“Astrophysics with E-LOFAR’’ 16-19 September 2008, Hamburg, Germany

Istituto di Radioastronomia, INAF- Bologna, ITALY

Cluster Radio Halos in the LOFAR eraCluster Radio Halos in the LOFAR era

Rossella CassanoRossella Cassano

Credit: S. Giacintucci

Origin of giant radio halos in galaxy clustersOrigin of giant radio halos in galaxy clusters

GeV electrons on Mpc scales

μG magnetic fields on Mpc scales

Tdiff

(~1010 yr) >> Tv (~108 yr)

(Jaffe 1976)

The Diffusion Problem:

One possibility to explain RH is given by the re-acceleration model in which relic or secondary relativistic electrons are in situ re-accelerated due to the interection with MHD turbulence generated in the cluster volume during merger events (e.g., 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)

Govoni et al. 2004

“bullet” cluster

Basic expectations of the re-acceleration scenario Basic expectations of the re-acceleration scenario

• RHs should be “transient” phenomena in merging GC, with a lifetime 1 Gyr derived from the combination of different timescales (crossing time between GC, dissipation timescale of the injected turbulence) => RH are not common in galaxy clusters.

• The synchrotron spectra of RHs should be caracterized by the presence of a cut-off at high frequency. The presence of the cut-off affects our ability to detect RH in the Universe, introducing a strong bias against observing them at frequency substantially larger than the cut-off frequency.

Rad

io P

ower

Frequency

obs. ν not observable RH

Observations: statisticsObservations: statistics

- 30% of GC with RH => RH not common

- RH are only found in merging clusters

- Bi-modality of GC=> empty region => fast evolution (in 108 yr) of GC from a “radio quiet” to a “radio loud” region! => Tacc108 yr

GMRT observations of a complete sample of 50 galaxy clusters with similar LX and z (Venturi et al. 2007, 2008; Venturi et al. 2007, 2008; see also talk by S. Giacintuccisee also talk by S. Giacintucci this conference)

blue: GMRT GC magenta : other RH

Brunetti et al. 2007

Statistical expectations for RHs in the framework of the re-Statistical expectations for RHs in the framework of the re-acceleration scenarioacceleration scenario (Cassano & Brunetti 2005; Cassano et al. 2006)(Cassano & Brunetti 2005; Cassano et al. 2006)

TurbulenceTurbulence

merger trees

χ-1 =τacc

γbχ/β

νbB γb2

Fraction of galaxy clusters with radio halosFraction of galaxy clusters with radio halos Cassano et al. 2008Cassano et al. 2008

The expected fraction of galaxy clusters with RHs increases with cluster mass (and LX) in line with present data.

NVSSGMRT

Observed

Predicted

0.410.11

0.080.04

What about radio halos at low radio frequency?

LWA

LOFAR

Obs. frequency range

Rad

io P

ower

Frequency

Acceleration

efficiency

Radio halos at lower radio frequenciesRadio halos at lower radio frequencies

Probability

NOWNOW NOW we see RH associated with the most energetic phenomena.

LOFARLOFAR

LOFAR should discover those RH associated with the most common and less energetic phenomena, caracterized by very steep radio spectra.

USSRH

“classical” RH

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

1.4 GHz1.4 GHz

240 MHz

150 MHz 150 MHz

240 MHz

The expected fraction of clusters with radio halos increases at low ν.

This increase is even stronger for smaller clusters (M<1015 M ⊙ ).

The increase of the fraction of galaxy clusters with radio halos as a function of the cluster mass become less striking at low radio frequency.

Cassano et al. 2008

Number Counts of RHs at low radio frequencyNumber Counts of RHs at low radio frequency

1.4 GHz

150 MHz

240 MHz

1.4 GHz

240 MHz

150 MHz

The expected number of RHs increases at lower frequencies by about a factor 10.

LOFARLOFAR should be able to detect the bulk of RHs.

The bulk of RHs emitting at GHz freq. is expected at relatively low z: 0.1-0.3. At low radio frequency a number of RHs is expected to be discovered at relatively higher redshifts z>0.4.

z

Rad

io P

ower

Frequency

The main expectation of the re-acceleration scenario is the presence The main expectation of the re-acceleration scenario is the presence of of Ultra Steep Spectrum Radio HalosUltra Steep Spectrum Radio Halos ( (USSRHUSSRH) emerging at low ) emerging at low υυ

1 GHz74 MHz

USSRHUSSRH

““classical” RHclassical” RH

An exampleAn example

Ultra Steep Spectrum Radio halos (USSRH) at 74 MHzUltra Steep Spectrum Radio halos (USSRH) at 74 MHz

74-600 MHz

600 MHz -1.4 GHz

>1.4 or “classical” >1.4 >1.4

600-1.4

74-600 74-600

600-1.4

At higher redshift USSRH start to become dominant with respect to “classical” RH also in massive clusters (M> 2·1015 M⊙).

74 MHz observations will reveal different populations of RH: USSRH + “classical RH”.

100 % of 74 MHz RH in cluster with M < 1015 M⊙ would be USSRH, while

50 % of 74 MHz RH in clusters with M > 2·1015 M⊙ would be “classical” RH.

Cassano et al., in prep.

Summary Basic expectations of the re-acceleration scenario seem to be in agreement with present observations of giant radio halosgiant radio halos in GC and some clear features of this model can be tested with LOFAR.

A unique expectation of this scenario is the existence of a population of radio halos emerging only at low radio frequency (USSRH). These USSRH are –indeed- not expected in the framework of any other model.

The USSRH in A521 may be the first of many RH that are hidden to present observations but that would be discovered by LOFARLOFAR. These USSRH are expected to be ~10 times more common than classical RH being activated by less energetic merging events, which are more common in the Universe.

The prototype of these USSRH may be the halo in the galaxy cluster A521A521 discovered with GMRT observations at 330-240 MHz .

Thierbach +al. 2002

Schlickeiser et al. 1987

Evidence of break in the spectrum of the emitting electrons at energies of few GeV.

Electron spectra

lossesF()

Acceleration Acceleration mechanism notmechanism notefficient !efficient !

acceleration

Observations: Spectral Cut-OffObservations: Spectral Cut-OffComa

Evidences for a cut-off in the synchrotron spectra of some RHs: ComaComa (Schlickeiser et al. 1987) A 3562A 3562 (Giacintucci et al. 2005) A 521A 521 (Brunetti et al. 2008)

Conclusions An increasing occurrence of giant RHs with cluster mass is expected in the framework of the re-acceleration model. This is in agreement with results of a statistical unbiased analisys of NVSS+GMRT observations for M>1015 M⊙ (LX> ~3·1044 h70

-2 erg/s) clusters.

In this scenario the number of giant radio halos in the whole universe at 1.4 GHz is expected to be ~100. This number is expected to increases up to a factor of ~10 at low radio frequency (150 –74) MHz (LOFAR, LWA). A new populations of USSRH is expected to be discovered at low radio frequency.

A prototype of this new class of radio halos (USSRH) have been discovered at 610-330-235 MHz in the galaxy clusters A521!!!