Cluster Abell 520: a perspective based on member galaxies

Abstract

Context. The connection of cluster mergers with the presence of extended, diffuse radio sources in galaxy clusters is still debated. An interesting case is the rich, merging cluster Abell 520, containing a radio halo. A recent gravitational analysis has shown in this cluster the presence of a massive dark core suggested to be a possible problem for the current cold dark matter paradigm. Aims. We aim to obtain new insights into the internal dynamics of Abell 520 analyzing velocities and positions of member galaxies. Methods. Our analysis is based on redshift data for 293 galaxies in the cluster field obtained combining new redshift data for 8 galaxies acquired at the TNG with data obtained by CNOC team and other few data from the literature. We also use new photometric data obtained at the INT telescope. We combine galaxy velocities and positions to select 167 cluster members around z ∼ 0.201. We analyze the cluster structure using the weighted gap analysis, the KMM method, the Dressler-Shectman statistics and the analysis of the velocity dispersion profiles. We compare our results with those from X-ray, radio and gravitational lensing analyses. Results. We compute a global line-of-sight (LOS) velocity dispersion of galaxies, σ v = 1066 +67 −61 km s −1 . We detect the presence of a high velocity group (HVG) with a rest-frame relative LOS velocity of v rf ∼ 2000 km s −1 with respect to the main system (MS). Using two alternative cluster models we estimate a mass range M(<1 h 70 M . We also find that the MS shows evidence of subclumps along two preferred directions. The main, complex structure NE1 + NE2 (with a velocity comparable to that of the MS) and the SW structure (at v rf ∼ +1100 km s −1 ) define the NE-SW direction, the same of the merger suggested by X-ray and radio data. The E and W structures (at v rf ∼ −1150 and v rf ∼ −300 km s −1 ) define the E-W direction. Moreover, we find no dynamical trace of an important structure around the lensing dark core. Rather, the HVG and a minor MS group, having different velocities, are roughly centered in the same position of the lensing dark core, i.e. are somewhat aligned with the LOS. Conclusions. We find that Abell 520 is definitely a very complex system. Our results suggest that we are looking at a cluster forming at the crossing of three filaments of the large scale structure. The filament aligned with the LOS and projected onto the center of the forming cluster might explain the apparent massive dark core shown by gravitational lensing analysis.

Structure of Abell 1995 from optical and X-ray data: a galaxy cluster with an elongated radio halo

“…We find that the elongation of the radio halo roughly agrees with the direction of the merger, as detected in several clusters, e.g. the Bullet Cluster (1E0657-56; Markevitch et al 2002) and Abell 520 (Girardi et al 2008); but see Abell 523 (Giovannini et al 2011). …”

“…We assumed t = 0.2 Gyr for the time elapsed from the core crossing (a typical value for clusters hosting radio halos; e.g. Markevitch et al 2002;Girardi et al 2008). Figure 16 illustrates the solutions of the bimodal model, where the mass of the system M sys is plotted against α, the projection angle between the direction defined by the subclumps and the plane of the sky.…”

Internal dynamics of Abell 2254: a merging galaxy cluster with a clumpy, diffuse radio emission

“…Finally, we stress the absence of a connection between bright galaxies and subclusters, while this is often found in complex systems in 2D (e.g., , in 3D (e.g., Abell 520, Girardi et al 2008), and also when a subcluster is partially destroyed (see the beautiful example of the tidal debris in A2744 by Owers et al 2011). In A2254, the high-velocity E peak, the one that is detected best through our optical analysis, hosts a bright, dominant galaxy.…”

“…1), as generally shown in other clusters not only by their large scale morphology but also by the point-to-point correlation between radio and X-ray brightness (see the case of the four clusters analyzed by Govoni et al 2001a). Moreover, halos are round or elongated in the direction of the merger (the bullet cluster by Markevitch et al 2002;Abell 520 by Girardi et al 2008;Abell 754 by Macario et al 2011): this is not the case for the A2254 halo.…”

“…The comparison between the observed shape of the X-ray surface brightness isophotes and the simulated ones (Poole et al 2006) suggests we can assume we see the merger after the core crossing and not too long after the apocenter. For comparison with other clusters hosting radio halos, the time t elapsed from the core crossing is generally few fractions of Gyr (e.g., Markevitch et al 2002;Girardi et al 2008). Figure 24 compares the bimodalmodel solutions as a function of α, where α is the projection angle between the plane of the sky and the line connecting the centers of the two clumps, with the observed mass of the system M sys .…”