Evolution of the grain size distribution in galactic discs
  Dust is formed out of stellar material and is constantly affected by
different mechanisms occurring in the ISM. Dust grains behave differently under
these mechanisms depending on their sizes, and therefore the dust grain size
distribution also evolves as part of the dust evolution itself. Following how
the grain size distribution evolves is a difficult computing task that is just
recently being overtaking. Smoothed particle hydrodynamic (SPH) simulations of
a single galaxy as well as cosmological simulations are producing the first
predictions of the evolution of the dust grain size distribution. We compare
for the first time the evolution of the dust grain size distribution predicted
by the SPH simulations with the results provided by the observations. We
analyse how the radial distribution of the small to large grain mass ratio
(D(S)/D(L)) changes over the whole discs in three galaxies: M 101, NGC 628 and
M 33. We find good agreement between the observed radial distribution of
D(S)/D(L) and what is obtained from the SPH simulations of a single galaxy. The
central parts of NGC 628, at high metallicity and with a high molecular gas
fraction, are mainly affected not only by accretion but also by coagulation of
dust grains. The centre of M 33, having lower metallicity and lower molecular
gas fraction, presents an increase of D(S)/D(L), showing that shattering is
very effective in creating a large fraction of small grains. Observational
results provided by our galaxies confirm the general relations predicted by the
cosmological simulations based on the two grain size approximation. However, we
present evidence that the simulations could be overestimating the amount of
large grains in high massive galaxies.
