The color-absolute magnitude relation for E and S0 galaxies. I - Calibration and tests for universality using Virgo and eight other nearby clusters

Abstract

Spectrum scanner observations of E and SO galaxies in the Virgo cluster confirm the previously known corrclation between color and absolute magnitude, and show it to be strongly wavelength- dependent, with a maximum near A3500. Observations with discrete filter bands show a tight relation between u - V color ( A and 5500 A) and absolute visual magnitude, in the sense of bluer colors for fainter galaxies. The color-magnitude (C-M) gradient is 0.1 mag color change in u - V per unit absolute magnitude change, and is the same for E and SO galaxies. Galaxies in eight other groups and clusters show a color-magnitude effect that has the same wavelength dependence, slope, absolute magnitude calibration, and cosmic scatter as the Virgo cluster. The C-M effect in all groups and clusters studied here can be made to coincide to form a common composite C-M correlation by shifts in magnitudes that are in the ratio of the redshifts (5 log V!VvIRGo), which demonstrates the universality of the C-M effect, assuming that the local velocity-distance relation is linear. On the other hand, if the magnitude shifts are determined independently by forcing the individual C-M relations to coincide with that for the Virgo cluster, photometric distances relative to Virgo can be determined that are red shift-independent. Such distances to nine groups and clusters correlate well with the red shifts, showing that the velocity-distance relation is linear, on the assumption now that the absolute magnitude calibration of the C-M effect is universal. The Virgo cluster falls within 25 km s ' of the ridge-line solution of the velocity-distance relation. This is within a velocity residual of 0.3 a of the variance of the mean velocity of the cluster itself (a = 68 km s-'). From these data, the Virgo cluster appears to have no measurable peculiar motion at a level of 25 km s -` relative to the underlying uniform Hubble flow. If we adopt the Virgo cluster modulus to be m - M = 31.70, the absolute calibration of the C-M effect for E and SO galaxies becomes Mv26 = -10.327(u - V)c + 2.19 with an intrinsic dispersion of a(M) = +0.5 mag for a single galaxy. Hence photometric distances can be determined for single E and SO galaxies by the color-magnitude method within errors that are distributed as a( r) = 10.22. Subject headings: galaxies: clusters of galaxies: photometry