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“ Surveying the low frequency sky with LOFAR ” 8-12 March 20 10 , Leiden , The Netherlands

Rossella Cassano. INAF- Istitut o di Radioastronomia, Bologna, ITALY. Cluster Radio Halos in the LOFAR era. Coll.: G. Brunetti, H.J.A. R öttgering, M. Brüggen. “ Surveying the low frequency sky with LOFAR ” 8-12 March 20 10 , Leiden , The Netherlands. Radio Halos in Clusters of Galaxies.

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“ Surveying the low frequency sky with LOFAR ” 8-12 March 20 10 , Leiden , The Netherlands

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  1. Rossella Cassano INAF-Istituto di Radioastronomia, Bologna, ITALY Cluster Radio Halos in the LOFAR era Coll.: G. Brunetti, H.J.A. Röttgering, M. Brüggen “Surveying the low frequency sky with LOFAR”8-12 March 2010, Leiden, The Netherlands

  2. Radio Halos in Clusters of Galaxies Radio Halo Coma Cluster Optical X- ray stars + dark matter hot diffuse gas Radio Relic Diffuse synchrotron radio sources from the ICM: Halosand Relicsprove the presence of non-thermal componenets, GeV electrons (~104)and Gmagnetic field,mixed with the thermal ICM on Mpc scales. Galaxy cluster mass: Barions Dark Matter70% 10%of stars in galaxies 15-20% of hot diffuse gas Important ingredients to understand the physics of the ICM.

  3. The Origin of Radio Halos Radio Halos are the most spectacular non thermal diffuse sources in clusters : Tdiff (~1010 yr) >> Tv (~108 yr) e.g. Jaffe (1977) The electron-diffusion time necessary to cover Mpc distances is >> than theelectron-radiative life-time! e-Diffusion length= =1.2 Mpc Need for injection/acceleration in situ

  4. “Rarity” of Radio Halos & connection with cluster mergers Brunetti et al. 2007 “Radio loud” GC Abell 754 Henry et al. 2004 “Bullet” cluster Govoni et al. 2004 blue GMRT GC magenta other RH “Radio Quiet” GC The majorityof Radio Quietclusters are found in a relaxed dynamical status. A 611 RXCJ 2003-2525 Giacintucci et al. 2007 Abell 2163 Feretti et al. 2001 Radio Halosare only found in non-relaxedclusters with evidences forrecent /ongoing cluster mergers (e.g. Buote 2001) …work in progress…

  5. Cluster-Cluster Mergers & re-acceleration scenario Abell 754 Henry et al. 2004 “Bullet” cluster Govoni et al. 2004 One possibility to explain Radio Halos is theturbulent re-acceleration scenariothat assumes that electrons are accelerated by MHD turbulence generated in the cluster volume during merger events(Brunetti et al. 2001, 2004; Petrosian 2001; Ohno et al. 2002; Fujita et al. 2003; Brunetti & Blasi 2005; Cassano & Brunetti 2005; Brunetti & Lazarian 2007; Petrosian & Bykov 2008) RXCJ 2003-2525 Giacintucci et al. 2007 Abell 2163 Feretti et al. 2001

  6. We expect a population of RH with different radio spectra, depending on the efficiency of the particle acceleration process LOFAR VLA RH detectable at GHz are mainly halos with larger υs( > 1 GHz) or relatively flatter spectra(α1.1-1.5) that areassociated with the most energetic and rare phenomena . νobs < νs Rare events νs LOFAR should discover a complex population of RH, including a large number of very steep spectrum sources (α>1.5) (USSRH) that areassociated with more common and less energetic phenomena . Radio Power More common events νs Frequency νs

  7. Abell 521: the prototype of USSRH ? (Brunetti +al. 2008, Nature 455,944) Low frequency High frequency Dallacasa et al. 2009 =1.9 =1.5 Macario et al 2010 =1.77 A697

  8. Statistical Modeling of cluster RH: ingredients (cosmological “version” of turbulent-acceleration model) ( Cassano & Brunetti 2005, Cassano et al 2006 ) Semi-analityc model of cluster formation  merger trees(Press & Schecther 1974; Lacey & Cole 1993) Estimate of the turbulent energy injected in the cluster volume during merger events (Ram Pressure Stripping) and the acceleration efficiency (τacc–1) due to MS waves.  χ-1 =τacc The cosmological evolution of the magnetic field is accounted for by scaling the field with the cluster mass (cosmological MHD simulations; e.g. Dolag et al. 2002). Calculate the accelerationof fossil edue to the interaction with the turbulent waves and the ensuing Synchrotron and Inverse Comptonemission spectra from the resulting electron spectra  νbB γb2 γbχ/β

  9. Model Expectations at 0=GHz vs Observations Cassano +al 2006 Cassano +al 2010 fRH M fRH The expected number of GHz RH is consistent with RH number counts from theNVSS(z=0.05-0.2; Giovannini et al. 1999) and from theGMRT RH Survey(z=0.2-0.4; Venturi et al. 2007; 2008). The fraction of GC with RH with νs>1.4 GHz is expected to increase with the cluster mass(and LX)in line with present data (from NVSS+GMRT; Cassano et al. 2008).

  10. Fraction of galaxy clusters with radio halos at low ν 74 MHz 120 MHz 150 MHz 240 MHz νs>1.4GHz • The expected fraction of clusters with radio halos increases at low ν. • This increase is even stronger forsmaller clusters(M<1015 M⊙ ).

  11. Radio halo luminosity functions at 120 MHz For a given cluster mass (and B), radio halos with smaller values of s have lower monochromatic radio luminosity at a given frequency 0< s Cassano et al. 2010 s < 600 MHz tot at 120 MHz s < 600 MHz s > 600 MHz nH(P)xP [Gpc h70-1]-3 s > 600 MHz P120 [Watt/Hz] P120 [Watt/Hz] The low-power end of the RHLF is dominated by RH with s < 600 MHz, i.e., halos with a synchrotron radio spectrum >1.9 between 240 and 600 MHz (f-).

  12. LOFAR surveys & detection of RH MS3 commisioning survey  120 MHz, rms  0.5 mJy, beam  30"30" Tier 1: The Large Area Survey  120 MHz, rms  0.1 mJy, beam 5"5" We consider a beam of 25"25" to increase the sensitivity to extended emission: About half of RH flux is emitted within 0.5 RH Govoni et al. 2004 Brunetti et al. 2007 Given the typical brightness profiles of RH this approach would lead to the detection, in the case  1 , at several , of the central part of the halos => we would identify (at least) candidates RH.

  13. (Brunetti et al. 2007, Cassano et al. 2008, Venturi et al. 2008) Injection of “fake” RH in the u-v data set “empty” field NVSS field 0=1.4 GHz, rms=0.45 mJy/b beam= 45"45" • 1 for the NVSS survey  2 for the GMRT RH surveys fH=0.28 mJy fH=0.32 mJy fH =0.45 mJy RH 2) 3) 1)

  14. RH Number Counts in LOFAR surveys: I • fmin(z) of Mpc scale RH assuming different values of “ rms” that mimic possible LOFAR observations; • LOFAR sky coverage: Northern hemisphere (>0) and high Galactic latitudes (|b|>20) s > 120 MHz  rms=0.25, 0.5, 1, 1.5 mJy/b The expected number of RH with120 <s < 600 MHz(USSRH)increases with increasing the survey sensitivity 120 <s < 600 MHz 300RH at z <0.650%with s <600 MHz  rms=0.25 mJy/b=> rms=0.2 mJy/b and =1-1.3 Tier 1?  rms=0.5 mJy/b=> rms=0.2-0.25 mJy/b and =2.5-2 200RH at z <0.633%with s <600 MHz => rms=0.5 mJy/b and =1 70RH at z <0.630%with s <600 MHz  rms= 1 mJy/b=> rms=0.5 mJy/b and =2 MS3 ?

  15. RH Number Counts in LOFAR surveys: II • Searching for RH in X-ray selected cluster samples with LOFAR surveys; • Catalogs of X-ray clusters in the northern hemisphere: z<0.3eBCS (Ebeling et al. 1998, 2000) NORAS (Böhringer et al. 2000) : 33% of the sky: |b|> 20 and  0° • 0.3<z<0.6MACS (Ebeling et al. 2001) 55% of the sky: |b|> 20 and -40° 80° Ebeling, Edge & Henry 2001 fx>310-12 erg s-1cm-2 fx>3·10-12 erg s-1cm-2 fx>10-12 erg s-1cm-2

  16. RH Number Counts in LOFAR surveys: II Combining radio and X-ray selection criteria we derive:  rms=1 mJy/b  rms=0.25 mJy/b s > 120 MHz 600 <s < 1400 MHz s > 1.4 GHz 240 <s < 600 MHz 120 <s < 240 MHz 130 RH at z<0.6 (out of 400 clusters in eBCS and MACS sample) 40% with s< 600 MHz  rms=0.25 mJy/b=> rms=0.2 mJy/b and 1-1.3  rms= 1 mJy/b=> rms=0.5 mJy/b and =2 70 RH at z<0.6, 20 RH with s< 600 MHz

  17. LR-LX luminosity correlation at low frequency Cassano 2010 0=120 MHz • observed halos  rms=0.25 mJy/b  rms=1 mJy/b >1400 MHz 600-1400 MHz 120-240 MHz 240-600 MHz USS halos observed at 120 MHz should be less luminous than those with flatter spectra in clusters with the same M (or Lx). The bulk of USS halos visible at low frequency are expected to be associated with galaxy clusters of intermediate X-ray luminosity, LX3-51044. By making use of Monte-Carlo procedures we show that the presence of these RH in LOFAR surveys at 120 MHz would cause a steepening and a broadening of the L120-LXcorrelation with respect to that observed at 1.4 GHz.

  18. Conclusions RadioHalos are possibly due to turbulence acceleration occuring in clusters during merger events Radio Halos are expected to be a complex population of radio sources whose spectral properties should be intrinsically related to the dynamical status of the hosting clusters Future is bright ! …. LOFAR is expected to discover >300 RH in the Tier 1 Large Area Survey and up to  50-100 in the MSSS survey (20-25 RH are presently known) LOFAR follow-up of eBCS+MACS clusters  130 RH in the Tier1 LAS The radio—X-ray luminosity correlations should steepen at lower freq.

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