What determines the grain size distribution in galaxies?

Dust in galaxies forms and evolves by various processes, and these dust processes change the grain size distribution and amount of dust in the interstellar medium (ISM). We construct a dust evolution model taking into account the grain size distribution, and investigate what kind of dust processes determine the grain size distribution at each stage of galaxy evolution. In addition to the dust production by Type II supernovae (SNe II) and asymptotic giant branch (AGB) stars, we consider three processes in the ISM: (i) dust destruction by SN shocks, (ii) metal accretion on to the surface of pre-existing grains in the cold neutral medium (CNM; called grain growth) and (iii) grain-grain collisions (shattering and coagulation) in the warm neutral medium and CNM. We found that the grain size distribution in galaxies is controlled by stellar sources in the early stage of galaxy evolution, and that afterwards the main processes that govern the size distribution changes to those in the ISM, and this change occurs at earlier stage of galaxy evolution for a shorter star formation time-scale (for star formation time-scales = 0.5, 5 and 50 Gyr, the change occurs about galactic age t similar to 0.6, 2 and 5 Gyr, respectively). If we only take into account the processes which directly affect the total dust mass (dust production by SNe II and AGB stars, dust destruction by SN shocks and grain growth), the grain size distribution is biased to large grains (a similar to 0.2-0.5 mu m, where a is the grain radius). Therefore, shattering is crucial to produce small (a less than or similar to 0.01 mu m) grains. Since shattering produces a large abundance of small grains (consequently, the surface-to-volume ratio of grains increases), it enhances the efficiency of grain growth, contributing to the significant increase of the total dust mass. Grain growth creates a large bump in the grain size distribution around a similar to 0.01 mu m. Coagulation occurs effectively after the number of small grains is enhanced by shattering, and the grain size distribution is deformed to have a bump at a similar to 0.03-0.05 mu m at t similar to 10 Gyr. We conclude that the evolutions of the total dust mass and the grain size distribution in galaxies are closely related to each other, and the grain size distribution changes considerably through the galaxy evolution because the dominant dust processes which regulate the grain size distribution change.