CLUSTERS OF GALAXIES-THERMAL AND NON-THERMAL PHENOMENA

Involved IRA Scientists and Collaborators: G. Brunetti, L. Feretti, I.M. Gioia, K.-H. Mack, P. Parma, T. Venturi, A. Zanichelli, A. Bonafede, M. Branchesi, R. Cassano, D. Dallacasa, C. Fanti, R. Fanti, S. Giacintucci, G. Giovannini, F. Govoni, L. Gregorini, E. Liuzzo, M. Murgia, E. Orru', G.  Setti, F. Vazza

The IRA scientists involved in this research study the physical conditions in the Intra-Cluster Medium (ICM) focussing on the interaction between the thermal and non-thermal components and on their evolution with cosmic epochs. Several studies on clusters and cluster galaxies are not confined to the radio domain but they use a multiband approach which includes the X-ray and optical regimes since in each band a different component of these large systems is dominant.
For a general approach see Feretti, Gioia & Giovannini 2002: ''Merging Processes in Galaxy Clusters'', Kluwer Academic Publishers.

HALOS AND RELICS

Clusters of galaxies represent the largest collapsed structures in the Universe and provide a powerful probe of the evolution of structure formation. They form through energetic merging of substructures (with energy release of order 10^(64) erg/s) and accretion of matter. Clusters of galaxies host manifold components, such as individual galaxies, hot intracluster gas and invisible dark matter. In a fraction of clusters, in addition to the X-ray emitting thermal plasma, there are also relativistic electrons and large-scale magnetic fields (commonly referred to as “non-thermal components'') as demonstrated by the detection of large-scale synchrotron radio sources: halos and relics. The IRA has been a leader for several years now in the study of the properties of halos and relics in the radio domain and in the comparison with the X-ray emission. Main results include the radio-X-ray correlations and the relation to the cluster under the hypothesis that these phenomena are found in clusters which have recently undergone a merger event. The hypothesis that radio relics are connected to cluster shocks which form during mergers is being tested through deep X-ray observations of cluster peripheral regions (e.g. Coma cluster, Feretti & Neumann 2006).
Research in this area includes: a) studies to understand the diffuse and extended radio emission at the center, or periphery, of the clusters by combining radio with X-ray and data at other wavelengths; b) building of theoretical models; c) analyses performed using the X-ray band where the thermal hot gas radiates.

OBSERVATIONAL STUDY OF THE PROPERTIES OF RADIO HALOS

Spectral index maps of radio halos are a powerful tool to investigate the connection between thermal and non-thermal components and to understand the origin of the radio emitting particles. This type of investigation began at IRA a few years ago and has produced a major advancement in our knowledge of the properties of these sources. Radio observations at 327 MHz of the radio halos in A665, A2163, A2219 and A2744, combined with images at 1.4 GHz, have allowed us to derive the radio spectral index which is an important indicator of the energy of the radio emitting particles. It is also an indicator of the processes of energy losses and re-accelerations of such particles. The results of this project comfirm that radio halos and merging events are related. In addition the hottest regions of the cluster are associated with regions of flatter spectral index.  Details and results in: Feretti et al. 2004; Orru' et al.  2007.

A second observational project involves the use of the Giant Metrewave Radio Telescope (GMRT, India) to obtain data on a complete sample of 50 massive and X-ray luminous clusters. This project is designed to obtain information on the fraction of galaxy clusters with radio halos. Clusters with no radio information were observed at 610 MHz at high sensitivity to look for extended emission. To fully test the cluster merging paradigm and provide an accurate number for the fraction of merging clusters hosting a radio halo, for each cluster in the sample, X-ray archives and optical databases are under inspection. The data will allow us to obtain information on the dynamical state from the gas density distribution and from the velocity distribution of the cluster galaxies. The results of this large observational project are still preliminary, and only partly published. The fraction of radio halos found so far is of order 25% (see Figure 5a and 5b), in reasonable agreement with expectations.

Fig 5a - GMRT 610 MHz contours (green) of the extended radio emission in RCXJ1314.4-2515 (z=0.2439) overlaid on the ASCA X-ray emission (colors). Two elongated relics and one radio halo are clearly visible.

Fig 5b - The giant radio halo in RXCJ2003.5-2323 at 610 MHz. The cluster redshift is 0.3171, and the halo has a largest linear size of 1.4 Mpc (from Venturi et al. 2007).

The work led to the detection of a number of radio relics and other intriguing diffuse sources, which are at present the subject of follow-up GMRT observations at 327 MHz and 235 MHz, in order to study their spectral index distribution. Details and preliminary results in: Venturi et al.  2007; Giacintucci et al. 2006.

THE ANALYSIS OF MAGNETIC FIELDS IN CLUSTERS

The intensity of magnetic fields in clusters can be derived from the diffuse radio emission but also from the analysis of the polarization properties of the radio sources embedded in the cluster. This is based on the fact that when the emitted polarized radiation travels across the magnetized ICM it becomes depolarized and changes the rotation of the polarization vector as a function of the observation wavelength (Faraday rotation). A program was undertaken to perform a polarimetric study of some clusters with and without radio halos and with different temperatures. We have developed a numerical approach to interpret the observational data under the hypothesis that the magnetic field has an intensity that decreases radially, and that its structure presents different coherent scales. Among the clusters studied so far there is the cluster A2255 which is the only cluster where polarized filaments have been detected within the radio halo. Details and results in: Govoni et al.  2006.

THE RLF OF DISTANT X-RAY SELECTED CLUSTER RADIO GALAXIES

The VLA is an excellent instrument to survey X-ray extragalactic objects and compare the radio properties of X-ray selected objects, for instance clusters of galaxies, with those selected at other wavelengths. Recently a complete sample of 18 X-ray selected clusters of galaxies chosen from among the most distant clusters (0.3 < z < 0.8) in the ROSAT North Ecliptic Pole survey was observed with the Very Large Array at 1.4 GHz. The radio survey revealed for the first time evolution both in amplitude and slope of the Radio Luminosity Function (RLF) of distant cluster galaxies with respect to local cluster samples (see Figure 6).

Fig 6 - The filled triangles indicate the NEP distant cluster radio luminosity function while the open pentagons and crosses represent the RLFs of local clusters by Fanti et al. (1984) and Ledlow & Owen (1996), respectively. A clear evolution of the distant cluster RLF is evident (from Branchesi et al. 2006).

An optical program necessary to derive and study the bivariate luminosity function of the sample is ongoing. Results of the VLA survey can be found in: Branchesi et al. 2006.

THE EFFECT OF X-RAY POINT SOURCES ON CLUSTER SCALING RELATIONS

Thanks to the superb angular resolution of Chandra, it is now possible to detect X-ray point sources, either embedded in galaxy clusters or along the cluster line-of-sight, thus allowing the study of source counts in distant cluster fields. Branchesi et al. 2007a analyzed the inner regions of 18 clusters of galaxies observed by Chandra to check for the presence of any overdensity of X-ray point sources embedded in the diffuse gas emission. These point sources are possible AGN (and sometimes powerful starburst galaxies) belonging to the clusters and could contaminate the cluster emission. A 2-sigma excess of cluster region sources was found at the bright end of the logN-logS of cluster sources.   The result suggests the possible existence of X-ray sources belonging to the cluster (most probably AGN, given their high luminosity) which might affect considerably the estimates of X-ray observables like cluster temperature and luminosity. The analysis has shown that the cluster temperature is affected by an amount of up to 13% and the luminosity by 18%. These percentages become larger if one considers clusters with z>0.7 where both temperature and luminosity increase by up to 24%. Thus point sources should be removed for cosmological studies using clusters of galaxies. However the inclusion of point sources does not significantly impact the slope and normalization of the the L_X-T relationship since for each cluster the correction to be applied to T and L_X produces a moderate shift in the L-T plane almost parallel to the best-fit of the ``correct'' L_X-T relation.

A second study consists in excluding the point sources from the analysis of the cluster emission to construct the ``correct'' cluster L_X-T scaling relation. The data used are either Chandra or XMM-Newton observations of ~40 clusters of galaxies which enable a determination of the L_X-T correlation to be compared to the theoretical models. The main result is that the we find a strong evolution, similar or stronger than the self-similar model, from z=0 to z<0.3, followed by a much weaker, if any, evolution at higher redshift. See for details and results: Branchesi et al. 2007b; Branchesi et al.  2007c.

DOMINANT DUMBBELL GALAXIES IN NEARBY CLUSTERS AND  CENTRAL GALAXIES IN POOR GROUPS

An interesting class of dominant galaxies found in the central regions of clusters is represented by Dumbbell systems (DBs): two galaxies of similar magnitude inside a common halo, at relatively small projected separation. Many questions on their origin, evolution and dynamical stability are still open.  Their radio emission properties are puzzling with complex morphologies arising from (or related to) the system dynamics. A multi-wavelength approach has been adopted to an optically selected sample of DBs: radio data show that 50\% of these DB host a radio source in the optically brightest nucleus. Spatially resolved spectroscopy with the VLT-VIMOS Integral Field Unit allows us to probe the role of the gas in the DB internal dynamics and in the formation of complex radio morphologies as well as the merger signatures in the envelope. Future mm/sub-mm and radio observations will permit the study of the DB dust and molecular gas properties.

In 2007 we started a program to study the pc-scale properties of the BCGs (Brightest Cluster Galaxies) using VLBI data to analyze if the peculiarities of these systems influence the activity in the radio band of the central AGN.

In order to investigate the role of radio loud AGNs located at the centre of poor groups on the properties of the intergalactic medium, a sample of poor groups is being observed with the GMRT over 4 radio frequency bands (from 1.4 GHz to 235 MHz). All central galaxies show complicated radio morphologies with long tails, winding lobes, abrupt changes of direction in the jets and lobes, emission from companions. At present the radio data are analyzed together with high resolution X ray images (Chandra) given the similar resolutions in the two different bands.