Evolution of the extinction curves in galaxies

We investigate the evolution of extinction curves in galaxies based on our evolution model of grain size distribution. In this model, we considered various processes: dust formation by SNe II and AGB stars, dust destruction by SN shocks in the ISM, metal accretion onto the surface of grains (referred to as grain growth), shattering and coagulation. We find that the extinction curve is flat in the earliest stage of galaxy evolution. As the galaxy is enriched with dust, shattering becomes effective to produce a large abundance of small grains ($a \la 0.01\;\mu$m). Then, grain growth becomes effective at small grain radii, forming a bump at $a \sim 10^{-3}${--}$10^{-2}\;\mu$m on the grain size distribution. Consequently, the extinction curve at ultraviolet (UV) wavelengths becomes steep, and a bump at $1/\lambda \sim 4.5\;\mu{\rm m}^{-1}\;(\lambda: \mbox{wavelength})$ on the extinction curve becomes prominent. Once coagulation becomes effective, the extinction curves become flatter, but the UV extinction remains overproduced when compared with the Milky Way extinction curve. This discrepancy can be resolved by introducing a stronger contribution of coagulation. Consequently, an interplay between shattering and coagulation could be important to reproduce the Milky Way extinction curve.