Observational signatures of the dust size evolution in isolated galaxy simulations

We aim to provide observational signatures of the dust size evolution in the ISM, in particular exploring indicators of polycyclic aromatic hydrocarbon (PAH) mass fraction (q_PAH) defined as the mass fraction of PAHs relative to total dust grains. Additionally, we validate our dust evolution model by comparing the observational signatures from our simulations to those from observations. We model the evolution of grain size distribution of Milky Way-like and NGC 628-like galaxies representing star-forming galaxies with a hydrodynamic simulation code, GADGET4-OSAKA, which considers dust production and interstellar processing. Furthermore, we perform post-processing dust radiative transfer with SKIRT based on the simulations to predict the observational properties. We find that the intensity ratio between 8 um and 24 um correlates with q_PAH and can be used as an indicator of PAH mass fraction. However, this ratio is influenced by the radiation field. As another indicator, we suggest the 8 um-to-total infrared intensity ratio (ν I_ν(8 μ m)/I(TIR)), which tightly correlates with q_PAH. Furthermore, we explore the spatial evolution of q_PAH in the simulated Milky Way-like galaxy using ν I_ν(8 μ m)/I(TIR). We find that the spatially resolved q_PAH increases with metallicity at lower metallicity (Z<0.2 Zsun) due to the interplay between accretion and shattering while it decreases with metallicity at higher metallicity (Z>0.2 Zsun) due to coagulation. Finally, we compare the above indicators in the NGC 628-like simulation with those observed in NGC 628. Consequently, our simulation underestimates the PAH mass fraction throughout the entire galaxy. This is probably because PAH is too efficiently lost by coagulation in the interstellar medium in our model, which suggests that the inhibition of coagulation of the PAHs is key to enhancing PAH formation.