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

d

HE

E

λ

RM*2

RM

Faraday rotation: IF the cluster acts like an external screen:

dlHnRMd

e0

cos

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 .