The Effects of Seeing on the Photometric Properties of Elliptical Galaxies

Abstract

The effects of seeing on the photometric properties of elliptical galaxies and, in particular, on the D_n_-{sigma} and fundamental plane distance estimators are studied. A new family of functions for the analytical approximation of the point spread function (psf) is introduced and is shown to fit stellar images better than the widely used Gaussian and Moffat functions. Seeing effects are computed for circular mean surface brightness distributions using the R^1/4^ law and models with more and less prominent central cusps. A prescription to correct raw half-luminosity radii, D_n_ diameters, and surface brightnesses at and within R_e_ is given. It is shown that seeing effects become important for distant ellipticals (cz >= 8000 km s^-1^). Uncorrected D_n_-{sigma} or fundamental plane distances are systematically too high, leading to spurious negative peculiar velocities of the order of 1000 km s^-1^ at a distance of 12000 km s^-1^. The large negative peculiar velocities of the most distant clusters of the Faber et al. sample are reduced when seeing effects are taken into account, as is the scatter in the D_n_-{sigma} relation. Seeing corrections are small for galaxies included in the flow solutions of Lynden-Bell et al. (1988) and are unlikely to affect them.