Structure and origin of S0 galaxies. II - Disk-to-bulge ratios

Abstract

The major-axis luminosity profiles of 12 ordinary S0 galaxies are decomposed into an exponential disk and an R to the 1/4 law bulge via a technique developed by Kormendy. The values of disk central-surface brightness are evenly distributed between 20.1 and 22.1 muB, but do not cluster around the value of 21.65 muB as found by Freeman. It is suggested that disks rarely attain the luminosity of typical giant ellipticals, and that the distribution function for the masses of ellipticals is different from that for disks. A new method for obtaining disk-to-bulge (D/B) ratios by using the results of the decomposition procedure for the S0s shows D/B between 0.2 and 3.0, while a second new method which explicitly corrects for star formation and dust in the disks of spirals indicates that no S0 galaxies in this sample have D/B ratios as large as Sc spirals. Though not entirely inconsistent with the 'parallel sequence', the data presented best fit the 'intrinsic formation' theories of the origin of the S0 class of galaxies.