The relative distance between the clusters of galaxies A2634 and coma

The Tully-Fisher (TF) and Fundamental Plane (FP) relations are used to obtain two independent estimates of the relative distance between the clusters A2634 and Coma. Previously published studies of A2634 showed a large discrepancy between the distance estimates obtained with the TF and the D-n-sigma relations, questioning the reliability of redshift-independent distances obtained using these relations. Because of the importance of this issue, we have obtained new distance estimates for A2634, based on much larger samples than previously used; and selected according to rigorous membership criteria. New I band CCD photometry for 175 galaxies, new 21 cm observations of 11 galaxies, and new velocity dispersion measurements for 62 galaxies are used together with previously published data in building these samples. As part of a larger project to compare the TF and FP distance-scales, we have obtained a new FP template using for the first time I band photometry. The template is derived using a sample of 109 E and SO galaxies that are members of the Coma cluster. Its parameters are in very good agreement with recent determinations of the FP obtained at shorter wavelengths. The uncertainty with which the FP can provide peculiar velocity estimates for single galaxies is similar or equal to 0.43 mag in the distance modulus, or 20% of the distance. This uncertainty is slightly larger than the typical uncertainty that characterizes TF estimates. However, this disadvantage is partly compensated by the fact that the sample incompleteness bias has a less severe effect on FP cluster distance estimates than it has on the corresponding TF distance estimates. Also, cluster membership is more readily established for early-type objects than for spirals. After the appropriate corrections for sample incompleteness have been taken into account, we find the TF and FP distance estimates to be in good agreement, both indicating that A2634 has a negligibly small peculiar velocity with respect to the Cosmic Microwave Background reference frame. Because of the high accuracy with which the two distance estimates have been obtained, their agreement strongly supports the universality of the TF and FP relations, and therefore their reliability for the estimate of redshift-independent distances. (C) 1997 American Astronomical Society.