| Abstract
| - This paper is aimed at giving an overview of the global properties of the rich cluster of galaxies ABCG 209. This is achieved by complementing the already available data with new medium-resolution spectroscopy and near-infrared (NIR) photometry which allow us to (i) analyse in detail the cluster dynamics, distinguishing among galaxies belonging to different substructures and deriving their individual velocity distributions, using a total sample of 148 galaxies in the cluster region, of which 134 belonging to the cluster; (ii) derive the cluster NIR luminosity function; (iii) study the Kormendy relation and the photometric plane of cluster early-type galaxies (ETGs). Finally we provide an extensive photometric (optical and NIR) and spectroscopic data set for such a complex system to be used in further analyses investigating the nature, formation and evolution of rich clusters of galaxies. The present study shows that the cluster is characterized by a very high value of the line-of-sight velocity dispersion: σv= 1268+93−84 km s−1, that results in a virial mass of Mvir= 2.95+0.80−0.78× 1015h−170 M⊙ within Rvir= 3.42 h−170 Mpc. The analysis of the velocity dispersion profile shows that such high value of σv is already reached in the central cluster region. There is evidence of three significant substructures, the primary one having a velocity dispersion of σv= 847+52−49 km s−1, which makes it consistent with mass estimates from weak lensing analyses. This observational scenario confirms that ABCG 209 is presently undergoing strong dynamical evolution with the merging of two or more subclumps. This interpretation is also supported by the detection of a radio halo suggesting that there is a recent or ongoing merging. Cluster ETGs follow a Kormendy relation whose slope is consistent with previous studies both at optical and NIR wavelengths. We investigate the origin of the intrinsic scatter of the photometric plane due to trends of stellar populations, using line indices as indicators of age, metallicity and α/Fe enhancement. We find that the chemical evolution of galaxies could be responsible for the intrinsic dispersion of the photometric plane.
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