ABSTRACT

We have found possible evidence that boxy-type elliptical galaxies favor the environment of clusters of galaxies, while disky-type ellipticals prefer the field environment.

MOTIVATION, DATA, AND RESULTS

Recent high-quality imaging and spectroscopic studies have shown that there is a fundamental distinction in elliptical galaxies : the isophotal shapes of elliptical galaxies (cf. Bender et al. 1989). Elliptical galaxies have three types of isophotes: boxy, elliptic, and disky. It should be stressed that these types are more physically related to the dynamical properties of the elliptical galaxies than are the fine morphologically peculiar features such as shells. The boxy elliptical galaxies owe their shape to the anisotropy of velocity dispersion of stars while the disky type are flattened by rotational motion. This dynamical difference between them suggests that the two types of galaxies have different formation histories. It is therefore natural to ask whether the difference in the isophotal shapes of elliptical galaxies may have some relation to their environments.

Our study is based on the data in Bender et al. (1989) who compiled published data as well as their own results of detailed CCD photometry of 109 elliptical galaxies. Among the 109 elliptical galaxies, we assigned the environments for 96 galaxies based on the paper by Faber et al. (1989). The elliptical galaxies in Virgo, Coma, Abell 194, and Abell 1367 clusters of galaxies are classified as cluster ellipticals. The elliptical galaxies to which no group identification has been given are classified as field ellipticals. The remaining ellipticals belong to the group of galaxies. In this way, we obtained a sample of 96 elliptical galaxies.