Shiau-Jie Rau (Department of Physics, National Tsing Hua University, Hsinchu 30043, Taiwan); Hiroyuki Hirashita (Institute of Astronomy and Astrophysics, Academia Sinica, Astronomy-Mathematics Building, AS/NTU, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan)
Polycyclic aromatic hydrocarbons (PAHs) are considered to show prominent emission spectral features in the mid-infrared, and they could be a useful tracer of galaxy evolutionary properties. In order to clarify the origin and evolution of PAH abundance in galaxies, we model the evolution of grain size distribution and regard PAHs as small carbonaceous grains. The evolution of grain size distribution is calculated in a manner consistent with the physical condition of the interstellar medium by post-processing the hydrodynamical simulation of an isolated disc galaxy. We also differentiate between aromatic and non-aromatic grains for carbonaceous dust species and explicitly considered the aromatization reaction. As a consequence, we find that the observed trend of the PAH abundance in nearby galaxies along the metallicity is consistently understood together with the grain size distribution in galaxy evolution. The fast aromatization guarantees that the small carbonaceous grains trace PAHs very well. Since small grain abundance is increased by the interplay between grain disruption by shattering and dust growth by accretion, it is essential to consider the evolution of grain size distribution in interpreting the PAH abundance in nearby galaxies.