Star Formation and AGN in the Core of the Shapley Supercluster: A VLA Survey of A3556, A3558, SC1327-312, SC1329-313, and A3562

The core of the Shapley supercluster (A3556, A3558, SC1327-312, SC1329-313, and A3562) is an ideal region in which to study the effects of cluster mergers on the activity of individual galaxies. This paper presents the most comprehensive radio continuum investigation of the region, relying on a 63-pointing mosaic obtained with the Very Large Array yielding an areal coverage of nearly 7 square degrees. The mosaic provides a typical sensitivity of about 80 uJy at a resolution of 16", enabling detection of galaxies with star formation rates as low as 1 solar mass per year. The radio data are complemented by optical imaging in B and R, producing a catalog of 210 radio-detected galaxies with m_R <= 17.36 (M_R <= -19). At least 104 of these radio-detected galaxies are members of the supercluster on the basis of public velocity measurements. Across the entire core of the supercluster, there appears to be a significant deficit of radio galaxies at intermediate optical magnitudes (M_R between -21 and -22). This deficit is offset somewhat by an increase in the frequency with which brighter galaxies (M_R less than -22) host radio sources. More dramatic is the highly significant increase in the probability for fainter galaxies (M_R between -20 and -21) in the vicinity of A3562 and SC1329-313 to be associated with radio emission. The radio and optical data for these sources strongly suggest that these active galaxies are powered by star formation. In conjunction with recent X-ray analysis, this is interpreted as young starbursts related to the recent merger of SC1329-313 with A3562 and the rest of the supercluster.Comment: Accepted by AJ; 50 pages, including 16 figures (for full resolution PDF, see http://mywebpages.comcast.net/nealamiller2/Shapley_pp.pdf

Abell 3560, a galaxy cluster at the edge of a major merging event

In this paper we study A3560, a rich cluster at the southern periphery of the A3558 complex, a chain of interacting clusters in the central part of the Shapley Concentration supercluster. From a ROSAT-PSPC map we find that the X-ray surface brightness distribution of A3560 is well described by two components, an elliptical King law and a more peaked and fainter structure, which has been modeled with a Gaussian. The main component, corresponding to the cluster, is elongated with the major axis pointing toward the A3558 complex. The second component, centered on the Dumb-bell galaxy which dominates the cluster, appears significantly offset (by about 0.15 Mpc) from the cluster X-ray centroid. From a Beppo-SAX observation we derive the radial temperature profile, finding that the temperature is constant (at kT~3.7 keV) up to 8 arcmin, corresponding to 0.3 Mpc: for larger distances, the temperature significantly drops to kT~1.7 keV. We analyze also temperature maps, dividing the cluster in 4 sectors and deriving the temperature profiles in each sector: we find that the temperature drop is more sudden in the sectors which point towards the A3558 complex. From VLA radio data, at 20 and 6 cm, we find a peculiar bright extended radio source (J1332-3308), composed by a core (centered on the northern component of the Dumb-bell galaxy), two lobes, a "filament" and a diffuse component. The morphology of the source could be interpreted either with a strong interaction of the radio source with the intracluster medium or with the model of intermittency of the central engine.Comment: 8 pages with encapsulated figures, A&A in pres

Radio Properties of the Shapley Concentration. III. Merging Clusters in the A3558 Complex

We present the results of a 22 cm radio survey carried out with the A3558 complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and thetwo groups SC1327-312 and SC1323-313, located in the central region of the complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and the two groups SC1327-312 and SC1323-313, located in the central region of the Shapley Concentration. The purpose of our survey is to study the effects of cluster mergers on the statistical properties of radio galaxies and to investigate the connection between mergers and the presence of radio halos and relic sources. We found that the radio source counts in the A3558 complex are consistent with the background source counts. Furthermore, we found that no correlation exists between the local density and the radio source power, and that steep spectrum radio galaxies are not segregated in denser optical regions. The radio luminosity function for elliptical and S0 galaxies is significantly lower than that for cluster type galaxies and for those not selected to be in clusters at radio powers logP(1.4) > 22.5, implying that the probability of a galaxy becoming a radio source above this power limit is lower in the Shapley Concentration compared with any other environment. The detection of a head-tail source in the centre of A3562, coupled with careful inspection of the 20 cm NRAO VLA Sky Survey (NVSS) and of 36 cm MOST observations, allowed us to spot two extended sources in the region between A3562 and SC1329-313, i.e. a candidate radio halo at the centre of A3562, and low brightness extended emission around a 14.96 magnitude Shapley galaxy.Comment: 18 pages, 14 figures. Accepted for publication on MNRA

XMM-Newton observation of the interacting cluster Abell 3528

We analyze the XMM dataset of the interacting cluster of galaxies Abell 3528 located westward in the core of the Shapley Supercluster, the largest concentration of mass in the nearby Universe. A3528 is formed by two interacting clumps (A3528-N at North and A3528-S at South) separated by 0.9 $h_{70}^{-1}$ Mpc at redshift 0.053. XMM data describe these clumps as relaxed structure with an overall temperature of $4.14 \pm 0.09$ and $4.29 \pm 0.07$ keV in A3528-N and A3528-S, respectively, and a core cooler by a factor 1.4--1.5 and super-solar metal abundance in the inner 30 arcsec. These clumps are connected by a X-ray soft, bridge-like emission and present asymmetric surface brightness with significant excess in the North--West region of A3528-N and in the North--East area of A3528-S. However, we do not observe any evidence of shock heated gas, both in the surface brightness and in the temperature map. Considering also that the optical light distribution is more concentrated around A3528-N and makes A3528-S barely detectable, we do not find support to the originally suggested head-on pre-merging scenario and conclude that A3528 is in a off-axis post-merging phase, where the closest cores encounter happened about 1--2 Gyrs ago.Comment: 15 pages, 16 figures (10 in colors), accepted for publication on A&A. For a Poscript version with high resolution figures see http://www.mi.iasf.cnr.it/~gasta/www/a3528.htm

The cluster relic source in A521

We present high sensitivity radio observations of the merging cluster A521, at a mean redsfhit z=0.247. The observations were carried out with the GMRT at 610 MHz and cover a region of $\sim$1 square degree, with a sensitivity limit of $1\sigma$ = 35 $\mu$Jy b$^{-1}$. The most relevant result of these observations is the presence of a radio relic at the cluster periphery, at the edge of a region where group infalling into the main cluster is taking place. Thanks to the wealth of information available in the literature in the optical and X-ray bands, a multi--band study of the relic and its surroundings was performed. Our analysis is suggestive of a connection between this source and the complex ongoing merger in the A521 region. The relic might be ``revived' fossil radio plasma through adiabatic compression of the magnetic field or shock re--acceleration due to the merger events. We also briefly discussed the possibility that this source is the result of induced ram pressure stripping of radio lobes associated with the nearby cluster radio galaxy J0454--1016a. Allowing for the large uncertainties due to the small statistics, the number of radio emitting early--type galaxies found in A521 is consistent with the expectations from the standard radio luminosity function for local (z$\le$0.09) cluster ellipticals.Comment: 30 pages 8 figures, 5 tables, accepted by New Astronom

High sensitivity low frequency radio observations of cD galaxies

We present the GMRT 235 MHz images of three radio galaxies and 610 MHz images of two sources belonging to a complete sample of cD galaxies in rich and poor galaxy clusters. The analysis of the spectral properties confirms the presence of aged radio emission in two of the presented sources.Comment: 3 pages, 2 figures, To appear in the Proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching (Germany

Radio emission at the centre of the galaxy cluster Abell 3560: evidence for core sloshing?

Previous radio observations of the galaxy cluster A3560 in the Shapley Concentration showed complex radio emission associated with the brightest cluster member.To understand its origin we observed it with the GMRT, the VLA and ATCA at 240 and 610 MHz, 1.28,1.4, 2.3,4.8 and 8.4 GHz, and performed a detailed morphological and spectral study of the radio emission associated with the BCG. We also observed the cluster with XMM-Newton and Chandra to derive the properties of the ICM. The radio emission of the N-E nucleus of the dumb-bell BCG shows an active radio galaxy, plus aged diffuse emission, which is not refurbished at present. Our Chandra data show that the radio active nucleus of the BCG has extended X-ray emission, which we classify as a low-luminosity corona. A residual image of the XMM-Newton brightness shows the presence of a spiral-like feature, which we interpret as the signature of gas sloshing. The presence of a subgroup is clear in the surface brightness residual map, and in the XMM-Newton temperature analysis. The optical 2D analysis shows substructure in A3560. A galaxy clump was found at the location of the X-ray subgroup, and another group is present south of the cluster core, close to the spiral-like feature. The aged part of the radio emission closely follows the spiral pattern of the X-ray residual brightness distribution, while the two active radio lobes are bent in a completely different direction. We conclude that the complex radio emission associated with the cluster BCG is the result of a minor merger event in A3560. The aged diffuse emission is strongly affected by the sloshing motion in the ICM. On the other hand, the bent jets and lobes of the current radio AGN activity may reflect a complex gas velocity field in the innermost cluster regions and/or sloshing-induced oscillations in the motion of the cD galaxy.Comment: 15 pages, 8 figures, 5 tables, A&A in pres

GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4. II.The eBCS clusters and analysis of the complete sample

We present the results of the GMRT cluster radio halo survey. The main purposes of our observational project are to measure which fraction of massive galaxy clusters in the redshift range z=0.2--0.4 hosts a radio halo, and to constrain the expectations of the particle re--acceleration model for the origin of the non--thermal radio emission. We selected a complete sample of 50 clusters in the X-ray band from the REFLEX (27) and the eBCS (23) catalogues. In this paper we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for all clusters still lacking high sensitivity radio information, i.e. 16 eBCS and 7 REFLEX clusters, thus completing the radio information for the whole sample. The typical sensitivity in our images is in the range 1$\sigma \sim 35-100 \mu$Jy b$^{-1}$. We found a radio halo in A697, a diffuse peripheral source of unclear nature in A781, a core--halo source in Z7160, a candidate radio halo in A1682 and ``suspect'' central emission in Z2661. Including the literature information, a total of 10 clusters in the sample host a radio halo. A very important result of our work is that 25 out of the 34 clusters observed with the GMRT do not host extended central emission at the sensitivity level of our observations, and for 20 of them firm upper limits to the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey shows that radio halos are not common, and our findings on the fraction of giant radio halos in massive clusters are consistent with the statistical expectations based on the re--acceleration model. Our results favour primary to secondary electron models.Comment: A&A in press, 17 pages, 12 figures, 4 tables Version with high quality figures available on web at http://www.ira.inaf.it/~tventuri/pap/Venturi_web.pd