Statistics of Giant Radio Halos and Electron Reacceleration Models

1
Rossella Cassano
INAF-Istituto di Radioastronomia,
Bologna, ITALY
Large scale diffuse radio emission from clusters
of galaxies
The Low Frequency Radio Universe 8-12
December 2008, Pune-INDIA
2
Outline of the talk
Clusters of Galaxies a brief intro
Diffuse Radio emission from the ICM Radio Relics
Radio Halos
Theoretical Models
The importance of low frequency observations
(GMRT, LOFAR, LWA)
3
Clusters of galaxies
what is a galaxy cluster made of ?

• Galaxy Clusters are the largest concetrations of
  matter in our Universe.
• They extend over 2-4 Mpc and have a total mass
  of 1014-1015 M?
• They contain thousands of galaxies, hot diffuse
  gas and expecially dark matter

Optical
X- ray
Coma Cluster
Galaxy cluster mass
Barions Dark Matter
70
10 of stars in galaxies 15-20 of hot diffuse
gas
hot diffuse gas
stars dark matter
4
Clusters of galaxies
How galaxy clusters form?
Borgani et al. 2004

• Cosmic structures forms as a result of the
  gravitational amplification of primordial density
  fluctuations.
• Clusters form by accretion of matter and mergers
  between sub-units at the intersection of
  filaments which make up the cosmic web.
• Gas falling into deep potential wells is heated
  to 107-108 K by shocks (and adiabatic
  compression)

5
Non-thermal components
Observational evidences
Diffuse synchrotron radio sources from the ICM
(not associated with any individual galaxy)

• Radio Halos (L1.4GHz 1024-1026 h70 Watt/Hz)
• steep spectrum sources (? 1.1-1.5)
• low surface brightness (µJy arcsec-2 at 1.4 GHz)
• at the cluster centre
• generally regular shape (mimic the X-ray
  morphology) (Mpc size)

-2
Radio Halo
Coma Cluster

• Radio Relics (L1.4GHz 1023-1025 h70 Watt/Hz)
• steep spectrum sources (? 1.1-1.5)
• at the cluster outskirts
• elongated morphology polarised

-2
Radio Relic
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.
6
Non-thermal components
cluster mergers
Radio Halos and Radio Relics are only found in
non-relaxed clusters with recent /ongoing cluster
mergers (e.g. Buote 2001)
Mergings are the most energetic events in the
Universe few 1063ergs in a crossing time!
Abell 3376 Bagchi et al. 2005
Abell 754 Henry et al. 2004
Abell 2163 Feretti et al. 2001
7
Non-thermal components
Cosmic rays in Galaxy Clusters

• INJECTION of CRs in GC may occur due to AGN
  (e.g., Ensslin et al. 1997, Biermann et al.
  2002) Starbursts / Galactic Winds (e.g., Voelk
  Atoyan 1999, 2000) Cosmological Shocks (e.g.,
  Roettiger et al. 1999, Miniati et al. 2001,
  Gabici Blasi 2003, Ryu et al. 2003, Pfrommer et
  al. 2006)

CR protons are long living particles and are
confined
(Voelk et al 1996 Berezinsky et
al 1997)
Diffusion time
protons
CR electrons are short living particles and
accumulated at ??100-300
(e.g., Sarazin 1999)
electrons
Radio emitting electrons ??104 have lifetime of
108 yrs)
Blasi, Gabici, Brunetti 2007
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What is the origin of the emitting electrons in
radio halo and relic sources?
9
Radio Relics
merger shock
subcluster
As suggested by observations radio relics seem to
be connected with merger shocks
main cluster
Shock (re-) acceleration (e.g.,Ensslinal.1998
Hoeft Bruggen 2007)
Relic emission produced by (thermal or
fossil) electrons accelerated by the passage of a
merger shock waves. The steady state energy
spectrum of the electrons in the relic is a power
law
Giacintucci et al. 2008
Abell 521
1
Adiabatic compression (e.g., Ensslin
Gopal-Krishna al. 2001)
Large scale shocks may
efficiently boost up ghost-radio plasma via
adiabatic compression in case this plasma is
not efficiently mixed with the ICM possibly
curved spectrum
GMRT 610 MHz radio contours on Chandra X-ray
image
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Radio Halos
the diffusion problem
Radio Halos are the most spectacular non thermal
diffuse sources in clusters
Tdiff (1010 yr) gtgt Tv (108 yr)
e.g. Jaffe (1977)
The electron-diffusion time necessary to cover
Mpc distances is gtgt than the electron-radiative
life-time!
1.2 Mpc
e-Diffusion length
Origin of relativistic electrons?
11
Radio Halos
secondary models
secondary modelsrelativistic electrons
continuosly injected in the ICM by inelastic
proton-proton collisions through production and
decay of charged pions (e.g., Dennison 1980,
Blasi Colafrancesco 1999, Dolag Ensslin 2000)
pth pCR ? ?? ?0 ? ?0 ? ?? ? ? gamma ray
emission
?? ? e? ? synchrotron radio emission

The fact that CR protons are long living
particles and are confined (Voelk et al 1996
Berezinsky et al 1997) solves of the diffusion
problem !
12
Radio Halos
secondary models
The drawback of secondary models is that diffuse
radio emission should be common in galaxy
clusters.. regardless of their present
dynamical properties!
Radio loud GC
Brunetti et al. 2007
blue GMRT GC
magenta other RH
Recently, GMRT observations of a complete sample
of 50 galaxy clusters with similar LX and z
(Venturi et al. 2007, 2008) confirmed that only
30 of X-ray luminous galaxy clusters host Radio
Halos Halos are not common in clusters
Radio Quite GC
see also talk by T. Venturi
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Radio Halos
Re-acceleration model
Only 10-30 of massive galaxy clusters have Radio
Halos (Giovannini al.1999, Cassano al.
2008) Radio Halos are detected only in galaxy
clusters with recent / ongoing merging
activity (Buote 2001, Venturi al 2007,08)
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Radio Halos
Re-acceleration model
Only 10-30 of massive galaxy clusters have Radio
Halos (Giovannini al.1999, Cassano al.
2008) Radio Halos are detected only in galaxy
clusters with recent / ongoing merging
activity (Buote 2001, Venturi al 2007,08)
Possible transient nature of RHalos
In situ re-acceleration by MHD turbulence
developed 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 Brunetti
Lazarian 2007 Petrosian Bykov 2008)
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Hystorical motivation Spectral steepening
Coma
Electron spectra
Schlickeiser et al. 1987
F(?)
losses
acceleration
Thierbach al. 2002
Evidence of break in the spectrum of the emitting
electrons at energies of (only!) a few GeV.
Acceleration mechanism is not efficient . Fermi
II (?)
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The general picture
merger history
SHOCKS accelerate e?, pcr
B
e?, p
pcr pth ? ?0 ? ? rays
? ? e?
clusters increase their mass via merger with
smaller subclusters
TURBULENCE reaccelerates fossil e? and
secondaries e? on Mpc scales
17
The general picture
?
merger history
SHOCKS accelerate e? , pcr

B
e?, p
pcr pth ? ?0 ? ? rays
clusters increase their mass via merger with
smaller subclusters
? ? e?
TURBULENCE reaccelerates fossil e? and
secondaries e? on Mpc scales

?
18
The general picture
merger history
SHOCKS accelerate e? , pcr

B
e?, p
pcr pth ? ?0 ? ? rays
clusters increase their mass via merger with
smaller subclusters
? ? e?
TURBULENCE reaccelerates fossil e? and
secondaries e? on Mpc scales

Synchrotron ICompton from secondary and
reaccelerated e? (Radio, X-rays, Gamma
rays) Gamma rays from ?0 decay
Broad band spectrum Statistical properities
In general complex
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General multiwavelength expectations
Brunetti, 2008
IC
po
MERGING CLUSTERS
Syn
4.5
1.5
ALL CLUSTERS
Ecr0.8 Eth
20
General multiwavelength expectations
Brunetti, 2008
blue GMRT GC
IC
magenta other RH
po
Syn
TRANSIENT PHENOMENA
4.5
1.5
Ecr0.8 Eth
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Statistical expectations fraction of GC with
radio halos
Cassano al 2006
GMRT
NVSS
NVSS data (from Giovannini et al. 1999) and
deep GMRT observations.

In the turbulent reacceleration model the
fraction of GC with RHalos is expected to
increase with the cluster mass (and LX).
More massive clusters develop more
turbulence (Eturb? Eth).
0.41?0.11 for Lxgt1044.9 erg/s 0.08?0.04 for
Lxlt1044.9 erg/s
(Venturi et al. 2007, 2008 Cassano et al. 2008)
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Why low frequency ?
Ensslin Roettgering 2002
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.
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Radio halos at lower radio frequencies
Obs. frequency range
NOW we see RH associated with the most energetic
phenomena!
LOFAR LWA
NOW
Acceleration efficiency
LOFAR should discover those RH associated with
the most common and less energetic phenomena!
Radio Power
Probability
Frequency
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Fraction of galaxy clusters with radio halos at
low ?
Cassano et al. 2008
150 MHz
150 MHz
240 MHz
240 MHz
1.4 GHz
1.4 GHz

• The expected fraction of clusters with radio
  halos increases at low ?
• This increase is even stronger for smaller
  clusters (Mlt1015 M? )

25
Radio halos at lower radio frequencies
classical RH
NOW we see RH associated with the most energetic
phenomena!
LOFAR LWA
NOW
LOFAR should discover those RH associated with
the most common and less energetic phenomena!
Radio Power
USSRH
GHz
Frequency
26
GMRT reveals the first USSRH in Abell 521
Brunetti, et al., 2008, Nature, 455, 944
USSRH
radio flux
frequency
TURBULENCE ..
27
Number Counts of RHs at low radio frequency
total
150 MHz
240 MHz
1.4 GHz
USS
15 rms1.62 mJy/b
We expect that the number of RHs detectable by
deep low frequency observations is 10 times
larger than that from present VLA observations at
1.4 GHz LOFAR and LWA would catch these RHs !
beam25x 25
Number counts at 120 MHz decresing the flux
limit of the surveys increases the fraction of
USSRH (Cassano et al., in prep.).
28
Number Counts of RHs at low radio frequency
total
150 MHz
240 MHz
1.4 GHz
USS
25 rms1 mJy/b
We expect that the number of RHs detectable by
deep low frequency observations is 10 times
larger than that from present VLA observations at
1.4 GHz LOFAR and LWA would catch these RHs !
beam25x 25
Number counts at 120 MHz decresing the flux
limit of the surveys increases the fraction of
USSRH (Cassano et al., in prep.).
29
Number Counts of RHs at low radio frequency
total
150 MHz
240 MHz
1.4 GHz
USS
50 rms0.5 mJy/b
We expect that the number of RHs detectable by
deep low frequency observations is 10 times
larger than that from present VLA observations at
1.4 GHz LOFAR and LWA would catch these RHs !
beam25x 25
Number counts at 120 MHz decresing the flux
limit of the surveys increases the fraction of
USSRH (Cassano et al., in prep.).
30
Conclusions
The presence of non-thermal components in the ICM
is now well established
Radio Halos (turbulence?) and Radio Relics
(shocks?) are connected with cluster mergers
Complex physical picture protons, secondaries,
different reacceleration processes that implies a
complex broad band spectrum
Statistical properties of Radio Halos are also
expected to be complex
Future is bright ! .
Low frequency radio observations are expected to
discover 1000 Radio Halos and many Radio relics
(LOFAR, LWA, GMRT)
Also high energy observatories (FERMI, Cherenkov
Arrays) will provide crucial constraints