Structural diversity of disc galaxies originating in the cold gas inflow
  from cosmic webs
  Disc galaxies show a large morphological diversity with varying contribution
of three major structural components; thin discs, thick discs, and central
bulges. Dominance of bulges increases with the galaxy mass (Hubble sequence)
whereas thick discs are more prominent in lower mass galaxies. Because galaxies
grow with the accretion of matter, this observed variety should reflect
diversity in accretion history. On the basis of the prediction by the cold-flow
theory for galactic gas accretion and inspired by the results of previous
studies, we put a hypothesis that associates different accretion modes with
different components. Namely, thin discs form as the shock-heated hot gas in
high-mass halos gradually accretes to the central part, thick discs grow by the
direct accretion of cold gas from cosmic webs when the halo mass is low, and
finally bulges form by the inflow of cold gas through the shock-heated gas in
high-redshift massive halos. We show that this simple hypothesis reproduces the
mean observed variation of galaxy morphology with the galaxy mass. This
scenario also predicts that thick discs are older and poorer in metals than
thin discs, in agreement with the currently available observational data.
