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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
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