Cluster Abell 520: a perspective based on member galaxies

Free access article

Issue		A&A Volume 491, Number 2, November IV 2008


Page(s)		379 - 395
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361:200810549
Published online		17 September 2008

A&A 491, 379-395 (2008)
DOI: 10.1051/0004-6361:200810549

A cluster forming at the crossing of three filaments?

M. Girardi^{1, 2}, R. Barrena³, W. Boschin^{1, 4}, and E. Ellingson⁵

¹ Dipartimento di Astronomia of the Università degli Studi di Trieste, via Tiepolo 11, 34143 Trieste, Italy
e-mail: girardi@oats.inaf.it
² INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
³ Instituto de Astrofísica de Canarias, C/vía Láctea s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
⁴ Fundación Galileo Galilei - INAF, Rambla José Ana Fernández Perez 7, 38712 Breña Baja (La Palma), Canary Islands, Spain
⁵ Center for Astrophysics and Space Astronomy, 389 UCB, University of Colorado, Boulder, CO 80309, USA

Received 9 July 2008 / Accepted 10 September 2008

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 $z \sim0.201$ . 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, $\sigma_{ v}=1066_{-61}^{+67}$ km s^-1. We detect the presence of a high velocity group (HVG) with a rest-frame relative LOS velocity of ${ v_{\rm rf}}\sim 2000$ km s^-1 with respect to the main system (MS). Using two alternative cluster models we estimate a mass range $M(\mbox{<}1 \;h_{70}^{-1}~\mathrm{Mpc})=(4.0$ –9.6) $~\times 10^{14}\;h_{70}^{-1}~M_{\odot}$ . We also find that the MS shows evidence of subclumps along two preferred directions. The main, complex structure ${\cal NE}1+{\cal NE}2$ (with a velocity comparable to that of the MS) and the ${\cal SW}$ structure (at ${ v_{\rm rf}}\sim +1100$ km s^-1) define the NE-SW direction, the same of the merger suggested by X-ray and radio data. The ${\cal E}$ and ${\cal W}$ structures (at ${ v_{\rm rf}}\sim -1150$ and ${v_{\rm rf}}\sim - 300$ 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.

Key words: galaxies: clusters: individual: Abell 520 -- galaxies: clusters: general -- galaxies: distances and redshifts

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