THE EFFECTS OF SEEING ON THE PHOTOMETRIC PROPERTIES OF ELLIPTIC GALAXIES

The effects of seeing on the photometric properties of elliptical gala xies and, in particular, on the D(n)-sigma and fundamental plane dista nce estimators are studied. A new family of functions for the analytic al approximation of the point spread function (psf) is introduced and is shown to fit stellar images better than the widely used Gaussian an d Moffat functions. Seeing effects are computed for circular mean surf ace brightness distributions using the R1/4 law and models with more a nd less prominent central cusps. A prescription to correct raw half-lu minosity radii, D(n) diameters, and surface brightnesses at and within R(e) is given. It is shown that seeing effects become important for d istant ellipticals (cz greater-than-or-equal-to 8000 km s-1). Uncorrec ted D(n)-sigma or fundamental plane distances are systematically too h igh, leading to spurious negative peculiar velocities of the order of 1000 km s-1 at a distance of 12 000 km s-1. The large negative peculia r velocities of the most distant clusters of the Faber et al. sample a re reduced when seeing effects are taken into account, as is the scatt er in the D(n)-sigma relation. Seeing corrections are small for galaxi es included in the flow solutions of Lynden-Bell et al. (1988) and are unlikely to affect them.