The GMRT Radio Halo Survey

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The GMRT Radio Halo Survey

Tiziana Venturi
INAF-IRA, Bologna
t.venturi_at_ira.inaf.it
GMRT Radio Halo Team S. Giacintucci (CfA), G.
Brunetti (IRA), R. Cassano (IRA), D. Dallacasa
(University of Bologna), S. Bardelli (Bologna
Astronomical Observatory)
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What are we talking about Extended (Mpc)
diffuse emission in galaxy clusters
RADIO RELICS Cluster periphery, elongated (but
also other morphologies), polarized (20 -30) ,
a 1.2 1.4 ORIGIN Shock (re-acceleration)
of low energy relativistic electrons or
adiabatic compression of fossil radio plasma
(Enßlin et al. 1998 Enßlin Gopal-Krishna 2001
Markevitch et al. 2005)
RADIO HALOS Centrally located regular
morphology good spatial coincidence with the
X-ray brightness distribution unpolarized
a1.2-1.4 ORIGIN Re-acceleration by turbulence
injected in the clusters by mergers (Brunetti et
al. 2001 Petrosian 2001) is the most appealing
scenario nowadays
RXCJ1314.5-2515- Venturi et al. 2007
Observational evidence for a connection between
diffuse cluster sources and cluster mergers,
though qualitative so far.
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Statistical expectations for radio halos
Statistical calculations in the framework of
the electron re-acceleration induced by
turbulence predict the number of galaxy clusters
with a giant radio halo as a function of the
cluster mass and redshift
(Cassano Brunetti, 2005 Cassano, Brunetti
Setti, 2006)
It is expected that a large fraction of the most
massive clusters (M gt 2x1015 MSun) in the
redshift range z 0.2-0.4 hosts a giant radio
halo
Literature data for clusters at z0 0.2 is in
agreement with the statistical expectations
(Giovannini et al. 1999)
GMRT Radio Halo Survey test the statistical
predictions of the re-acceleration by turbulence
for the formation of giant radio halos
in the redshift interval z0.2 0.4
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Aims of the GMRT Radio Halo Survey
- Determine the fraction of clusters in the
redshift range 0.2- 0.4 hosting a custer radio
halo through the discovery of new radio halo
sources - Combine our results with the literature
information available at lower redshift,
so as to fill the whole range z0 0.4 - Better
constrain the dependence with the cluster mass
- Better investigate the connection between
diffuse cluster sources and cluster mergers -
New and unexpected findings GMRT at 610
MHz Ideal combination of frequency, sensitivity
and resolution to detect low brightness extended
radio sources with steep spectrum (a 1 )
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Sample selection and observations
50 clusters selected on the basis of the X-ray
luminosity from REFLEX and eBCS
0.2 lt z lt 0.4 Lx 5 x 1044 erg s-1 -30lt d lt
2.5 (27 REFLEX) 15 lt d lt 60 (23 eBCS)
16 clusters with literature information
or belonging to other projects 34 observed
with the GMRT at 610 MHz
- Each cluster observed for 2 - 3 .5 hours with
USBLSB (32 MHz) - Average rms (1s) 60 70
µJy/b (from 30 to 100 µJy/b) - Resolution range
from 5 (full resolution) to 20 30
(tapered images)
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From the literature/other projects 7 giant
radio halos A2744 A1300 A2163
A773 A1758a A2219 A2390 3 non
detections, 1 unclear, 5 with no information
GMRT Observational Summary
Among the 34 clusters observed as part of the
GMRT Radio Halo Survey 4 new radio halos, 3 of
them giant - A209 A697 RXCJ1314-2515
RCXJ2003.5-2323 1 candidate giant halo
A1682 1 suspect radio halo Z2661 1 relic,
A521, and 1 cluster with a double relic,
RXCJ1314-2515 Evidence of
merger in all these clusters Moreover - 1
mini-halo (cooling core) - 1 candidate mini-halo
(candidate cooling core)
Halos and relics Are RARE!!!
26 CLUSTERS WITH NO DETECTED DIFFUSE EMISSION
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From non detections to radio power upper limits
Undetections
Injection of families of extended components
(fake radio halos) in the u-v data imaging
Flux density/radio power upper limits
S decreases
LAS
increases
Venturi et al. 2008
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GMRT radio halo survey Comparison with the
statistical expectations 0 lt z lt 0.4
Significant increase in the number of clusters
with radio halo with increasing X-ray luminosity
Bimodal distribution of clusters with and without
radio halo
Cassano et al., 2008, AA, 480, 327
Brunetti et al. 2007, ApJ 670L, 5
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Halos, relics and cluster mergers
RXCJ1314-2515
RXCJ2003.5-2323
A209
A697
Z2661
GMRT _at_ 610 MHz over Chandra
Cluster dynamical state and presence/absence of
halos and/or relics
A1682
perturbed
relaxed
Venturi et al. 2007 2008
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Classical and Ultra Steep
Spectrum Radio Halos Discovery in A521
GMRT 325 MHz follow up observations of the radio
halos and relics in the GMRT cluster sample led
to the discovery of a new type of cluster
sources ULTRA STEEP SPECTRUM RADIO HALOS
A521 GMRT _at_ 325 MHz
A521 GMRT _at_ 610 MHz
A521 GMRT _at_ 240 MHz over Chandra
Giacintucci et al. 2008 Macario, Master thesis
Brunetti et al. 2008
Giacintucci et al. 2006
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Ultra Steep Radio Halo in A521 and the
classical halo in RXCJ2003.5-2323

235 MHz - GMRT - 610 MHz
RXCJ2003.5-2323
240 MHz - GMRT
Relic
Halo a2
VLA 1.4 GHz
1.4 GHZ VLA overlaid on 610 MHz GMRT
Expected in the re-acceleration model less
energetic merger events
A521
Giacintucci et al. To be submitted
Brunetti et al. 2008
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Low frequency follow up
Work in progress PhD thesis
A natural evolution of the GMRT radio halo survey
is the study of diffuse cluster emission, relics,
classical radio halos, candidate radio halos and
candidate steep spectrum radio halos at 325 and
240 MHz
A2744 Classical radio halo
A1682 Candidate USSRH
325 MHz GMRT over DSS2
240 MHz GMRT over VLSS
A697 Radio halo with possible steep spectrum
A781 Diffuse perihperal emission of unclear
nature
325 MHZ GMRT over Chandra
325 MHz GMRT over DSS2
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GMRT Radio Halo Survey Summary
Discovery of new diffuse cluster radio sources,
in the form of radio halos, relics, mini-halos,
and candidates Support to the connection between
halos/relics and cluster merger -
clusters with halos and/or relics
perturbed
- lack of cluster scale emission
relaxed/perturbed Support to the
statistical expectations based on the
re-acceleration model by turbulence via cluster
merger

- bimodal distribution of clusters with an
without radio halo - increasing
fraction of radio halos for clusters with mass M
gt2x1015 MSun Discovery of cluster radio halos
best detectable at low frequency, i.e. lt 610 MHz,
whose spectrum is steeper than that of
classical radio halos, defined Ultra Steep
Spectrum Radio Halos