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Register a new userMultiple merging events in Abell 521
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We present a detailed spatial and dynamical analysis of the central $sim$~2.2~h~Mpc region of the galaxy cluster Abell~521 (z=0.247), based on 238 spectra obtained at the 3.6~m Telescope of ESO and at the CFHT. From the analysis of the 125 galaxies confirmed members of the cluster, we derive a mean velocity of $74019 ^{+112}_{-125}$ km/s and detect a complex velocity distribution with high velocity dispersion, $1325 ^{+145}_{-100}$ km/s), but clear departure from a single gaussian component. The general structure of the cluster follows a NW/SE direction, crossed by a perpendicular high density ``ridge of galaxies in the core region. The northern region of the cluster is characterized by a lower velocity dispersion as compared to the whole cluster value; it hosts the BCG and a dynamically bound complex of galaxies, and it is associated to a group detected in X-ray (Arnaud et al 2000). This region could be in pre-merger stage onto the main cluster nearly in the plane of the sky. These results, taken together with the fact that most of the clumps detected on the isodensity maps, as well as the early type galaxies and the brightest ones are aligned, suggest that this NW/SE direction is the preferred one for the formation of this cluster. The central high dense region shows a lower velocity location ($73625 ^{+344}_{-350}$ km/s) and significantly higher scale ($1780 ^{+234}_{-142}$ km/s) as compared to the whole cluster values. This is due to the presence of a low-velocity group of galaxies with a high fraction of emission line objects. This can be explained in a scenario in which a merging of subclusters has recently occurred along the direction of the ``ridge with a significant component along the line of sight.
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We present a dynamical analysis of the central ~1.3 square degrees of the cluster of galaxies Abell 1367, based on 273 redshift measurements (of which 119 are news). From the analysis of the 146 confirmed cluster members we derive a significantly non-Gaussian velocity distribution, with a mean location C_{BI} = 6484+/-81 km/s and a scale S_{BI} = 891+/-58 km/s. The cluster appears elongated from the North-West to the South-East with two main density peaks associated with two substructures. The North-West subcluster is probably in the early phase of merging into the South-East substructure (~ 0.2 Gyr before core crossing). A dynamical study of the two subclouds points out the existence of a group of star-forming galaxies infalling into the core of the South-East subcloud and suggests that two other groups are infalling into the NW and SE subclusters respectively. These three subgroups contain a higher fraction of star-forming galaxies than the cluster core, as expected during merging events. Abell 1367 appears as a young cluster currently forming at the intersection of two filaments.
The pre-merging system of galaxy clusters Abell 3391-Abell 3395 located at a mean redshift of 0.053 has been observed at 1 GHz in an ASKAP/EMU Early Science observation as well as in X-rays with eROSITA. The projected separation of the X-ray peaks of the two clusters is $sim$50$$ or $sim$ 3.1 Mpc. Here we present an inventory of interesting radio sources in this field around this cluster merger. While the eROSITA observations provide clear indications of a bridge of thermal gas between the clusters, neither ASKAP nor MWA observations show any diffuse radio emission coinciding with the X-ray bridge. We derive an upper limit on the radio emissivity in the bridge region of $langle J rangle_{1,{rm GHz}}< 1.2 times 10^{-44} {rm W}, {rm Hz}^{-1} {rm m}^{-3}$. A non-detection of diffuse radio emission in the X-ray bridge between these two clusters has implications for particle-acceleration mechanisms in cosmological large-scale structure. We also report extended or otherwise noteworthy radio sources in the 30 deg$^2$ field around Abell 3391-Abell 3395. We identified 20 Giant Radio Galaxies, plus 7 candidates, with linear projected sizes greater than 1 Mpc. The sky density of field radio galaxies with largest linear sizes of $>0.7$ Mpc is $approx 1.7$ deg$^{-2}$, three times higher than previously reported. We find no evidence for a cosmological evolution of the population of Giant Radio Galaxies. Moreover, we find seven candidates for cluster radio relics and radio halos.
Using analytical and numerical methods (fluid and particle-in-cell simulations) we study a number of model problems involving merger of magnetic flux tubes in relativistic magnetically-dominated plasma. Mergers of current-carrying flux tubes (exemplified by the two dimensional `ABC structures) and zero total current magnetic flux tubes are considered. In all cases regimes of spontaneous and driven evolution are investigated. We identify two stages of particle acceleration during flux mergers: (i) fast explosive prompt X-point collapse and (ii) ensuing island merger. The fastest acceleration occurs during the initial catastrophic X-point collapse, with the reconnection electric field of the order of the magnetic field. During the X-point collapse particles are accelerated by charge-starved electric fields, which can reach (and even exceed) values of the local magnetic field. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization $sigma$. For plasma magnetization $sigma leq 10^2$ the spectrum power law index is $p> 2$; in this case the maximal energy depends linearly on the size of the reconnecting islands. For higher magnetization, $sigma geq 10^2$, the spectra are hard, $p< 2$, yet the maximal energy $gamma_{max}$ can still exceed the average magnetic energy per particle, $ sim sigma$, by orders of magnitude (if $p$ is not too close to unity). The X-point collapse stage is followed by magnetic island merger that dissipates a large fraction of the initial magnetic energy in a regime of forced magnetic reconnection, further accelerating the particles, but proceeds at a slower reconnection rate.
The galaxy cluster Abell 3266 is one of the X-ray brightest in the sky and is a well-known merging system. Using the ability of the eROSITA telescope onboard SRG (Spectrum Rontgen Gamma) to observe a wide field with a single pointing, we analyse a new observation of the cluster out to a radius of R_200. The X-ray images highlight substructures present in the cluster, including the northeast-southwest merger seen in previous ASCA, Chandra and XMM-Newton data, a merging group towards the northwest and filamentary structures between the core and one or more groups towards the west. We compute spatially-resolved spectroscopic maps of the thermodynamic properties of the cluster, including the metallicity. The merging subclusters are seen as low entropy material within the cluster. The filamentary structures could be the rims of a powerful AGN outburst, or most likely material stripped from the western group(s) as they passed through the cluster core. Seen in two directions is a pressure jump at a radius of 1.1 Mpc consistent with a shock with a Mach number of ~1.5-1.7. The eROSITA data confirm that the cluster is not a simple merging system, but is made up of several subclusters which are merging or will shortly merge. For the first time we find a radio halo associated with the system detected in GLEAM data. We compute a hydrostatic mass from the eROSITA data, finding good agreement with a previous XMM-Newton result. With this pointing we detect several extended sources, where we find for seven of them secure associations between z=0.36-1.0; i.e., background galaxy groups and clusters, highlighting the power of eROSITA to find such systems.
Deep radio observations of the galaxy cluster Abell 781 have been carried out using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to previous 610 MHz observations and to archival VLA 1.4 GHz data. The radio emission from the cluster is dominated by a diffuse source located at the outskirts of the X-ray emission, which we tentatively classify as a radio relic. We detected residual diffuse emission at the cluster centre at the level of S(325 MHz)~15-20 mJy. Our analysis disagrees with Govoni et al. (2011), and on the basis of simple spectral considerations we do not support their claim of a radio halo with flux density of 20-30 mJy at 1.4 GHz. Abell 781, a massive and merging cluster, is an intriguing case. Assuming that the residual emission is indicative of the presence of a radio halo barely detectable at our sensitivity level, it could be a very steep spectrum source.
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