“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 Cluster Radio Halos in the LOFAR eraLOFAR era

Rossella Cassano

Coll.: G. Brunetti, H.J.A. Röttgering, M. Brüggen

Radio Halos in Clusters of GalaxiesRadio 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 (~104) 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 HalosThe 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!

Tdiff

(~1010 yr) >> Tv (~108 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 mergersRarity” of Radio Halos & connection with cluster mergers

RXCJ 2003-2525 Giacintucci et al. 2007

Abell 754Henry et al. 2004

Abell 2163Feretti et al. 2001

Brunetti et al. 2007 “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. 2004

A 611

…work in progress…

Cluster-Cluster Mergers & re-acceleration scenarioCluster-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 2003-2525 Giacintucci et al. 2007

Abell 754Henry et al. 2004

Abell 2163Feretti et al. 2001

“Bullet” cluster Govoni et al. 2004

Rad

io P

ower

Frequency

νs

νs

VLALOFAR RH detectable at GHz are mainly halos with larger υs ( > 1 GHz) or relatively flatter spectra (α1.1-1.5) 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

We expect a population of RH with different radio spectra, depending on the efficiency of the particle acceleration process

Rare events

More common events

νobs < νs

Low frequency

Abell 521Abell 521 : the prototype of USSRH ?

=1.9

=1.5

High frequency

Macario et al 2010

(Brunetti +al. 2008, Nature 455, 944)

=1.77A697

Dallacasa et al. 2009

Statistical Modeling of cluster RH: ingredients

χ-1 =τacc

γbχ/β νbB γb2

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.

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 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).

Cassano +al 2006

Model Expectations at Model Expectations at 00=GHz vs Observations=GHz vs Observations

fRH

The expected number of GHz RH is consistent with RH number counts from the NVSS (z=0.05-0.2; Giovannini et al. 1999) and from the GMRT RH Survey (z=0.2-0.4; Venturi et al. 2007; 2008).

Cassano +al 2010

fRH M

Fraction of galaxy clusters with radio halos at low Fraction of galaxy clusters with radio halos at low νν

νs>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<1015 M ⊙ ).

Radio halo luminosity functions at 120 MHzRadio halo luminosity functions at 120 MHz

tot at 120 MHzs < 600 MHz

s < 600 MHz

s > 600 MHz

s > 600 MHz

n H(P

)xP

[G

pc h

70-1]-3

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-).

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 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"

About half of RH flux is emitted within 0.5 RH

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. 2004 Brunetti et al. 2007

We consider a beam of 25"25" to increase the sensitivity to extended emission:

Injection of “fake” RH in the u-v data set

RH

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

fH=0.28 mJy fH=0.32 mJy fH =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: IRH 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

• 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)

rms=0.25, 0.5, 1, 1.5 mJy/b

rms=0.25 mJy/b => rms=0.2 mJy/b and =1-1.3300 RH at z <0.6 50% with s <600 MHz

rms=0.5 mJy/b => rms=0.2-0.25 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 MS3 ?

Tier 1?

Ebeling, Edge & Henry 2001

fx>3·10-12 erg s-1cm-2 fx>10-12 erg s-1cm-2fx>310-12 erg s-1cm-2

RH Number Counts in LOFAR surveys: IIRH 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<0.3 eBCS (Ebeling et al. 1998, 2000) NORAS (Böhringer et al. 2000) : 33% of the sky: |b|> 20 and 0°

0.3<z<0.6 MACS (Ebeling et al. 2001) 55% of the sky: |b|> 20 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: IIRH Number Counts in LOFAR surveys: II

rms=0.25 mJy/b => rms=0.2 mJy/b and 1-1.3 130 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

LLRR-L-LXX luminosity correlation at low frequency 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 Lx).

The bulk of USS halos visible at low frequency are expected to be associated with galaxy clusters of intermediate X-ray luminosity, LX3-51044.

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 LL120120-L-LXX correlation with respect to that observed at 1.4 GHz.

0=120 MHz • observed halos

120-240 MHz

240-600 MHz

600-1400 MHz

>1400 MHz

Cassano 2010

ConclusionsConclusions

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 50-100 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.