Radio Observations of Galaxy Clusters

T. VenturiINAF, Istituto di

Radioastronomia

SnowCluster 2015 19 March 2015

Outline

Diffuse sources in galaxy clusters and their origin

Results from the Extended GMRT Radio Halo SurveyNew discoveries & statisticsBimodality and connection with cluster mergersSpectra of radio halosRadio properties of BCGs in merging and relaxed clusters

Present and future prospects Faint diffuse cluster sourcesSearch for radio halos in nearby clusters with KAT7

Cluster centres: GIANT RADIO

HALOS

Present in ~ 20-25 galaxy clusters

~ Mpc size- Steep spectrum (α~1.1-1.4 and steeper)- (sub) μJy/”2 surface brightness - Follow X-ray emission- Unpolarized

Evidence of at least Mpcscale magnetic fields

A1914

A209

RXCJ 2003.5-2323 Cluster outskirts: RELICS

~25 relics & candidates

~ Mpc size- Steep spectrum- (α~1.2-1.5)- Low surface brightness- Elongated morphology - Polarized (up to 30%)

Evidence of at least Mpcscale magnetic fields with some degree of

order

A521

The Sausage

Cluster cores: RADIO MINI-

HALOS

Present so far in 15 galaxy clusters

~ 100 – 400 kpc in size- Steep spectrum (α~1.2-1.5 and steeper)- (sub) μJy/”2 surface

brightness - Located at the peak of

central X-ray brightness distribution

- Central BCG always is always a radio source

A1914

A209Perseus RXCJ1532 over Chandra

RXCJ1532 over HST

A478

Most recent update and radio images from Giacintucci+ 2014a

and Giacintucci+ 2014b

Correlation between mini-halos, relaxed clusters and cold fronts (Mazzotta & Giacintucci 2008, Giacintucci+ 2014b)

Diffusion problem

Crossing time of electrons ~ 10 Gyrbut radiative lifetime ~ 0.1 Gyr

Observed halos

Cassano+ 2008

Cassano+ 2008

tdiff >> trad

Re-acceleration needed

Rarity: why?

Vazza et al. 2009

Merger Shocks and turbulence Particle acceleration

Diffuse radio emission

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 & Bykov2008…)

Origin of relativistic electrons

Secondary models

Vazza et al. 2009

Merger Shocks and turbulence Particle acceleration

Diffuse radio emission

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 & Bykov2008…)

Origin of relativistic electrons

Secondary models

A study of a large cluster sample for statistical investigations

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, 2015)

GMRT follow-up observations at 325 MHz of clusters with radio halo (Venturi et al. 2013)

Total of 64 clusters

Detection of new GRH, relics, mini-halos, and UPPER LIMITS

A sistematic search for diffuse cluster scale emission10 years of E-GMRT Radio Halo Cluster Survey

FINAL RESULTSCollaborators: G. Brunetti, R. Cassano, D. Dallacasa, S. Giacintucci, R. Kale

Many hundreds hours of GMRT observing time

13 radio halos (22%)2 relics & 1 double relic9 mini-halos (16%)

43% of clusters host diffuse emission of some kind3 candidate radio halos

Upper limits

Mini-halo + RH fraction

10%

54%

Kale+ 2015

Findings from the EGRHS - I.Upper limits and bimodality

UNRELAXED

RELAXED Final bimodality plot (Kale et al. 2015)

More by Cassano

Findings from the EGRHS - I.Upper limits and bimodality for radio halos

Bimodality and mass dependance confirmed by SZ parametersCassano et al.2013

Investigation of the bimodality and mass dependendance under test for a larger (80% complete) mass selected catalogue from the latest SZ cluster catalogue

release ongoing (GMRT observations under analysis)See poster by V. Cuciti

Findings from the EGRHS - II.Radio Halo/Merger connection

Mini-halo in RXCJ1532.9+3021

Giant radio halo in A1758a

More by Cassano

Findings from the EGRHS - III.Giant “ classical” and Ultra steep radio halos

RXCJ2003.5-2323 Abell 209

Venturi+ 2007

Macario+ 2010

Abell 521

Brunetti+ 2008

RXCJ 1314.5-2515

Venturi+ 2007

Abell 697 RCXJ1514 Abell 1300

Giacintucci+ 2011

Venturi+ 2013

From Cassano

Wide range of values for α, from ≈ 1.2 to 2

Abell 521

Giacintucci+ 2009

Findings from the EGRHS - III.Giant “ classical” and Ultra steep radio halos

Predictions from turbulent re-acceleration: “GHz” radio halos and ultra steep spectrum RHs

From Cassano

Ultra steep spectrum radio halos, best detectable at frequencies below 610 MHz as due to less energetic/minor mergers/accretion of smaller systems

From Cassano

Venturi et al. 2013

A broad range of spectral slopes is indeed found and the mass & type of merger do seem to play a key role

Wide range of values for α, from ≈ 1.2 to 2 Brown & Rudnick 2011

ComaM~1015 Msun

Major merger

A3562 – Giacintucci+ 2005

A3562 – M~ few x1014 MSun

Minor merger

Kale et al. 2015, Giacintucci et al. 2014 & in prep.

Findings from the EGRHS - IV.Mini-halos

Findings from the EGRHS - V.Radio properties of the BCGs in the sample

Sample of BCGs selected from the EGRHS for statistical investigations(Kale et al. to be subm.)

Probability of chance detection ≤3.4x10-4

Radio loud BCGs more aboundant in relaxed clusters in almost all

radio power bins

57 BCGs in 55 clusters – Fraction of radio loud BCGs is 49%Of the 55 clusters, 51% are mergers and 49% are relaxed

Clusters and BCGS in the merger/relaxedparameter space

Radio powers & upper limits vsconcentration parameter Fractional radio luminosity function

Findings from the EGRHS - VI.The tip of the iceberg

A697

“Old” VLA and JVLAMacario et al 2010

A781 – Venturi+ 2011 A1682 – Venturi+ 2013

A2552 – Kale+ 2015

Findings from the EGRHS - VI.The tip of the iceberg

A697

“Old” VLA and JVLAMacario et al 2010

A781 – Venturi+ 2011 A1682 – Venturi+ 2013

A2552 – Kale+ 2015

MWA 80 – 300 MHz95, 155, 215 MHz

θ≈2.5’

KAT-7 1 – 2 GHz1250 MHz - 1900 MHz

θ≈2.5’

Observations of a mass & distance limited sample (M>4x1014 MSun, 0.05<z<0.1) of Southern Clusters

MWA Collaborators:

M. Johnston-HollittK.S. DwarakanathR. Kale et al.

KAT-7 Collaborators:

G. BernardiN. Oozeer et al.

Diffuse emission in galaxy clusters with SKA precursors

with R. Cassano, D. Dallacasa, G. Brunetti

KAT-7 Observations of 14 clusters

3 candidates and upper limits for 6 clusters (Bernardi et al. to be subm.)

CONCLUSIONS AND FUTURE PROSPECTS

The Exended GMRT Radio halo survey has consolidated our understanding of the origin of giant radio halos, providing observational support to the turbulent re-acceleration model dependance with mass, USSRHs, quantitave connection with cluster mergers

Striking predominance of radio loud BCGs in relaxed clusters

We may have picked up only the tip of the iceberg

Investigation of minor mergers and less massive systems as next step: JVLA, LOFAR and SKA precursors (MWA and MeerKAT) prove to be crucial

Thank you for your attention