The Carnegie-Irvine Galaxy Survey. IX. Classification of Bulge Types and
  Statistical Properties of Pseudo Bulges
  We study the statistical properties of 320 bulges of disk galaxies in the
Carnegie-Irvine Galaxy Survey, using robust structural parameters of galaxies
derived from image fitting. We apply the Kormendy relation to classify
classical and pseudo bulges and characterize bulge dichotomy with respect to
bulge structural properties and physical properties of host galaxies. We
confirm previous findings that pseudo bulges on average have smaller S\'{e}rsic
indices, smaller bulge-to-total ratios, and fainter surface brightnesses when
compared with classical bulges. Our sizable sample statistically shows that
pseudo bulges are more intrinsically flattened than classical bulges. Pseudo
bulges are most frequent (incidence $\gtrsim 80\%$) in late-type spirals (later
than Sc). Our measurements support the picture in which pseudo bulges arose
from star formation induced by inflowing gas, while classical bulges were born
out of violent processes such as mergers and coalescence of clumps. We reveal
differences with the literature that warrant attention: (1) the bimodal
distribution of S\'{e}rsic indices presented by previous studies is not
reproduced in our study; (2) classical and pseudo bulges have similar relative
bulge sizes; and (3) the pseudo bulge fraction is considerably smaller in
early-type disks compared with previous studies based on one-dimensional
surface brightness profile fitting. We attribute the above differences to our
improved image quality, more robust bulge-to-disk decomposition technique, and
different classification criteria applied. Moreover, we find that barred
galaxies do not host more pseudo bulges or more prominent pseudo bulges than
unbarred galaxies. Various implications of these findings are discussed.
