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

Abstract

We investigate recent claims that the Hubble sequence of spiral galaxies is scale-free. Fundamental to this investigation is the fact that within the photometric data of 86 spiral galaxies from de Jong & van der Kruit - from which these claims were made - a trend exists between morphological type and bulge profile shape. While late-type spiral bulges are described by an exponential 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 spirals have a larger r_e/h ratio than late-type spiral galaxies. In contrast to this, fitting an exponential profile to the bulges of all spirals 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).