10.6 Future Needs and Directions
There are several topics which need to be addressed to provide a firmer basis for the use of the method.
(1) Perhaps the most pressing need is for more high quality, multiple observations of galaxies in clusters to test the reproducibility and precision of the diameter and dispersion measurements and to determine the limiting scatter in the relationship. Apparent differences between authors on the distance to the same cluster (e.g., Abell 2199) and differences between the IRTF and Dn- method (e.g., Abell 2634) must be resolved. A substantial amount of work is underway that will address these points: a survey of clusters in the Hercules and Pisces-Cetus region is being undertaken by the EFAR collaboration of Baggley, Bertschinger, Burstein, Colless, Davies, McMahan, Saglia, and Wegner; Lucey and collaborators are also studying a sample of more distant clusters.
(2) Investigations into possible systematic effects that may affect the Dn- method must be continued. Differences in star formation history, structure or kinematics need to be explored.
(3) The effect of seeing on the measurement of Dn needs to be quantified in a realistic way. Model point-spread-functions may not adequately describe the smoothing that occurs. Work by the EFAR group is underway in this area.
(4) The canonical form of the Dn- relation must be determined more precisely. Is a simple power law an adequate representation? Is curvature present at low luminosities? The Coma cluster appears to provide the best opportunity for establishing these relationships.
(5) Is it possible to include other early-type galaxies to improve the estimation of distance to galaxy aggregates? Several studies have utilized lenticular galaxies: Dressler's (1987) discussion of Coma and Virgo, the exploration of the far-side of the Great Attractor by Dressler and Faber (1990b) and the recent work on distant clusters by Lucey et al. (1991a) suggests that disk galaxies can be used. The expansion of the technique to include galaxies spanning a wider range of properties has two positive implications. The number of galaxies in an aggregate would typically be doubled if early type disk galaxies could be included, leading to more accurate distance estimates for those aggregates. Further if the selection criterion could be used to include galaxies such as M31 and M32, then the method could be calibrated independently allowing absolute rather than relative distances to be determined.
(6) Finally, a better understanding of the bias introduced by the highly non-uniform distribution of galaxies needs to be developed. The simulations reported by Dekel et al. (1990) suggest that any residual errors which remain after making the correction of equation (24) are small. Lynden-Bell (1992) developed Schechter's (1980) method of correcting for the Malmquist-like bias and finds that the correction for the bias depends only on the dispersion in field galaxies and not on distance. Applying it to the elliptical galaxy data he finds results that are almost identical to the original analysis. On the other hand Landy and Szalay (1991), claim that much of the Great Attractor signal may be due to the Malmquist correction. We look forward to Bertschinger's (1992) review of the subject to clarify the situation.