Evolution of 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 Type II supernovae and asymptotic giant branch stars, dust destruction by supernova shocks in the interstellar medium, 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, producing a large abundance of small grains (a ≲ 0.01 μm). Then, grain growth becomes effective at small grain radii, leading to the formation of a bump at a ∼ 10− 3–10− 2 μm on the grain-size distribution. Consequently, the extinction curve at ultraviolet (UV) wavelengths becomes steep, and a bump at 1/λ ∼ 4.5 μm− 1 (where λ is the 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, the interplay between shattering and coagulation could be important in reproducing the Milky Way extinction curve.