Author: Alister W. Graham & Mercedes Prieto
Abstract: Recent studies have claimed that the Hubble sequence of late-type spirals, and spirals in general, is scale-free. Within the photometric data used in these works, a trend exists between morphological type and bulge profile shape such that late-type spiral bulges are described by an exponential luminosity profile, where-as the early-type spiral bulges are better described by an $r^{1/2}$ or $r^{1/4}$ law. The universal application of an exponential light-profile to the disk {\it and} bulge of {\it all} spiral galaxies is not justified. Taking structural parameters from exponential disk models and the best-fitting $r^{1/n}$ bulge models (with n=1, 2 or 4), the mean effective-bulge-radius to disk scale-height ratio ($r_e/h$) increases at the 3 $\sigma$ significance level. Additionally, the early-type spiral galaxies are shown to have a larger $r_e/h$ ratio than the late-type spirals in all passbands.