Formation of late-type spiral galaxies: gas return from stellar
  populations regulates disk destruction and bulge growth
  Spiral galaxies have most of their stellar mass in a large rotating disk, and
only a modest fraction in a central spheroidal bulge. This poses a major
challenge for cosmological models of galaxy formation. Galaxies form at the
centre of dark matter halos through a combination of hierarchical merging and
gas accretion along cold streams, and should rapidly grow their bulge through
mergers and instabilities. Cosmological simulations predict galaxies to have
most of their mass in the central bulge, and therefore an angular momentum much
below the observed level, except in dwarf galaxies. We propose that the
continuous return of fresh gas by stellar populations over cosmic times could
solve this issue. A population of stars formed at a given instant typically
returns half of its initial mass in the form of gas over 10 billion years, and
the process is not dominated by rapid supernovae explosions but by the
long-term mass-loss from low- and intermediate-mass stars. Using simulations of
galaxy formation, we show that this recycling of gas can strongly affect the
structural evolution of massive galaxies, potentially solving the bulge
fraction issue: we find that the bulge-to-disk ratio of a massive galaxy can be
divided by a factor of 3. The continuous recycling of baryons through star
formation and stellar mass loss helps the growth of disks and their survival to
interactions and mergers. Instead of forming only early-type,
spheroid-dominated galaxies (S0 and ellipticals), the standard cosmological
model can then successfully account for massive late-type, disk-dominated
spiral galaxies (Sb-Sc).
