Research On Ellipticities And Position Angles Of Cluster Galaxies

An investigation of the orientation of galaxies in clusters is regardedas a standard test of theories of galaxy and large scale structureformation and a probe of the dynamical state of clusters. A wellestablished alignment effect is that between the orientation of nearbyclusters and between the orientation of BCG (or cD) and that of theirparent cluster. Furthermore, recent evidence has shown thatsubstructures in clusters are aligned with the cluster orientation as wellas along the large-scale filamentary structures within which they areembedded. The alignments between BCG and cluster galaxies exit onthe scale~Mpc. Cluster and field galaxies have different environments,and the ellipticity and position angle distributions reflect thediscrepancies between Cluster and field galaxies. A1689is a youngdynamical rich cluster and has two mass clumps. The aim of this paper isto detect the alignment between BCG and cluster galaxies in Abell1689on a smaller scale. This paper utilizes the software SEXtractor to getobjects on two bands, and eliminates data by setting some conditionscorresponding to the ellipticity, the magnitude and the position angle,and then gets cluster members by the red selection. Finally, I researchinto the alignment in the cluster central region by the statistical dataincluding galaxy ellipticities and position angles，and find that:(i) thealignment exits in the central region,(ii) the galaxies in the inner partsare rounder than those in the outskirts，(iii) the ellipticity and positionangle distributions of field galaxies haven’t any trends. According to thestatistical results, the possible fact infuencing the distributions is theeffect of tidal force. Meanwhile the paper proves the fact that thealignments between BCG and cluster galaxies appear distinct in activeclusters.