giant radio halos



giant radio halos
Dynamics of galaxy clusters
A radio perspective
Tiziana Venturi, INAF, Istituto di Radioastronomia
S. Giacintucci, D. Dallacasa, R. Kale, G. Brunetti, R. Cassano, M. Rossetti
GEE 3 – Padova, 12 November 2013
Diffuse radio emission in galaxy clusters as signpost for cluster
 Radio halos and relics: massive cluster mergers
 Radio mini-halos: massive cool core clusters
Radio emission in galaxy
over the
cluster scale
Emission from
individual radio
Cluster weather
and AGN
3C75 in A401
Hydra A
Radio emission
from the
central galaxy
Radio halos
& Relics
Cluster centers and outskirts
in unrelaxed clusters
Coma cluster @325 MHz with WSRT
Brown & Rudnick 2011
Present in ~ 20 galaxy clusters
~ Mpc size
- Steep spectrum (α~1.1-1.4… but wait)
- μJy/”2 surface brightness
- Follow X-ray emission
- Unpolarized
Central regions in relaxed
clusters - mini halos
~25 relics and candidates
~ Mpc size
- Steep spectrum (α~1.2-1.5)
- Low surface brightness
- Elongated morphology
- Polarized (up to 30%)
Giacintucci et al. subm.
Abell 521
Abell 209
Abell 520
Abell 1300
Abell 1240
Abell 754
Abell 2744
Abell 2163
Abell 3667
Slide from Giacintucci
Historical problems to account for the
origin of halos
The diffusion problem
1 Mpc
The rarity
Crossing time of electrons ~ 10 Gyr
but radiative lifetime ~ 0.1 Gyr
Cassano+ 2008
tdiff >> trad
Re-acceleration needed
What makes those
clusters special?
Origin of the relativistic electrons in
radio halos
Primary models - in-situ re-acceleration of relativistic electrons (Jaffe 1977)
Secondary models – Relativistic electrons are continuously injected in the ICM
by inelastic proton-proton collisions through productions and decay of charged
pions (e.g., Dennison 1980, Dolag & Ensslin 2000, Keshet & Loeb 2010)
CR protons are long lived particles confined over the whole cluster volume and for
this reason this model solves the diffusion problem
Primary models - in-situ re-acceleration of relativistic electrons by MHD
turbulence (e.g., Brunetti et al. 2001, 2004; Petrosian 2001; Fujita et al. 2003; Petrosian & Bykov
Shocks and turbulence
Particle acceleration
Diffuse radio emission
Vazza et al. 2009
How to address the problem of the origin
and occurrence of radio halos
 Studies of “individual” clusters for detailed analysis
 Studies of large cluster samples for statistical investigations
Results shown here based on:
 GMRT RH Survey (observations at 610
MHz, Venturi et al 2007, 2008)
 Extended GMRT RH Survey (observations
at 610/240 MHz, Kale et al. 2013)
 GMRT 325 MHz follow up of radio halos
(Venturi et al. 2013)
 GMRT 153 MHz follow up of special cases
(Macario et al. 2013)
Total of 67 clusters (50+17)
Detection of new GRH, relics,
mini-halos, and UPPER LIMITS
Observational results
RHs and cluster mass
z=0 – 0.4
The fraction of radio halos is a strong
function of mass: only ~ 5% for Mcluster
<1015 MSun, around 40% for Mcluster ≥1015
Msun (z ≤ 0.4; NVSS+GMRT radio halo
cluster samples, Cassano+ 2008). EGMRT RHS confirms this result (Kale et
al. 2013)
Bimodality – Not due to limited
Latest bimodality
plot with E-GMRT
upper limits
included (Kale et
al. 2013)
Bimodality and mass dependance confirmed by SZ parameters
Cassano et al.2013
Giant RH
Cassano et al. 2010
 Radio Halos are a transient phenomenon (timescale ~ Gyr). They are
rare and are found only in massive non-relaxed clusters with evidence
for recent or ongoing cluster mergers.
 Outliers (unrelaxed clusters without radio halo) do exists
Spectra of radio halos
Coma: prototype of “GHz” radio halo
A521: prototype of USSRH
Predictions from turbulent
re-acceleration model
From Cassano
Venturi et al. 2013
USSRHs as due to less energetic mergers, i.e. major mergers
between less massive systems, or accretion of small systems
Key area for new generation of radio interferometers (LOFAR
and SKA precursors
Radio mini-halos
Diffuse faint synchrotron radio sources surrounding the radio
BCG in an increasing number of relaxed cool core clusters
Ongoing study using archive and proprietary VLA & GMRT data starting from the
ACCEPT and HIFLUGCS sample – Giacintucci et al. subm.
15 mini-halos and 6
candidates known to
date: sources usually
confined within the cluster
cool core, with size of the
order of 100-300 kpc
Mini-halos are found in the
most extreme and massive
cool core clusters: i.e. low
central entropy, short
cooling time, high global
Origin of relativistic electrons in mini-halos
The case of RXJ1720.1+26
Giacintucci et al. submitted
ZuHone et al. 2013
The mini-halo extends
within the cluster core
and is bounded by the
sloshing cold fronts –
May sloshing induced
turbulence reaccelerate
the relativistic electrons?
Radio brightness
consistent with
simulations of sloshing
cores; spectral
steepening consistent
with electron reacceleration
The presence of diffuse radio sources, or lack thereof, is a strong indication of
the dynamics of the hosting cluster:
 Giant radio halos, as well as radio relics, are ALWAYS found in massive
clusters with ongoing major mergers
 Giant radio halos with ultrasteep radio spectrum are found in massive
clusters undergoing minor mergers
The statistical results based on the E-GMRT radio halo survey provide strong
support to the merger-induced particle reacceleration at the origin of radio
 Radio mini-halos seem to be fairly common in relaxed massive clusters with
strong cool cores
Turbulence induced by sloshing is a possible source of particle reacceleration
More from the E-GMRT Radio Halo Survey
The survey has delivered a huge amount of data and information on
individual galaxies (the resolution at 610 MHz is ~5”, comparable to
FIRST at 1.4 GHz)
Ongoing work
RLF of BGCs in the sample of 67 clusters with the following goals:
 Check for possible evolutionary effects compared to local BCGs
 RLF of BCGs in merger and relaxed systems to check for
environmental effects

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