DYNAMICAL EVOLUTION AND GALAXY POPULATIONS IN THE CLUSTER ABCG209 AT Z = 0.2
The thesis work is focused on the analysis of the galaxy clusters ABCG 209, at zrv 0.2, which is characterized by a strong dynamical evolution. The data sample used is based mainly on new optical data (EMMI-NTT: B, V and R band images and MOS spectra), acquired in October 2001 at the European Southern Observatory in Chile. Archive optical data ( CFHR12k: B and R images), and X-ray (Chandra) and radio (VLA) observations are also analysed. The n1ain goal of this analysis is the investigation of the connection between internal cluster clynamics and star formation history, aimed at understanding the complex mechanisms of cluster formation and evolution. The internal dynamics of the cluster was studied through a spectroscopic survey of 112 cluster Inembers. The dynamical analysis has pointed out that ABCG 209 is characterized by a very high value of the line of sight velocity dispersion: av = 1250-1400 km s- 1that results in a virial mass of M = 1.6-2.2 x 1015 h-1 M0 within Rvir· A preferential SE-NW direction is indicated by: a) the presence of a velocity gradient in the velocity field; b) the elongation in the spatial distribution of colour-selected cluster members; c) the elongation of the X-ray contour levels in the Chandra image; d) the elongation of the cD galaxy. T h ere is evidence of substructure, as shown by i) significant cleviation of the velocity distribution from a Gaussian, with evidence for two secondary clumps at z = 0.199 and z = 0.215, which appear spatially segregated from the main cluster, ii) the Dressler & Schectman test and iii) the two-dimensional distribution of the colour-selected members shows a strong luminosity segregation: bright galaxies R < 19.5 are centered around the cD galaxy, while faint galaxies R > 19.5 show some clumps. The main one, Eastern with respect to the cD galaxy, is well coincident with the secondary X -ray peak. ' The study of the galaxy luminosity function (LF) in B, V and R bands has pointed out that ABCG 209 is a cluster with intermediate characteristic between a dynamically-evolved, rich clusters and clusters with central dominant galaxies having bright characteristic luminosities and shallow faint-end slopes and less evolved clusters, characterized by steep faint-end slopes, thus reconciling the asynnnetric properties of X-ray emission with the non flat-LF shape of irregular systen1s. This observational scenario suggests that ABCG 209 is undergoing a strong dynan1ical evolution with the Inerging of two or more subclumps along the SE-NW direction in a plane which is not parallel to the plane of sky. The merging might be in a more advanced status, where luminous galaxies trace the remnant of the core-halo structure of a pre-merging clump hosting the cD galaxy. The elongation and asymmetry of the galaxy distribution (of the X-ray emission) and the shape of the LFs show that ABCG 209 is not yet a fully relaxed system. The effect of cluster environment (as measured in terms of the local surface density of R < 23.0 galaxies) o n the global properties of the cluster galaxies is examined through the analysis of the L Fs, colour-magnitude relations, and average colours by using wide field (30'x42') B- and R-band images. The faint-end slope of the LF, a, shows a strong dependence on environment, becoming steeper at > 3a significance level from high- to low-density environments. The red sequence is found to be 0.022 ± 0.014 mag redder in the high-density region than for the intermediate-density region by fixing the slope. In contrast no correlation between the slope of the red sequence and environment was observed. Studying the effect of the cluster environment on galaxy star-formation, we find that the blue galaxy fraction decreases monotonically with density, in agreement with other studies. The observed trends of steepening of the faint-end slope, faintening of the characteristic luminosity, and increasing blue galaxy fraction, from high- to low-density environments, are manifestations of the morphology-density relation, where the fraction of early-type galaxies decreases smoothly and monotonically from the cluster core to the periphery, while the fraction of late-type galaxies increases in the same manner. The observed trends in the composite LF reflect this morphology-density relation: the galaxy population in the cluster core is dominateci by early-type galaxies and so the cornposite LF resembles that of this type of galaxy, with a shallow faint-end slope and a bright characteristic lun1inosity; whereas in lower density regions the fraction ~f late-type galaxies increases, and so the composite LF increasingly resembles that of the late-type, with a steep faint-end slope and a fainter characteristic magnitude. The analysis of the mean colour of luminous (R < 21) cluster galaxies as a function of their spatial position shows clearly the complex effects of the cluster environment and dynamics on their constituent galaxies. The reddest galaxies are concentrateci around the cD galaxy (main cluster) and a more diffuse region 5 arcmin to the north is coincident with the structure predicted from weak lensing analysis. The effect of the preferential SE-NW direction for ABCG 209 is apparent in the presence of bright blue galaxies near the cD galaxy perpendicular to the axis and hence unaffected by the cluster merger, and an extension of red galaxies to the SE which may indicate the irifall of galaxies into the cluster along a filament. This preferential SE-NW direction appears also related to the large-scale structure in which ABCG 209 is embedded, with two rich (Abell class R=3) clusters ABCG 222 at z = 0.211 and ABCG 223 at z = 0.2070 are located 1.5° (15M pc) to the NW along this preferential axis. Cluster dynamics and large-scale structure clearly have a strong influence on galaxy evolution, so we have performed a detailed study of spectroscopic properties of 102 luminous member galaxies. W e find five different galaxy types: i) passive evolving galaxies (E), which exhibit red colours and no emission lines, ii) emission line galaxies (ELG), which are blue and have prominent emission lines, iiiiv) strong Hc5 galaxies, that are characterized by the presence of strong Hc5 equivalent width and can be divided into blue (HDSblue) and red (HDSred), according to the break at 4000 A and the B-R colours, v) and finally anemie spirals (Ab-spirals), that have spectral properties equal to passive evolving galaxies, but are disk-dominateci systems. These different spectral classes are strongly segregated in the phase-space, as indicated by the two dimensionai Kolmogorov-Smirnov test. Passive evolving galaxies represent the I"V 74% of the cluster members. This population formed very early, during the initial collapse of the cluster. They lie mainly in high density regions and ha ve a velocity dispersion fully consistent with those of the whole cluster. This result is understandable in terms of cosmological models of structure formation, in which early-type galaxies form in the highestdensity regions corresponding to the cores of rich clusters. HDSred galaxies are distributed along the elongation of the cluster mainly in intermediate density regions and have a significant low velocity dispersion, suggesting that this population could be the remnant of an infalling group. According to the evolution models, the presence of a strong Hc5 absorption line in their spectra indicates that these galaxies have experimented a short starburst of star formation in the past few Gyrs. In the starburst nwdel [Hc5] and Dn(4000) declines on a timescale of I"V 2Gyr after the burst has ceased, irregardless of their SFR before the burst. Thus the fact that we see these galaxies, detecting [Hc5] > 3.0 A implies that the burst has occurred no more than 2 Gyr ago. HDSblue galaxies are found in intermediate density regions in a direction perpendicular to the cluster elongation, dose to the secondary peak of the X-ray flux andina region where the intracluster Inedium (ICM) is dense. Moreover galaxies belonging to this class seem to be aligned Thus the burst in the star formation seen in these galaxies could be due to the interaction with the hot dense ICM. They have high velocity dispersion. ELGs lie in low density regions and have high line-of-sight velocity dispersion. Both the spatial position and the velocity dispersion suggest that these two populations of galaxies have recently fallen into the cluster from the field. All these results support an evolutionary scenario in which ABCG 209 is characterized by a sum of two components: an old galaxy population, formed very earlier (zJ ;:G 3), and a younger population of infalling galaxies. Moreover this cluster may have experimented l or 2 Gyrs ago a merging with an infalling galaxy group, as indicated also by the previous dynamical analysis. This detailed study has showed clearly the importance to have multi-band data and to perform a n1ulti-directional analysis, in order to precisely characterize the different cluster components. It is now fundamental to extend this kind of analysis to other clusters at higher redshift and with different dynamical properties. To address the issue if clusters are generally young or old one needs to have measurements of subclustering properties of a large sample of clusters and at the same time it is fundamental to precisely characterize cluster components belonging to different structures and environments inside a single cluster.