  Spiral galaxies comprise three major structural components; thin discs, thick
discs, and central bulges. Relative dominance of these components is known to
correlate with the total mass of the galaxy, and produces a remarkable
morphological variety of spiral galaxies. Although there are many formation
scenarios regarding individual components, no unified theory exists which
explains this systematic variation. The cold-flow hypothesis predicts that
galaxies grow by accretion of cold gas from cosmic webs (cold accretion) when
their mass is below a certain threshold, whereas in the high-mass regime the
gas that entered the dark matter halo is first heated by shock waves to high
temperatures and then accretes to the forming galaxy as it cools emitting
radiation (cooling flow). This hypothesis also predicts that massive galaxies
at high redshifts have a hybrid accretion structure in which filaments of cold
inflowing gas penetrate surrounding hot gas. In the case of Milky Way, the
previous study suggested that the cold accretion created its thick disc in
early times and the cooling flow formed the thin disc in later epochs. Here we
report that extending this idea to galaxies with various masses and associating
the hybrid accretion with the formation of bulges reproduces the observed
mass-dependent structures of spiral galaxies: namely, more massive galaxies
have lower thick disc mass fractions and higher bulge mass fractions. The
proposed scenario predicts that thick discs are older in age and poorer in iron
than thin discs, the trend observed in the Milky Way (MW) and other spiral
galaxies.
