Edge-on disk galaxies in the SDSS DR6: Fractions of bulgeless and other
  disk galaxies
  The aim of this study is to determine the fractions of different spiral
galaxy types, especially bulgeless disks, from a complete and homogeneous
sample of 15127 edge-on disk galaxies extracted from the sixth data release
from the Sloan Digital Sky Survey. The sample is divided in broad morphological
classes and sub types consisting of galaxies with bulges, intermediate types
and galaxies which appear bulgeless. A small fraction of disky irregulars is
also detected. The morphological separation is based on automated
classification criteria which resemble the bulge sizes and the flatness of the
disks. Each of these broad classes contains about 1/3 of the total sample.
Using strict criteria for selecting pure bulgeless galaxies leads to a fraction
of 15% of simple disk galaxies. We compare this fraction to other galaxy
catalogs and find an excellent agreement of the observed frequency of bulgeless
galaxies. Although the fraction of simple disk galaxies in this study does not
represent a cosmic fraction of bulgeless galaxies, it shows that the relative
abundance of pure disks is comparable to other studies and offers a profound
value of the frequency of simple disks in the local Universe. This fraction of
simple disks emphasizes the challenge for formation and evolution models of
disk galaxies since these models are hard pressed to explain the observed
frequency of these objects.
