Transcript The GMRT Radio Halo survey Results and implications for LOFAR Simona Giacintucci Harvard-Smithsonian CfA, Cambridge, USA INAF-IRA, Bologna, Italy T.

The GMRT Radio Halo survey
Results and implications for LOFAR
Simona Giacintucci
Harvard-Smithsonian CfA, Cambridge, USA
INAF-IRA, Bologna, Italy
T. Venturi, R. Cassano, G. Brunetti, D. Dallacasa, G. Setti (INAF-IRA, Bologna)
S. Bardelli (OAB, Bologna)
Cluster scale diffuse radio emission
Abell 2256
RADIO RELICS: cluster outskirts, elongated
morphology, polarized up to 30%
Origin: shock (re)-acceleration of
relativistic electrons or shock adiabatic
compression of fossil radio plasma ?
e.g., Ensslin et al. 1998; Rottgering et al. 1997;
Ensslin & Gopal-Krishna 2001; Markevitch et al.
2005; Hoeft and Bruggen 2007…
RADIO HALOS: centrally located, regular
structure similar to the X-ray morphology,
unpolarized
VLA 1.4 GHz on Chandra
(discrete radio galaxies subtracted)
Clarke & Ensslin 2006
Origin: a promising possibility is the (re)acceleration of relativistic electrons by
merger driven turbulence (Brunetti et al. 2001,
Petrosian 2001, Fujita et al. 2003,…)
Statistical expectations for radio halos
Statistical calculations in the framework of the re-acceleration scenario
(Cassano & Brunetti 2005; Cassano, Brunetti & Setti 2006) allow to derive the
probability to form a radio halo as function of the cluster mass and redshift
Most of radio halos are expected in
massive and luminous (M ≥ 2 x
1015 Mo , Lx ≥ 5 x 1044 erg s-1 )
clusters in the redshift range z =
0.1 - 0.4
Agreement with the
observed statistics
at z ≤ 0.2 (Giovannini
et al. 1999)
Need for statistical
information for z > 0.2
The GMRT Radio halo survey
Aims
 discovery new radio halos (and relics)
 measure for the first time the occurrence of radio halos at z = 0.2 - 0.4
 constrain the dependence of their occurrence on the cluster mass
 combine the results with the statistics at z ≤ 0.2 and test the predictions of
the statistical calculations
 verify the connection between radio halos/relics and cluster mergers
WHY GMRT at 610 MHz?
Radio halos/relics have steep radio spectra (α ≥ 1 ) and low surface brigthness
→ GMRT is an ideal instrument for our goal, since it is capable of very high
sensitivity at low radio frequencies
We asked for 610 MHz observations, since this frequency offers the best
compromise among sensitivity, confusion and range of resolutions (from 5” to
“tapered” images with 20”-30” resolution)
Sample selection & observations
From the X-ray catalogues REFLEX, BCS and eBCS we extracted a complete
sample of 50 clusters (27 REFLEX e 23 BCS/eBCS) with:
0.2 < z < 0.4 ; Lx ≥ 5 x 1044 erg s-1
-30°< δ < 2.5° (REFLEX)
+15° < δ < 60° (BCS/eBCS)
16 clusters: literature + VLA archive +
GMRT Cluster Key Project
34 observed with the GMRT
(Jan 2005 – Aug 2006)
7 known radio halos
EXPECTED 5- 8 NEW DETECTIONS
 2 – 3 hrs observation at 610 MHz for each cluster (USB+LSB: 32 MHz tot. band)
 <rms> ~ 60 µJy/beam (35 – 100 μJy/beam)
RESULTS I. Detections and non-detections
7 known radio halos from the literature (A2744, A1300, A2163, A773, A1758a,
A2219, A2390)
34 clusters observed with the GMRT:
- 4 new halos:
3 giant, 1 halo with LLS ~ 500 kpc
- 1 candidate radio halo
- 1 cluster with possible diffuse emission
Evidence of
merger in all
these clusters
- 1 relic + 1 double relic
- 1 mini-halo (cool core)
- 1 candidate mini-halo (candidate cool core)
26 non-detections
Halos and relics
are rare
(mergers and relaxed)
9 remaining clusters (literature, VLA archive, GMRT cluster key project):
3 undetections, 1 deserves further investigation, 5 without information
Venturi et al. 2007 & 2008
0 < z < 0.4 : GMRT + literature
Bimodal distribution of clusters
with and without radio halo
Increase of the fraction of clusters
with radio halo with the X-ray
luminosity (mass)
Brunetti et al. 2007, ApJ 670L, 5
Cassano et al., 2008, A&A, 480, 327
Results II. Halos, relics and cluster mergers
All new halos/relics are in merging custers
RXCJ1314.4-2515
Venturi et al., 2007 & 2008
RXCJ2003-2323
MAJOR MERGERS
1 Mpc
GMRT 610 MHz + XMM-Newton + optical
1 Mpc
GMRT 610 MHz on Chandra
Results II. Halos, relics and cluster mergers
Multiple peaks
Cluster dynamical state and
presence of a radio halo/relic.
A209 - GMRT 610 MHz on Chandra
1 Mpc
ellipticity
Multiple-moment power ratio analysis
of the 2-dimensional potential
(Buote & Tsai 1995; Hart 2008)
Venturi et al., 2007 & 2008
Abell 521: Relic + first Ultra Steep Radio Halo
GMRT 610 MHz on Chandra
GMRT 327 MHz
HALO
Res. 35” (point sources
subtracted out)
RELIC
Res. 13” - rms = 0.04 mJy/b
RELIC
Res. 13” - rms = 0.1 mJy/b
Follow up at 327 MHz (GMRT) to study the relic revealed the existence
of a central radio halo with a very steep spectrum (α ~ 2)
Abell 521: Relic + first Ultra Steep Radio Halo
Spectral index steepening
across the relic
Resolution 35” - rms = 0.2 mJy/b
RELIC
α ~1.5
Electron acceleration
by a shock with Mach
number ~ 2.2
RELIC
Conclusions
 The GMRT Radio Halo Survey provided support to the re-acceleration
scenario, contributing to our understanding of the origin of diffuse
radio emission in galaxy clusters and its connection with the large
scale structure formation.
The GMRT Radio Halo Survey revelead the existence of ultra steep
radio halos which emerge only at low frequency ( < 1 GHz)  merging
events less energetic than those producing the standard “high
frequency” radio halos (~ GHz )
LOFAR will be important for:
- the study of the low-frequency spectrum (total and local)
of radio relics
- the discovery of other Ultra Steep Spectrum Radio Halos