Annalisa BonafedePhD student at IRA
Radio Astronomy Institute
Bologna (Italy) With:
L. Feretti, G. Giovannini, M. Murgia, F. Govoni, G. B .Taylor , H. Ebeling,
S. Allen, G. Gentile, Y. PhilstromAscona, 1/06/2009Cosmological magnetic field
OOUUTTLILINNEE
Magnetic Field in Galaxy Clusters
The interesting case of MACS J0717+3745
Radio observations: discovery of the most distant radio halo
Polarization properties
Constrain on the magnetic field strength and structure from polarization and under equipartition assumption
conclusions
Galaxy Clusters:
Intra Cluster Medium : Non thermal component magnetic field +relativistic particles
Radio Diffuse EmissionPolarization properties of radio sources
Abell 2163Abell 2163
Feretti et al. (2001), Govoni et al. (2004)Feretti et al. (2001), Govoni et al. (2004)
Abell 1240Abell 1240
Bonafede et al Bonafede et al (2009)(2009)
Optical emission:
Ma et al (2008) Edge et al (2003)
“A prime example of a complex major cluster merger” (Ma + 2009)
Z=0.55 1 arsec 6.394 kpc
07h17m25s
30s
35s
40s
37°43’30”
44’23”
45’00”
46’00”
30”
30”
30”
HST F814w filter
X-ray emission: Ma et al (2008)
Chandra 60 ksec ACIS-I
L =2.74±0.03 1045 erg/s [0.1 – 2.4]keV
<T> =11 kev T going from 5 to 20 keV
Joint Optical – X-ray analysis
4 distinct sub-clusters ongoing
triple merger
+ 6 Mpc long filament,
source of continuous and discrete accretion of matter by the cluster
Z=0.55 1 arsec 6.394 kpc
What happens to the non-thermal
component of the ICM?
Edge et al. 2003
VLA – B-array obs 4 freq in the 20 cm band tos ~ 2h per freq.
C-array obs 2 freq in the 6 cm band tos ~ 2h per freq
Radio observations:
In full polarization mode
+ VLA Archive obs at 8.5 GHz C array
obs at 4.8 GHz D-array
θ~ 5’’ x 5”
θ~ 2’’ x 2”
VLA – C-array obs 1.4 GHz, tos~ 2h θ~ 18’’ x15”
θ~ 18’’ x15”
Observing the compact features A
B
C
Filamentary structure
θ~ 5’’ x 5”
θ~ 2’’ x 2”
Chandra + VLA 1.4 GHZ
HST + VLA 8.5 GHZ
260 kpc
Discovery of the most distant & most powerful radio halo
P1.4 GHz~1.6 1026 W/Hz ; also emitting at 4.9 GHz
1.5
Mpc
Observing the extended emission
θ~ 18’’ x20”
Chandra [0.5-7keV]
+ VLA 1.4 GHZ
1.5
Mpc
Polarized emission from the radio halo
2nd case after Abell 2255 (Govoni e al. 2005)
1.4 GHZ 5% mean Pol
Max ~ 24%
Min ~ 0.6%
θ~ 18’’ x20”
Upper limits 4% con beam 45% quidi non è ‘’strano’’
1.4 GHZ4.9 GHZ
What are we seeing at high resolution?
8% mean Pol17% mean Pol
9% pol flux16% pol flux
20% pol flux0.4 % pol flux
But...
Faraday rotation internal to the emiting source
Strong beam depolarization
RM
RM*2
...we can verify the fit of Ψ versus λ2
Foreground structure RM due to our galaxy λ2 law
Background structure RM due to the cluster λ2
lawWithin the cluster icm structure Internal rotation NO λ2
law
RM fit with PACERMAN algorithm (Polarization Angle CorrRecting Rotation Measure ANalisys )
Dolag et al. (2005)
Filamentary part of the radio halo
% Polarization:
-Increases with increasing n
-Decreases toward the cluster center
nB 2||
Numerical Simulations by murgia et al. 2004
Vectorial form
n ≥3
Λmax 100s kpc
The magnetic field power spectrum
if Emin is constat with r
pEEN )(
Using the deprojected brightness profile
Fact
or
2.5
Under the equipartition assumption: L computed in a fixed
energy range:Emin Emax
2
3
2
2
0 1)(
cr
rBrB 1.1
2
3
2
2
0 1)(
cr
rnrn
Beta-model from x-ray analysis (Ma et al. 2008)1.1
2/)5()( pBj
Under the equipartition assumption: L computed in a fixed
energy range:Emin Emax
Emin =100 and Emax >> Emin
GBeq 2.1
2
3
2
2
0 1)(
cr
rBrB
1.1GB 30
ConclusionsDiscovery of a giant radio Discovery of a giant radio
halohalo emitting from emitting from 74 MHZ – 4.9 GHz74 MHZ – 4.9 GHz z=0.55 z=0.55 the most distant one the most distant one
P P1.4 GHz1.4 GHz~1.6 ~1.6 10102626 W/Hz W/Hz the most powerful one the most powerful one
Polarized emission from the radio Polarized emission from the radio halohalo 0.6 % – 24% 0.6 % – 24% second case after A2255 second case after A2255
(Govoni et al. 2005)(Govoni et al. 2005) Magnetic field Magnetic field power spectrum n> 3, power spectrum n> 3, ΛΛmaxmax≥ 100s≥ 100s kpckpc
BBeqeq~ 1.2 µG, B~ 1.2 µG, B00 ~3 µG, B decreases as the ~3 µG, B decreases as the gas densitygas density
Under Under equipartition equipartition
assumption assumptionBonafede et al, arXiv:0905.3552 .
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