Abstract details

We have derived radial sky-subtracted surface brightness profiles of a sample of 40 face-on nearby early-type spiral galaxies, classified as unbarred, from SDSS data as well as our own observations (INT,NOT) at La Palma. The main objective was to derive the fraction of truncated ("type II") profiles. We find that only a small minority ~10% of the galaxies are truncated within the limits of the measured surface brightness range. The majority of the discs show a single unbroken exponential ("type I") brightness profile out to the edge of detectability, while a significant minority show "antitruncations", external profiles less steep than those of the internal disc. We apply a simple photometric test to show that the slopes of the inner profiles in these antitruncated ("type III") profiles match those of type I, while the outer parts are significantly shallower. We conclude that the modification which produces the antitruncations operates on the outer part of the profile and is not the result of secular concentration of stars in the inner parts of the galaxies concerned. A general profile classification scheme based on these and other recent related measurements is presented, and will be of use in probing disc formation scenarios. 1.Instituto de Astrofisica de Canarias, Spain 2.UNAM, Ensenada, Mexico. 3.Max Planck Insitut fuer Astrophysik, Garching, Germany. 4. Consejo Superior de Investigaciones Cientificas, Spain. 5.Dpto.de Astrofisica, U.La Laguna, Spain.