The Dn-sigma relation for bulges of disk galaxies - A new, independent measure of the Hubble constant

It has recently been found (Dressler et al., 1987) that a new photometric parameter, Dn, correlates well with central velocity dispersion for a sample of elliptical galaxies in clusters. Here it is shown that this correlation extends, perhaps equally well, to the bulges of early-type galaxies with Hubble types S0 to Sb. With the data available at present for S0 and spiral galaxies in the coma and Virgo clusters, the slopes and zero points of the log Dn-log sigma relationships are identical to those for the elliptical galaxies in the two clusters. In contrast to earlier work using the Faber-Jackson relation, these data show that the inner structures of bulges and ellipticals are very similar, which suggests that their origins were the same. The Dn-sigma relationship provides a simple and accurate estimator of relative distance. An absolute calibration is possible if the method can be applied to galaxies for which distances have been determined with primary indicators like Cepheid variables (e.g., M31 and M81). Using these data, the Dn-sigma relation gives a Hubble constant of 67 + or - 10 km/s Mpc.