IRREGULAR clusters 5µG
Magnetic fields in REGULAR clusters are higher
10-30µG
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Magnetic Field Gas Density Relation
Correlation between clusters X-ray surface
brightness Sx and Faraday rotation measure
calculated in the sources location Dolag et al.
(2001)
IRREGULAR CLUSTERS
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Magnetic Field Gas Density Relation
Dolag et al., in prep.
Regular Clusters
Irregular Clusters
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Results from Rotation Measures
Lawler Dennison (1982) Tribble (1991)
Felten (1996) Sokoloff et al. (1998)
APPROXIMATION Magnetic field tangled on a
single scale ?c. Screen made by cells of
uniform size, electron density, and magnetic
field strength, with a field orientation at
random angles in each cell.
ANALYTICAL APPROXIMATION
ltRMgt-91 Rad/m2 sRM78 Rad/m2

• RM IMAGES WITH SMALL SPATIAL SCALES
• ltRMgt?0
• MAGNETO HYDRODYNAMIC SIMULATIONS Dolag et al.
  (2002)

CLUSTER MAGNETIC FIELDS FLUCTUATE OVER A WIDE
RANGE OF SPATIAL SCALES
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Magnetic Field Power Spectrum
USING A MULTI-SCALE MAGNETIC FIELD MODEL WITH A
POWER SPECTRUM WHICH FOLLOW
IT IS POSSIBLE TO STUDY THE BEHAVIOUR OF THE RM
FOR DIFFERENT VALUES OF THE SPECTRL INDEX OF THE
POWER SPECTRUM.
Murgia et al. (2004)
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Magnetic Field Power Spectrum

Cluster magnetic field strengths can be
calculated through their effects on the
polarization properties of radio galaxies by
using an analytical formulation based on the
approximation that the magnetic field is tangled
on a single scale
Efforts into extending the analysis of
the Faraday maps beyond the the tangled
cell model
Ennslin Vogt (2003) Vogt Ennslin(2003) Vogt
Ennslin (2005)
Murgia et al. (2004)

• DEVELOPED A NUMERICAL METHOD
• TO DETERMINE THE MAGNETIC POWER
• SPECTRUM BY SIMULATING THE
• FARADAY ROTATION EFFECTS
• ON THE POLARIZATION
• PROPERTIES OF RADIO
• GALAXIES AND RADIO HALOS

• DEVELOPED A FOURIER ANALYSIS
• OF RM MAPS TO DETERMINE THE
• MAGNETIC FIELD POWER
• SPECTRUM
• CORRELATION LENGTH
• STRENGTH

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Magnetic Field Power Spectrum

Vogt Ennslin (2005) A BAYESIAN VIEW ON FARADAY
ROTATION MAPS RM map (Vogt et al. 2005) with
PACERMAN algorithm Dolag et al. (2005)
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Magnetic Field Power Spectrum
A119 (Irregular Cluster)
Cells 10 kpc BB 5µG Feretti et al. (1999)
0
0.9
B?n BB 7.5µG Dolag et al. (2001) n2 6kpc -
770 kpc B 5µG ltBgt1.5µG
e
0
0
Murgia et al. (2004)
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Magnetic Field Power Spectrum
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Magnetic Field Power Spectrum
Simulated halo brigthness and polarization images
Murgia et al. (2004)
n3
n2
n4
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Diffuse Synchrotron Polarized Emission
A2255 The first detection of polarized emission
in a radio halo
Relic
Halo
Govoni et al. (2005)
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SYNTHESIS
Magnetic fields are common in regular and
irregular clusters and most of what we know
comes from the detection of radio waves

• DIFFUSE SYNCHROTRON RADIO EMISSIONS

Halos, Relics (Irregular Clusters) Mini-Halos
(Regular Clusters)

• ROTATION MEASURE OF RADIO GALAXIES

Analytical Approximation
CLUSTER MAGNETIC FIELDS
Equipartition
Reconciling magnetic fields values
GAS DENSITY RELATION
POWER SPECTRUM
STRENGTH
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SYNTHESIS
Magnetic fields are common in regular and
irregular clusters and most of what we know
comes from the detection of radio waves

• DIFFUSE SYNCHROTRON RADIO EMISSIONS

Halos, Relics (Irregular Clusters) Mini-Halos
(Regular Clusters)

• ROTATION MEASURE OF RADIO GALAXIES

Analytical Approximation
CLUSTER MAGNETIC FIELDS
Observations suggest that probably both small
and large scales structures coexist
Equipartition
Reconciling magnetic fields values
GAS DENSITY RELATION
POWER SPECTRUM
STRENGTH
Regular clusters gt Irregular clusters
If the magnetic field fluctuates up to scales of
some hundred kpc, then steep power spectra may
give rise to detectable polarized filaments.