The GMRT Radio Halo survey
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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).

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The GMRT Radio Halo survey Results and implications for LOFAR

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The gmrt radio halo survey results and implications for lofar

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

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

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,…)

VLA 1.4 GHz on Chandra(discrete radio galaxies subtracted) Clarke & Ensslin 2006


The gmrt radio halo survey results and implications for lofar

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

Need for statistical information for z > 0.2

Agreement with the observed statistics at z ≤ 0.2 (Giovannini et al. 1999)


The gmrt radio halo survey

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)


The gmrt radio halo survey results and implications for lofar

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)


The gmrt radio halo survey results and implications for lofar

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

- 1 relic + 1 double relic

- 1 mini-halo (cool core)

- 1 candidate mini-halo (candidate cool core)

Evidence of merger in all these clusters

Halos and relics

are rare

26 non-detections(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


The gmrt radio halo survey results and implications for lofar

0 < z < 0.4 : GMRT + literature

Increase of the fraction of clusters with radio halo with the X-ray luminosity (mass)

Bimodal distribution of clusters with and without radio halo

Brunetti et al. 2007, ApJ 670L, 5

Cassano et al., 2008, A&A, 480, 327


The gmrt radio halo survey results and implications for lofar

Results II. Halos, relics and cluster mergers

All new halos/relics are in merging custers

Venturi et al., 2007 & 2008

RXCJ2003-2323

RXCJ1314.4-2515

MAJOR MERGERS

1 Mpc

1 Mpc

GMRT 610 MHz + XMM-Newton + optical

GMRT 610 MHz on Chandra


The gmrt radio halo survey results and implications for lofar

Results II. Halos, relics and cluster mergers

Cluster dynamical state and presence of a radio halo/relic.

A209 - GMRT 610 MHz on Chandra

Multiple peaks

1 Mpc

ellipticity

Multiple-moment power ratio analysis of the 2-dimensional potential

(Buote & Tsai 1995; Hart 2008)

Venturi et al., 2007 & 2008


The gmrt radio halo survey results and implications for lofar

Abell 521: Relic + first Ultra Steep Radio Halo

GMRT 610 MHz on Chandra

GMRT 327 MHz

HALO

Res. 35” (point sources subtracted out)

RELIC

RELIC

Res. 13” - rms = 0.1 mJy/b

Res. 13” - rms = 0.04 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)


The gmrt radio halo survey results and implications for lofar

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


The gmrt radio halo survey results and implications for lofar

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


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