Formation History of Polycyclic Aromatic Hydrocarbons in Galaxies
  Polycyclic aromatic hydrocarbons (PAHs) are some of the major dust components
in the interstellar medium (ISM). We present our evolution models for the
abundance of PAHs in the ISM on a galaxy-evolution timescale. We consider
shattering of carbonaceous dust grains in interstellar turbulence as the
formation mechanism of PAHs while the PAH abundance can be reduced by
coagulation onto dust grains, destruction by supernova shocks, and
incorporation into stars. We implement these processes in a one-zone chemical
evolution model to obtain the evolution of the PAH abundance in a galaxy. We
find that PAH formation becomes accelerated above certain metallicity where
shattering becomes efficient. For PAH destruction, while supernova shock is the
primary mechanism in the metal-poor environment, coagulation is dominant in the
metal-rich environment. We compare the evolution of the PAH abundances in our
models with observed abundances in galaxies with a wide metallicity range. Our
models reproduce both the paucity of PAH detection in low metallicity galaxies
and the metallicity-dependence of the PAH abundance in high-metallicity
galaxies. The strong metallicity dependence of PAH abundance appears as a
result of the strong metallicity dependence of the dust mass increase by the
accretion of metals onto dust grains, which are eventually shattered into PAHs.
We conclude that the observational trend of the PAH abundance can be a natural
consequence of shattering of carbonaceous grains being the source of PAHs. To
establish our scenario of PAH formation, observational evidence of PAH
formation by shattering would be crucial.
