Observations of Magnetic Fields in Intracluster Medium. Govoni Federica INAF Osservatorio Astronomico di Cagliari

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1 Observations of Magnetic Fields in Intracluster Medium Govoni Federica INAF Osservatorio Astronomico di Cagliari Collaborators: M. Murgia, L. Feretti, G. Giovannini, V. Vacca, A. Bonafede IAU 2012 China SpS4: New era for studying interstellar and intergalactic magnetic fields

2 Galaxy Clusters Optical

3 Galaxy Clusters Optical - X-ray

4 Galaxy Clusters Our knowledge of intracluster magnetic fields has advanced significantly over the past decade, thanks to new radio observations and to more sophisticated interpretative tools. Reviews: Kronberg et al. (1994) Carilli & Taylor (2002) Govoni & Feretti (2004) Ferrari et al. (2008) Feretti et al. (2012)

5 Galaxy Clusters (First part of the talk) Polarized emission from RADIO GALAXIES Optical - X-ray - Radio Individual clusters Statistical studies

6 Galaxy Clusters (Second part of the talk) Total intensity and Polarized emission from RADIO HALOS Optical - X-ray - Radio

7 The Faraday rotation effect in galaxy clusters 4.8 GHz 10 kpc 1.4 GHz NGC 326 Murgia et al. (2001)

8 The Faraday rotation effect in galaxy clusters see e.g. Burn (1966)

9 Interpretation of Faraday rotation effect The magneto-ionic medium is approximated by uniform cells of size ΛC with random orientation in space The Faraday rotation from a physical depth L (>>ΛC) is expected to be a Gaussian with zero mean and dispersion given by: See e.g. Lawler & Dennison (1982), Tribble (1991), Feretti et al. (1995), Felten (1996), Sokoloff et al. (1998)

10 Interpretation of Faraday rotation effect - Dedicated software tools Murgia et al. (2004), Laing et al. (2008) - Semi analytical approach Ensslin & Vogt (2003), Kuchar & Ensslin (2011) (Bayesian approach) --> Constraint the magnetic field power spectrum parameters n=11/3 Λmax=32 kpc FARADAY code (Murgia et al. 2004)

11 Interpretation of Faraday rotation effect - Dedicated software tools Murgia et al. (2004), Laing et al. (2008) - Semi analytical approach Ensslin & Vogt (2003), Kuchar & Ensslin (2011) (Bayesian approach) --> Constraint the magnetic field power spectrum parameters n=11/3 Λmax=64 kpc FARADAY code (Murgia et al. 2004)

12 Interpretation of Faraday rotation effect - Dedicated software tools Murgia et al. (2004), Laing et al. (2008) - Semi analytical approach Ensslin & Vogt (2003), Kuchar & Ensslin (2011) (Bayesian approach) --> Constraint the magnetic field power spectrum parameters n=11/3 Λmax=128 kpc FARADAY code (Murgia et al. 2004)

13 Interpretation of Faraday rotation effect - Dedicated software tools Murgia et al. (2004), Laing et al. (2008) - Semi analytical approach Ensslin & Vogt (2003), Kuchar & Ensslin (2011) (Bayesian approach) --> Constraint the magnetic field power spectrum parameters Synthetic RM images which include the effects of window functions, noise, etc., that can be directly compared with the observations. FARADAY code (Murgia et al. 2004)

14 Interpretation of Faraday rotation effect Structure function S(dx,dy)=<[RM(x,y) RM(x+dx, y+dy)]²>(x,y) Autocorrelation function A(dx,dy)=<[RM(x,y)RM(x+dx, y+dy)]>(x,y) <RM> σrm

15 Individual clusters 3C449 GALAXY GROUP Feretti et al. (1999), Guidetti et al. (2010) <RM> = rad/m² σrm = 19 rad/m² 3C449 Central magnetic field strength B0~3.5 µg which fluctuates over scales from 0.2 kpc to 65 kpc.

16 Individual clusters 3C31 GALAXY GROUP Laing et al. (2008) In this inclined radio source (52 ), the far lobe shows larger RM (Laing-Garrington effect) TO OBSERVER Extra path

17 A400 Individual clusters A400 MERGING CLUSTER Eilek & Owen (2002) <RM> = - 8 rad/m² σrm = 100 rad/m² Magnetic field strength of a few µg at the center of merging galaxy clusters.

18 Individual clusters A2199 RELAXED CLUSTER Ge & Owen (1994), Vacca et al. (2012) <RM> = - 54 rad/m² σrm = 460 rad/m² Magnetic field strength larger than 10 µg at the center of cool-core clusters. HYDRA A Taylor & Perley (1993), Ensslin & Vogt (2005), Laing et al (2008)

19 Individual clusters A2199 RELAXED CLUSTER Vacca et al. (2012) Magnetic field power law power spectrum characterized by an index n=2.8 which fluctuate from 0.7 kpc to 35 kpc. Central magnetic field strength B0~11.7 µg which decrease with the radius following the gas density.

20 MERGING CLUSTER GovoniA2255 et al Govoni et al. (2006) Individual clusters η~0.5-1 COMA Bonafede et al. (2010) A119 Feretti at al. (1999) Dolag et al. (2001) A2382 Guidetti et al. (2008)

21 MERGING CLUSTER GovoniA2255 et al Govoni et al. (2006) Individual clusters The magnetic field power spectrum can be approximated with a power law with the slope close to the Kolmogorov index in some clusters but shallower indexes are also observed.

22 MERGING CLUSTER GovoniA2255 et al Govoni et al. (2006) Individual clusters

23 Statistical Studies Govoni et al Govoni et al (see also Dolag et al. 2001, Dolag 2006) These trends indicate that magnetic fields are common in galaxy clusters, in agreement with the results by Clarke et al. (2004) and Johnston-Hollitt & Ekers (2004) who analyzed the <RM> of sources located behind and within clusters.

24 Statistical Studies Govoni et al Auto-correlation length ΛB=25 kpc Govoni et al (see also Dolag et al. 2001, Dolag 2006) η 0.5

25 Statistical Studies Govoni et al Auto-correlation length ΛB=25 kpc Govoni et al (see also Dolag et al. 2001, Dolag 2006) η 0.5

26 Statistical Studies Govoni et al Auto-correlation length ΛB=25 kpc Govoni et al (see also Dolag et al. 2001, Dolag 2006) η 0.5

27 Statistical Studies Bonafede et al. (2011) SELECTED A SAMPLE OF 39 MASSIVE CLUSTERS AND USED THE NVSS TO ANALYZE THE FRACTIONAL POLARIZATION OF RADIO GALAXIES. This trend indicates that magnetic fields are common in galaxy clusters, in agreement with the results by Clarke et al. (2004) and Johnston-Hollitt & Ekers (2004) who analyzed the <RM> of sources located behind and within clusters

28 Statistical Studies Bonafede et al. (2011) Kolmogorov like power-spectrum n=11/3 with and auto-correlation length ΛB=25 kpc

29 Galaxy Clusters (Second part of the talk) Total intensity and Polarized emission from RADIO HALOS Optical - X-ray - Radio

30 Galaxy Clusters Relic (see talks by A. Bonafede and C. Pfrommer) Radio halo Radio halos are ideal sources for magnetic field studies. Different magnetic field power spectra determine different radio halo intensities, morphologies and polarization properties. (Tribble 1991).

31 Synthetic halo images and polarized vectors Λmax=64 kpc FARADAY code (Murgia et al. 2004) Magnetic field power-spectrum n=11/3

32 Synthetic halo images and polarized vectors Λmax=400 kpc FARADAY code (Murgia et al. 2004) Magnetic field power-spectrum n=11/3

33 Synthetic halo images and polarized vectors Λmax=1024 kpc FARADAY code (Murgia et al. 2004) Magnetic field power-spectrum n=11/3

34 Polarization of Radio Halos A2255 Govoni et al. (2005), see also Pizzo et al. (2009)

35 Polarization of Radio Halos MACS J Bonafede et al. (2009), see also Van Weeren et al. (2009) 0.4 Mpc

36 Polarization of Radio Halos A665 Vacca et al. (2010) <B0> 1.3µG η 0.5 Assumptions: - Gaussian magnetic field with a Kolmogorov index - Energy equipartition with a power law electron spectrum - Magnetic field strength decreasing following the thermal gas density

37 Polarization of Radio Halos A665 Vacca et al. (2010) Λ max~450 kpc

38 Polarization of Radio Halos Vacca et al. (2010) FPOL=24%

39 Polarization of Radio Halos Vacca et al. (2010) FPOL=7%

40 Polarization of Radio Halos Vacca et al. (2010)

41 Conclusions The power spectrum of the intracluster magnetic field can be constrained if detailed RM images of cluster radio sources are available. Typical magnetic field strength are of a few μg, but can be larger than 10 μg at the center of cool-core clusters. The magnetic field power spectrum can be approximated with a power law with the slope close to the Kolmogorov index in some clusters but shallower indexes are also observed. There also evidences that the magnetic field decreases following the gas density. It is likely that in the next future we will be able to analyse the polarization properties of radio halos and to obtain a clearer picture of the power spectrum of the magnetic field fluctuations over the entire volume of the cluster.