ABSTRACT

We use published data for 31 early-type members of binary systems in order to estimate the slope of the (Dnσ) relation. This is considered to be a representation of the Fundamental Plane (FP) for ellipticals. We find a slope for this relation of a = 0.92 ±0.18 when computed with a simple model for the distances. A similar slope has been obtained by others for galaxy groups and it is comparable to values obtained for clusters with a low Abell richness. The scatter of pair values around the FP does not correlate with galaxy properties such as ellipticity, isophotal twisting and total color index. The larger deviations from the (Dnσ) relation tend to involve pairs with smaller projected separations.

INTRODUCTION

Concerns about independence of the FP from environmental conditions have been expressed by Djorgovski et al. (1988), Lucey et al. (1991a, b) and De Carvalho and Djorgovski (1992). On the contrary, Burstein et al. (1990) suggest that the FP does not depend on environment demonstrating, in particular, that the (Dnσ) relation does not depend upon cluster properties.

Isolated binaries represent a different environment from that of a cluster center where galaxies have had time to homogenize. Binary galaxy evolution is driven by a combination of three time scales which are roughly of the same order: the orbital period, the rotational period of the individual galaxies and the burst duration of star formation (SF) triggered by the encounters. Theoretical simulations suggest that they merge rapidly (compared to a Hubble time) into E galaxies, because tidal friction is very efficient.