Dynamics of galaxy clusters A radio perspective

Transcription

1 Dynamics of galaxy clusters A radio perspective Tiziana Venturi, INAF, Istituto di Radioastronomia Collaborators S. Giacintucci, D. Dallacasa, R. Kale, G. Brunetti, R. Cassano, M. Rossetti GEE 3 Padova, 12 November 2013

2 Outline Diffuse radio emission in galaxy clusters as signpost for cluster dynamics Radio halos and relics: massive cluster mergers Radio mini-halos: massive cool core clusters

3 Radio emission in galaxy clusters 3C75 in A401 Emission from individual radio galaxies Cluster weather and AGN feedback Hydra A Radio emission from the central galaxy Perseus Mini-halos Diffuse emission over the cluster scale Radio halos & Relics RXCJ

4 Cluster centers and outskirts in unrelaxed clusters Coma MHz with WSRT Brown & Rudnick 2011 GIANT RADIO HALOS Present in ~ 20 galaxy clusters ~ Mpc size - Steep spectrum (α~ but wait) - μjy/ 2 surface brightness - Follow X-ray emission - Unpolarized Central regions in relaxed clusters - mini halos RADIO RELICS ~25 relics and candidates ~ Mpc size - Steep spectrum (α~ ) - Low surface brightness - Elongated morphology - Polarized (up to 30%) Giacintucci et al. subm.

5 RXCJ Abell 520 Abell 754 Bullet Abell 521 Abell 1300 Abell 2744 Abell 2163 Abell 209 Abell 1240 RXCJ Abell 3667 Slide from Giacintucci

6 Historical problems to account for the origin of halos The diffusion problem The rarity e - 1 Mpc Observed halos Crossing time of electrons ~ 10 Gyr but radiative lifetime ~ 0.1 Gyr t diff >> t rad Re-acceleration needed Cassano What makes those clusters special?

7 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 2008 ) Merger Shocks and turbulence Particle acceleration Diffuse radio emission Vazza et al. 2009

8 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

9 Observational results from E-GMRT RHS RHs and cluster mass z=0 0.4 Bimodality Not due to limited sensitivity UNRELAXED The fraction of radio halos is a strong function of mass: only ~ 5% for M cluster <10 15 M Sun, around 40% for M cluster M sun (z 0.4; NVSS+GMRT radio halo cluster samples, Cassano+ 2008). E- GMRT RHS confirms this result (Kale et al. 2013) Latest bimodality plot with E-GMRT upper limits included (Kale et al. 2013) RELAXED

10 Bimodality and mass dependance confirmed by SZ parameters Cassano et al.2013

11 A1758N RELAXED Giant RH UNRELAXED Cassano et al 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

12 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 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

13 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 kpc Mini-halos are found in the most extreme and massive cool core clusters: i.e. low central entropy, short cooling time, high global temperature

14 Origin of relativistic electrons in mini-halos The case of RXJ Giacintucci et al. submitted ZuHone et al 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

15 Conclusions 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 halos 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

16 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