The other side of Bulge Formation in a Lambda-CDM cosmology: Bulgeless
  Galaxies in the Local Universe
  We study the physical properties, formation histories, and environment of
galaxies without a significant "classical" spheroidal component, as predicted
by semi-analytical models of galaxy formation and evolution. This work is
complementary to the analysis presented in De Lucia et al., (2011), where we
focus on the relative contribution of various physical mechanisms responsible
for bulge assembly in a Lambda-CDM cosmology. We find that the fraction of
bulgeless galaxies is a strong decreasing function of stellar mass: they
represent a negligible fraction of the galaxy population with M* > 10^12 Msun,
but dominate at M* < 10^10 Msun. We find a clear dichotomy in this galaxy
population, between central galaxies of low-mass dark matter haloes, and
satellite galaxies in massive groups/clusters. We show that bulgeless galaxies
are relatively young systems, that assemble most of their mass at low-redshift,
but they can also host very old stellar populations. Since galaxy-galaxy
mergers are assumed to lead to the formation of a spheroidal component, in our
models these galaxies form preferentially in low-mass haloes that host a small
number of satellites galaxies. We show that the adopted modelling for galaxy
mergers represents a key ingredient in determining the actual number of
bulgeless galaxies. Our results show that these galaxies are not a rare
population in theoretical models: at z~0, galaxies with no classical bulge (but
often including galaxies with the equivalent of pseudo-bulges) account for up
to 14% of the galaxies with 10^11 < M*/Msun < 10^12.
