Alignment and morphology of elliptical galaxies: the influence of the cluster tidal field

We investigate two possible effects of the tidal field induced by a sphericalcluster on its elliptical galaxy members: the modification of the ellipticityof a spherical galaxy and the isophotal alignment in the cluster radialdirection of a misaligned prolate galaxy. Numerical N-body simulations havebeen performed for radial and circular galactic orbits. The properties of thestars' zero--velocity surfaces in the perturbed galaxies are explored briefly,and the adiabaticity of the galaxy to the external field is discussed. For achoice of parameters characteristic of rich clusters we find that the inducedellipticity on a spherical galaxy is below or close to the detectability level.But we find that the tidal torque can result in significant isophotal alignmentof the galaxies' major axis with the cluster radial direction if the galaxy isoutside the cluster core radius. The time required for the alignment is veryshort compared with the Hubble time. A significant increase in the ellipticityof the outer isophotes of the prolate model is also found, but with noobservable isophotal twisting. Our main prediction is an alignment segregationof the elliptical galaxy population according to whether their orbits liemostly outside or inside the cluster core radius. These results also suggestthat galactic alignment in rich clusters is not incompatible with a bottom-upgalaxy formation scenario.Comment: 20 pages, uuencoded compressed tarred postscrip