LOFAR & Particle acceleration in LOFAR & Particle acceleration in Galaxy ClustersGalaxy Clusters

Gianfranco Gianfranco BrunettiBrunettiInstitute of Radioastronomy –INAF, Bologna,

ITALY

Outline

- Galaxy Clusters & LS radio emission - Radio Halos (turbulence?) - Why low frequency ? - Ultra steep spectrum halos - Evolution of radio halos (and LS magnetic fields)

- Radio Relics (shock acceleration?) - Tracing formation of LSS - Constraining particle acceleration at weak shocks

Non-thermal components

Abell 3376Bagchi et al. 2005

& cluster mergers

Radio Halos and Radio Relics are only found in non-relaxed clusters with recent /ongoing cluster mergers (e.g. Buote 2001)

Abell 754Henry et al. 2004

Both Halos & Relics have steep spectrum, F()=Fo-, with 1.3

Abell 2163Feretti et al. 2001

Radio RelicsRadio Halos

The general picture merger history

clusters increase their mass via merger with smaller subclusters

e, p

TURBULENCE reaccelerates fossil e and secondaries e on Mpc scales

BSHOCKS accelerate e , pcr

± e

pcr pth 0 rays

The general picture merger history

clusters increase their mass via merger with smaller subclusters

e, p

TURBULENCE reaccelerates fossil e and secondaries e on Mpc scales

BSHOCKS accelerate e , pcr

± e

pcr pth 0 rays

?

?(eg., Brunetti et al. 2001, 2004, 2009; Petrosian 2001; Miniati et al. 2001; Fujita et al. 2003; Ryu et al. 2003; Pfrommer & Ensslin 2004; Brunetti & Blasi 2005; Cassano & Brunetti 2005; Cassano et al. 2006; Brunetti & Lazarian 2007; Hoeft & Bruggen 2007; Pfrommer et al. 2008; Petrosian & Bykov 2008)

Why low frequency ?

Regardless of the origin of Radio

Halos, extrapolations of their number

counts at 1.4 GHz based on the

Radio-X ray correlation observed for

Radio Halos suggest that a large

fraction of these Halos is at faint fluxes.

Due to their steep synchrotron spectrum, faint Radio Halos should appear

more luminous at low frequencies and thus LOFAR and LWA are expected

to discover a large number of these objects.

Ensslin & Roettgering 2002

Rad

io P

ower

Frequency

Low frequency High frequency

Are we biased (high freq) ? (Brunetti +al. 2008, Nature

455,944)

Ferrari et al.2003,06

Rad

io P

ower

Frequency

Dallacasa, GB, et al. 2009

=1.9

=1.5

N(E)=k E-4.8

lossesacceleration

Are we biased (high freq) ? (Brunetti +al. 2008, Nature

455,944)

Rad

io P

ow

er

Frequency

The case of the “ultra steep” The case of the “ultra steep” spectrm radio halosspectrm radio halos (turbulent acceleration model) (turbulent acceleration model)

Energy releaseAcceleration efficiency

Rare events

More common events

Rad

io P

ow

er

Frequency

The case of the “ultra steep” The case of the “ultra steep” spectrm radio halos spectrm radio halos

Rad

io P

ow

er

Frequency

We expect a populations of radio halos withsteeper spectrum that is (better) visible at low frequencies (Cassano, Brunetti, Setti 2006)

The case of the “ultra steep” The case of the “ultra steep” spectrm radio halos spectrm radio halos

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 MHz150 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 ⊙ )

Cassano et al. 2008

Rad

io P

ower

Frequency

Acc

loss

Montecarlo calculations

Rad

io P

ow

er

Frequency

( Cassano, GB, et al in prep )

z=0-0.1z=0.5-0.6

600 MHz

Expected number counts( Cassano, GB, et al in prep )

Totalb < 600 MHz

beam=20x20 arcsec

120 MHz

Mpc scale

Evolution of Radio HalosEvolution of Radio Halos

Brunetti et al 2007++

Which is the difference betweenRH-clusters and ULimits ?

Does non thermal emission evolve ?

Which time-scale ?

Evolution of Radio HalosEvolution of Radio Halos

Which is the difference betweenRH-clusters and ULimits ?

Does non thermal emission evolve ?

Which time-scale ?

Brunetti et al 2007++

Radio Emitting GC

Radio Quiet GC

Evolution of Radio HalosEvolution of Radio Halos

Which is the difference betweenRH-clusters and ULimits ?

Does non thermal emission evolve ?

Which time-scale ?

Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004, Venturi et al. 2008)

Brunetti et al 2007++

Radio Emitting GC

Radio Quiet GC

Evolution of Radio HalosEvolution of Radio Halos Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004)

Which is the difference betweenRH-clusters and ULimits ?

Does non thermal emission evolve ?

Which time-scale ?

Magnetic field dissipation ?

Brunetti et al 2007++

Radio Emitting GC

Radio Quiet GC

Brunetti et al. 2009

B+δB

B

Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004)

Which is the difference betweenRH-clusters and ULimits ?

Does non thermal emission evolve ?

Which time-scale ?

Magnetic field dissipation ?

0.4 Gyr

0.6 Gyr

1 Gyr

1.4 Gyr

Evolution of Radio HalosEvolution of Radio Halos

Brunetti et al. 2009

B+δB

B

diss << 1 Gyr

Connection with cluster mergers (e.g. Schuecher et al. 2001, Markevitch et al. 2002, Boschin et al. 2003 Govoni et al. 2004)

Which is the difference betweenRH-clusters and ULimits ?

Does non thermal emission evolve ?

Which time-scale ?

Magnetic field dissipation ?

1 Gyr

Subramanian et al.2006

Evolution of Radio HalosEvolution of Radio Halos

B+δB

B

Evolution of Radio HalosEvolution of Radio Halos

The acceleration of emitting particles

must be “transient” and particle cooling drives the transition …

consistent with turbulent acceleration (Brunetti et al. 2007, 2009)

1/2 Gyr

Evolution of Radio HalosEvolution of Radio Halos

The acceleration of emitting particles

must be “transient” and particle cooling drives the transition …

consistent with turbulent acceleration (Brunetti et al 2007, 2009)

1/2 Gyr

Cluster “bi-modality” is expectedless important at lower frequencies.

Also the spread of the correlationmust increase at lower frequencies

Radio Relics as tracers of shocks at LSS

Shock acceleration thermal (ICM) particles(Ensslin et al.1998; Roettiger et al.1999; Sarazin 1999; Miniati et al.2001; ..)

Compression of ghost radio plasma (by shocks)(Ensslin & Gopal-Krishna 2001; Bruggen & Ensslin 2002;..)

The potential is to trace LS shocks where thermal emission is fading awayThe potential is to trace LS shocks where thermal emission is fading away && to unveil the evolution of radio plasma (& its sources) in the ICMto unveil the evolution of radio plasma (& its sources) in the ICM

Abell 3376Bagchi et al. 2005

Abell 3667Roettiger et al 1999

Shocks in Galaxy Clusters

Vazza, Brunetti, Gheller 2008

Shocks in Galaxy Clusters

Vazza, Brunetti, Gheller 2008

Miniati et al. 2001;Ryu et al. 2003;Pfrommer et al. 2006,08;Hoeft & Bruggen 2007;Skillman et al. 2008

Hoeft & Bruggen 2007

First attempts to radio images from Cosmological simulations

also Miniati et al 2001; Pfrommer et al 2008; Donnert et al 2009

Magnetic field is compressed (and amplified)at shocks, consequently Radio Relics shouldbe polarised (relatively large polarisation)

Hoeft & Bruggen 2007

Complex situations …

Hoeft & Bruggen 2007

van Weeren et al. 2009Bonafede et al. 2009

Brentjens 2008

Uncertainties in CR accelerationKang & Jones 2007

Pfrommer et al. 2008

Vazza, Brunetti, Gheller 2008

Kang & Jones 2007

Pfrommer et al. 2008

Vazza, Brunetti, Gheller 2008

Kang & Jones 2002

Kang & Jones 2002

Uncertainties in CR acceleration

Kang & Jones 2007

Pfrommer et al. 2008

Vazza, Brunetti, Gheller 2008

Kang & Jones 2002

Kang & Jones 2002

Uncertainties in CR acceleration

Galaxy clusters are unique labs Galaxy clusters are unique labs to study particle acceleration to study particle acceleration

at at weak & LSweak & LS shocks shocks

Conclusions

- A fraction of the energy dissipated during cluster formation is channelled into non thermal components - Radio Halos (turbulence?) - Calculations suggest that we are missing the bulk of Halos ! - Calculations suggest that LOFAR will detect several 100+ Radio Halos (depending on rms….) - Radio Halos are “transient” sources connected with mergers and we claim that the synchrotron emission in “radio quiet” clusters is suppressed by particle cooling (test by LOFAR)

- Radio Relics (shock acceleration?) - Allow tracing formation of LSS… better(?) than X-rays - Unique: constraining particle acceleration at weak shocks