Secular evolution and the assembly of bulges
  Bulges are of different types, morphologies and kinematics, from
pseudo-bulges, close to disk properties (Sersic index, rotation fraction,
flatenning), to classical de Vaucouleurs bulges, close to elliptical galaxies.
Secular evolution and bar development can give rise to pseudo-bulges. To ensure
prolonged secular evolution, gas flows are required along the galaxy life-time.
There is growing evidence for cold gas accretion around spiral galaxies. This
can explain the bar cycle of destruction and reformation, together with
pseudo-bulge formation. However, bulges can also be formed through major
mergers, minor mergers, and massive clumps early in the galaxy evolution. Bulge
formation is so efficient that it is difficult to explain the presence of
bulgeless galaxies today.
