On the light-profiles of spiral galaxy bulges and the Hubble sequence for spirals

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 sh ape such that late-type spiral bulges are described by an exponential lumin osity profile, where-as the early-type spiral bulges are better described b y an r(1/2) or r(1/4) law. The universal application of an exponential ligh t-profile to the disk and bulge of all spiral galaxies is not justified. Ta king structural parameters from exponential disk models and the best-fittin g r(1)/n bulge models (with n = 1, 2 or 4), the mean effective-bulge-radius to disk scale-length ratio (r(e)/h) increases at the 3 sigma significance level. Additionally, the early-type spiral galaxies are shown to have a lar ger r(e)/h ratio than the late-type spirals in all passbands.