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. X-RAY – RADIO Govoni et al. (2004) Giovannini & Feretti (2000)
Diffuse Synchrotron Emission in Irregular Clusters:
HALOS and RELICS
X-RAY – RADIO Govoni et al. (2004)
A665
A520
A754
Giovannini & Feretti (2000)
Govoni et al. (2001)
Bacchi et al. (2003)
A2163
A1914
A2256 Clarke TALK
Eilek et al. POSTER
Delain & Rudnick POSTER
A Orru’ et al. POSTER
A Giovannini et al. POSTER
Feretti et al. (2001)
Bacchi et al. (2003)

5. 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

6. Brentjens & de Bruyn TALK
Diffuse Synchrotron Emission in Regular Clusters:
MINI-HALOS
PKS Baum & O’Dea (1991)
VIRGO Owen et al. (2000)
A Bacchi et al. (2003)
A Rizza et al. (2000),
Gitti et al. (2003)
Brentjens & de Bruyn TALK
PERSEUS CLUSTER
Fabian et al. (2000)
Burns et al. (1992), Sijbring (1993)

7. Beq∝(L k/Ф V) Diffuse Synchrotron Emission
SYNCHROTRON EMISSION  MAGNETIC FIELD STRENGTH
Magnetic field strength averaged over the entire source volume
CLASSICAL EQUIPARTITION ASSUMPTION
Beq∝(L k/Ф V)
2/7
Ф Magnetic field filling factor
k ratio of the energy in relativistic protons to that in electrons
V=Volume
L=Synchrotron 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)

8. Rotation Measure of Radio Galaxies in Clusters
NGC326 Murgia et al. (2001)
Kronberg , Physics Today (2002)
6 cm
20 cm

9. Rotation Measure of Radio Galaxies in Clusters
NGC326 Murgia et al., in prep.

10. Hydra A Taylor & Perley (1993)
Rotation Measures in Regular Clusters
Hydra A Taylor & Perley (1993)
Lane et al. (2004)
Rad/m/m
+5000 Rad/m/m

11. Rotation Measures in Irregular Clusters
A400 (3C75) Eilek & Owen (2002)
-170 Rad/m/m
+170 Rad/m/m

12. Rotation Measure of Radio Galaxies in Clusters
RM images obtained by using the
PACERMAN (Polarization Angle
CorrEcting Rotation Measure Analysis)
Algorithm Dolag et al. (2005)
A Feretti et al. (1999) 3 sources
3C129 Taylor et al. (2001) 2 sources
A514 Govoni et al. (2001) 5 sources

13. SAMPLE OF IRREGULAR CLUSTERS Clarke (2004)
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)
Valle’e et al. (1986)
Kim et al. (1991)
Clarke et al. (2001)
Johnston-Hollitt & Ekers (2004)

14. The Case of A2065 z=0.073 In collaboration with K. Dolag:
THE ROTATION MEASURE OF RADIO GALAXIES IS REPRESENTATIVE OF THE CLUSTER MAGNETIC FIELD
z=0.073
In collaboration with K. Dolag:
RM of radio sources in hot
galaxy clusters

15. Results from Rotation Measures
Lawler & Dennison (1982)
Tribble (1991)
Felten (1996)
Sokoloff et al. (1998)
APPROXIMATION:
Magnetic field tangled on a single scale Λc.
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:
<RM>=-91 Rad/m2
σRM=78 Rad/m2
RM Structures of 5-10 kpc
Magnetic fields in IRREGULAR clusters ~5μG
Magnetic fields in REGULAR clusters are higher ~10-30μG

16. 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

17. Magnetic Field – Gas Density Relation
Dolag et al., in prep.
Regular
Clusters
Irregular
Clusters

18. Results from Rotation Measures
Lawler & Dennison (1982)
Tribble (1991)
Felten (1996)
Sokoloff et al. (1998)
APPROXIMATION:
Magnetic field tangled on a single scale Λc.
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:
<RM>=-91 Rad/m2
σRM=78 Rad/m2
RM IMAGES WITH SMALL SPATIAL SCALES
<RM>≠0
MAGNETO HYDRODYNAMIC SIMULATIONS Dolag et al. (2002)
CLUSTER MAGNETIC FIELDS FLUCTUATE OVER A WIDE RANGE OF SPATIAL SCALES

19. 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)

20. 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

21. 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)

22. Magnetic Field Power Spectrum
(Irregular Cluster)
Cells 10 kpc
B=B =5μG
Feretti et al. (1999)
B∝n
B=B =7.5μG
Dolag et al. (2001)
n=2 6kpc kpc
B =5μG
<B>=1.5μG
0.9 e
Murgia et al. (2004)

23. Magnetic Field Power Spectrum

24. Magnetic Field Power Spectrum:
Simulated halo brigthness and polarization images
Murgia et al. (2004)
n=2
n=3
n=4

25. Diffuse Synchrotron Polarized Emission
A2255: The first detection of polarized emission in a radio halo
Relic
Halo
Govoni et al. (2005)

26. 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

27. 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 >
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.