The influence of bulge profile shapes on claims for a scale-free Hubble sequence for spiral galaxies

We investigate recent claim that the Hubble sequence of spiral galaxies is scale-free. Fundamental to this investigation is the fact that within the p hotometric data of 86 spiral galaxies from de Jong & van der Kruit-from whi ch these claims were made-a trend exists between morphological type and bul ge profile shape. While late-type spiral bulges are described by an exponen tial luminosity profile, the early-type spiral bulges are better described by an r(1/2) or r(1/4) law. Taking the scale lengths from the best-fitting surface brightness profile models (i.e., either using exponential, r(1/2), or r(1/4) law profile parameters), we show that in all six passbands used ( BVRIHK), the early-type spiral galaxies have a larger r(e)/h ratio than lat e-type spiral galaxies. In contrast to this, fitting an exponential profile to the bulges of all spiral galaxies results in the mean r(e)/h ratio for the early-type spiral galaxies actually being smaller than the mean r(e)/h ratio for the late-type spiral galaxies (at the 3 sigma significance level using K-band data).