We evaluate in a homogeneous way the optical masses of 170 nearby clusters (z< 0.15). The sample includes both data from the literature and the new ENACS data (Katgert et al. 1996, 1998). On the assumption that mass follows the galaxy distribution, we compute the masses of each cluster by applying the virial theorem to the member galaxies. We constrain the masses of very substructured clusters (about 10% of our clusters) between two limiting values. After appropriate rescaling to the X-ray radii, we compare our optical mass estimates to those derived from X-ray analyses, which we compiled from the literature (for 66 clusters). We find a good overall agreement. This agreement is expected in the framework of two common assumptions: that mass follows the galaxy distribution, and that clusters are not far from a situation of dynamical equilibrium with both gas and galaxies reflecting the same underlying mass distribution. We stress that our study strongly supports the reliability of present cluster mass estimates derived from X-ray analyses and/or (appropriate) optical analyses.Comment: 13 pages, 7 eps figures, tables are not included, USE LaTeX2e !!, accepted by Ap

Abstract. We present an objective and automated procedure for detecting clusters of galaxies in imaging galaxy surveys . Our Voronoi Galaxy Cluster Finder (VGCF) uses galaxy positions and magnitudes to find clusters and determine their main features: size, richness and contrast above the background. The VGCF uses the Voronoi tessellation to evaluate the local density and to identify clusters as significative density fluctuations above the background. The significance threshold needs to be set by the user, but experimenting with different choices is very easy since it does not require a whole new run of the algorithm. The VGCF is non-parametric and does not smooth the data. As a consequence, clusters are identified irrespective of their shape and their identification is only slightly affected by border effects and by holes in the galaxy distribution on the sky. The algorithm is fast, and automatically assigns members to structures. A test run of the VGCF on the PDCS field centered at α = 13 h 26 m and δ = +29• 52 (J2000) produces 37 clusters. Of these clusters, 12 are VGCF counterparts of the 13 PDCS clusters detected at the 3σ level and with estimated redshifts from z = 0.2 to z = 0.6. Of the remaining 25 systems, 2 are PDCS clusters with confidence level < 3σ and redshift z ≤ 0.6. Inspection of the 23 new VGCF clusters indicates that several of these clusters may have been missed by the matched filter algorithm for one or more of the following reasons: a) they are very poor, b) they are extremely elongated, c) they lie too close to a rich and/or low redshift cluster.

Aims. We present a detailed dynamical analysis of the rich galaxy cluster A2744, containing a powerful diffuse radio halo. Methods. Our analysis is based on redshift data for 102 galaxies, part of them recovered from unexplored spectra in the ESO archive. We combine galaxy velocity and position information to select the cluster members and determine global dynamical properties of the cluster. We use a variety of statistical tests to detect possible substructures. Results. We find that A2744 appears as a well isolated peak in the redshift space at z = 0.306, which includes 85 galaxies recognized as cluster members. We compute the line-of-sight (LOS) velocity dispersion of galaxies, σ V = 1767 +121 −99 km s −1 , which is significantly larger than what is expected in the case of a relaxed cluster with an observed X-ray temperature of 8 keV. We find evidence that this cluster is far from dynamical equilibrium, as shown by the non-Gaussian nature of the velocity distribution, the presence of a velocity gradient and a significant substructure. Our analysis shows the presence of two galaxy clumps of different mean LOS velocities ∆V ∼ 4000 km s −1 . We detect a main, low-velocity clump with σ V ∼ 1200−1300 km s −1 and a secondary, high-velocity clump with σ V = 500−800 km s −1 and located in the S-SW cluster region. We estimate a cluster mass within 1 Mpc of 1.4−2.4×10 15 M , depending on the model adopted to describe the cluster dynamics. Conclusions. Our results suggest a merging scenario of two clumps with a mass ratio of 3:1 and a LOS impact velocity of ∆V rf ∼ 3000 km s −1 , likely observed just after the core passage. The merging is occuring roughly in the NS direction with the axis close to the LOS. This scenario agrees with that proposed on the basis of recent Chandra results in its general lines although suggesting a somewhat more advanced merging phase. Our conclusions support the view of the connection between extended radio emission and energetic merging phenomena in galaxy clusters.

Blazars are active galactic nuclei, which are powerful sources of radiation whose central engine is located in the core of the host galaxy. Blazar emission is dominated by non-thermal radiation from a jet that moves relativistically towards us, and therefore undergoes Doppler beaming. This beaming causes flux enhancement and contraction of the variability timescales, so that most blazars appear as luminous sources characterized by noticeable and fast changes in brightness at all frequencies. The mechanism that produces this unpredictable variability is under debate, but proposed mechanisms include injection, acceleration and cooling of particles, with possible intervention of shock waves or turbulence. Changes in the viewing angle of the observed emitting knots or jet regions have also been suggested as an explanation of flaring events and can also explain specific properties of blazar emission, such as intra-day variability, quasi-periodicity and the delay of radio flux variations relative to optical changes. Such a geometric interpretation, however, is not universally accepted because alternative explanations based on changes in physical conditions-such as the size and speed of the emitting zone, the magnetic field, the number of emitting particles and their energy distribution-can explain snapshots of the spectral behaviour of blazars in many cases. Here we report the results of optical-to-radio-wavelength monitoring of the blazar CTA 102 and show that the observed long-term trends of the flux and spectral variability are best explained by an inhomogeneous, curved jet that undergoes changes in orientation over time. We propose that magnetohydrodynamic instabilities or rotation of the twisted jet cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the extreme optical outburst of 2016-2017 (brightness increase of six magnitudes) occurred when the corresponding emitting region had a small viewing angle. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory.

Aims. The goal of this work is to investigate the Faraday rotation measure (RM) of radio galaxies in hot galaxy clusters in order to establish a possible connection between the magnetic field strength and the gas temperature of the intracluster medium. Methods. We performed Very Large Array observations at 3.6 cm and 6 cm of two radio galaxies located in A401 and Ophiuchus, a radio galaxy in A2142, and a radio galaxy located in the background of A2065. All these galaxy clusters are characterized by high temperatures. Results. We obtained detailed RM images at an angular resolution of 3 for most of the observed radio galaxies. The RM images are patchy and reveal fine substructures of a few kpc in size. Under the assumption that the radio galaxies themselves have no effect on the measured RMs, these structures indicate that the intracluster magnetic fields fluctuate down to such small scales. These new data are compared with RM information present in the literature for cooler galaxy clusters. For a fixed projected distance from the cluster center, clusters with higher temperature show a higher dispersion of the RM distributions (σ RM ), mostly because of the higher gas density in these clusters. Although the previously known relation between the clusters X-ray surface brightness (S X ) at the radio galaxy location and σ RM is confirmed, a possible connection between the σ RM − S X relation and the cluster temperature, if present, is very weak. Therefore, in view of the current data, it is impossible to establish a strict link between the magnetic field strength and the gas temperature of the intracluster medium.

We study the structure of the galaxy cluster Abell 523 (A523) at z = 0.104 using new spectroscopic data for 132 galaxies acquired at the Telescopio Nazionale Galileo, new photometric data from the Isaac Newton Telescope, and X-ray and radio data from the Chandra and Very Large Array archives. We estimate the velocity dispersion of the galaxy population, σ V = 949 +80 −60 km s −1 , and the X-ray temperature of the hot intracluster medium, kT = 5.3 ± 0.3 keV. We infer that A523 is a massive system: M 200 ∼ 7 − 9 ×10 14 M ⊙ . The analysis of the optical data confirms the presence of two subclusters, 0.75 Mpc apart, tracing the SSW-NNE direction and dominated by the two brightest cluster galaxies (BCG1 and BCG2). The X-ray surface brightness is strongly elongated towards the NNE direction, and its peak is clearly offset from both the BCGs. We confirm the presence of a 1.3 Mpc large radio halo, elongated in the ESE-WNW direction and perpendicular to the optical/X-ray elongation. We detect a significant radio/X-ray offset and radio polarization, two features which might be the result of a magnetic field energy spread on large spatial scales. A523 is found consistent with most scaling relations followed by clusters hosting radio haloes, but quite peculiar in the P radio -L X relation: it is underluminous in the X-rays or overluminous in radio. A523 can be described as a binary head-on merger caught after a collision along the SSW-NNE direction. However, minor optical and radio features suggest a more complex cluster structure, with A523 forming at the crossing of two filaments along the SSW-NNE and ESE-WNW directions.

Abstract. We present the results of the dynamical analysis of the rich, hot, and X-ray very luminous galaxy cluster A2219, containing a powerful diffuse radio-halo. Our analysis is based on new redshift data for 27 galaxies in the cluster region, measured from spectra obtained at the TNG, with the addition of other 105 galaxies recovered from reduction of CFHT archive data in a cluster region of ∼5 radius (∼0.8 h −1 Mpc at the cluster distance) centered on the cD galaxy. The investigation of the dynamical status is also performed using X-ray data stored in the Chandra archive. Further, valuable information comes from other bands -optical photometric, infrared, and radio data -which are analyzed and/or discussed, too. We find that A2219 appears as a peak in the velocity space at z = 0.225, and select 113 cluster members. We compute a high value for the line-ofsight velocity dispersion, σ v = 1438 +109 −86 km s −1 , consistent with the high average X-ray temperature of 10.3 keV. If dynamical equilibrium is assumed, the virial theorem leads to M ∼ 2.8× 10 15 h −1 M for the global mass within the virial region. However, further investigation based on both optical and X-ray data shows significant signs of a young dynamical status. In fact, we find strong evidence for the elongation of the cluster in the SE-NW direction coupled with a significant velocity gradient, as well as for the presence of substructure both in optical data and X-ray data. Moreover, we point out the presence of several active galaxies. We discuss the results of our multi-wavelength investigation suggesting a complex merging scenario where the main, original structure is subject to an ongoing merger with a few clumps aligned in a filament in the foreground oriented in an oblique direction with respect to the line-of-sight. Our conclusion supports the view of the connection between extended radio emission and merging phenomena in galaxy clusters.

Context. The most accepted scenario for the origin of fossil groups is that they are galaxy associations in which the merging rate was fast and efficient. These systems have assembled half of their mass at early epoch of the Universe, subsequently growing by minor mergers, and therefore could contain a fossil record of the galaxy structure formation. Aims. We have started an observational project in order to characterize a large sample of fossil groups. In this paper we present the analysis of the fossil system RX J105453.3+552102. Methods. Optical deep images were used for studying the properties of the brightest group galaxy and for computing the photometric luminosity function of the group. We have also performed a detail dynamical analysis of the system based on redshift data for 116 galaxies. Combining galaxy velocities and positions we selected 78 group members. Results. RX J105453.3+552102 is located at z = 0.47, and shows a quite large line-of-sight velocity dispersion σ v ∼ 1000 km s −1 . Assuming the dynamical equilibrium, we estimated a virial mass of M(