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

Radio Halos in Clusters of Galaxies Optical X- ray Coma Cluster Galaxy cluster mass: Barions Dark Matter 70% stars + dark matter hot diffuse gas 10% of stars in galaxies 15-20% of hot diffuse gas Radio Halo Radio Relic Diffuse synchrotron radio sources from the ICM: Halos and Relics prove the presence of non-thermal componenets, GeV electrons (  ~10 4 ) and  G magnetic field, mixed with the thermal ICM on Mpc scales. Important ingredients to understand the physics of the ICM.

The Origin of Radio Halos =1.2 Mpce-Diffusion length= The electron-diffusion time necessary to cover Mpc distances is >> than the electron-radiative life-time! T diff (~10 10 yr) >> T v (~10 8 yr) e.g. Jaffe (1977) Need for injection/acceleration in situ Radio Halos are the most spectacular non thermal diffuse sources in clusters :

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

Cluster-Cluster Mergers & re-acceleration scenario One possibility to explain Radio Halos is the turbulent re-acceleration scenario that 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 Giacintucci et al Abell 754 Henry et al Abell 2163 Feretti et al “Bullet” cluster Govoni et al. 2004

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

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

Statistical Modeling of cluster RH: ingredients χ -1 =τ acc γbχ/βγbχ/β ν b  B γ b 2 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 ( τ acc –1 ) due to MS waves. Calculate the acceleration of fossil e  due to the interaction with the turbulent waves and the ensuing Synchrotron and Inverse Compton emission spectra from the resulting electron spectra   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). (cosmological “version” of turbulent-acceleration model) ( Cassano & Brunetti 2005, Cassano et al 2006 )

The fraction of GC with RH with ν s >1.4 GHz is expected to increase with the cluster mass (and L X ) in line with present data (from NVSS+GMRT; Cassano et al. 2008). Cassano +al 2006 Model Expectations at 0 =GHz vs Observations f RH The expected number of GHz RH is consistent with RH number counts from the NVSS (z= ; Giovannini et al. 1999) and from the GMRT RH Survey (z= ; Venturi et al. 2007; 2008). Cassano +al 2010 f RH M

Fraction of galaxy clusters with radio halos at low ν ν s >1.4GHz 240 MHz 150 MHz 120 MHz 74 MHz  The expected fraction of clusters with radio halos increases at low ν.  This increase is even stronger for smaller clusters (M<10 15 M ⊙ ).

Radio halo luminosity functions at 120 MHz tot at 120 MHz s < 600 MHz s > 600 MHz n H (P)xP [Gpc h ] -3 P 120 [Watt/Hz] The low-power end of the RHLF is dominated by RH with s 1.9 between 240 and 600 MHz (f - ). 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

LOFAR surveys & detection of RH MS 3 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" About half of RH flux is emitted within 0.5 R H 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. Govoni et al. 2004Brunetti et al We consider a beam of 25"  25" to increase the sensitivity to extended emission:

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

RH Number Counts in LOFAR surveys: I The expected number of RH with 120 < s < 600 MHz (USSRH) increases with increasing the survey sensitivity 120 < s < 600 MHz s > 120 MHz f min (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)  rms=0.25, 0.5, 1, 1.5 mJy/b  rms=0.25 mJy/b => rms=0.2 mJy/b and  =1-1.3  300 RH at z <0.6  50% with s <600 MHz  rms=0.5 mJy/b => rms= mJy/b and  =2.5-2 => rms=0.5 mJy/b and  =1  rms= 1 mJy/b => rms=0.5 mJy/b and  =2  200 RH at z <0.6  33% with s <600 MHz  70 RH at z <0.6  30% with s <600 MHz MS 3 ? Tier 1?

Ebeling, Edge & Henry 2001 f x >3· erg s -1 cm -2 f x > erg s -1 cm -2 f x >3  erg s -1 cm -2 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 20 and   0° and -40°   80°

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

 rms=1 mJy/b L R -L X luminosity correlation at low frequency  rms=0.25 mJy/b USS halos observed at 120 MHz should be less luminous than those with flatter spectra in clusters with the same M (or L x ). The bulk of USS halos visible at low frequency are expected to be associated with galaxy clusters of intermediate X-ray luminosity, L X  3-5  L 120 -L X 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 L 120 -L X correlation with respect to that observed at 1.4 GHz. 0 =120 MHz observed halos MHz MHz MHz >1400 MHz Cassano 2010

Conclusions Radio Halos 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 LOFAR is expected to discover >300 RH in the Tier 1 Large Area Survey and up to  in the MSSS survey (20-25 RH are presently known) Future is bright ! …. LOFAR follow-up of eBCS+MACS clusters   130 RH in the Tier1 LAS The radio—X-ray luminosity correlations should steepen at lower freq.