  Formation process(es) of galactic bulges are not yet clarified although
several mechanisms have been proposed. In a previous study, we suggested one
possibility that galactic bulges have been formed from the cold gas inflowing
through surrounding hot halo gas in massive dark matter halos at high
redshifts. It was shown that this scenario leads to the bulge-to-total stellar
mass ratio increasing with the galaxy mass, in agreement with the well-known
observed trend. We here indicate that it also reproduces recent observational
results that the mean stellar age of the bulge increases with the galaxy mass
while the age gradient across the bulge decreases. We infer that this formation
path applies mainly to high-mass galaxies and the bulges in lower-mass galaxies
have different origins such as secular formation from the disc material.
