Observational signatures of the dust size evolution in isolated galaxy simulations

Aims. We aim to provide observational signatures of the dust size evolution in the interstellar medium. In particular, we explore indicators of the polycyclic aromatic hydrocarbon (PAH) mass fraction (q_PAH), defined as the mass fraction of PAHs relative to the total dust grains. In addition, we validate our dust evolution model by comparing the observational signatures from our simulations to observations. Methods. We used the hydrodynamic simulation code, GADGET4-OSAKA to model the dust properties of Milky Way-like and NGC 628-like galaxies representing star-forming galaxies. This code incorporates the evolution of grain size distributions driven by dust production and interstellar processing. Furthermore, we performed post-processing dust radiative transfer calculations with SKIRT based on the hydrodynamic simulations to predict the observational properties of the simulations. Results. We find that the intensity ratio between 8 and 24 μm (I_ν(8 μm)/I_ν(24 μm)) is correlated with q_PAH and can be used as an indicator of the PAH mass fraction. However, this ratio is influenced by the local radiation field. We suggest the 8 μm-to-total infrared intensity ratio (νI_ν(8 μm)/I_TIR) as another indicator of the PAH mass fraction, since it is tightly correlated with the PAH mass fraction. Furthermore, we explored the spatially resolved evolutionary properties of the PAH mass fraction in the simulated Milky Way-like galaxy using νI_ν(8 μm)/I_TIR. We find that the spatially resolved PAH mass fraction increases with metallicity at Z ≲ 0.2 Z_⊙ due to the interplay between accretion and shattering, whereas it decreases at Z ≳ 0.2 Z_⊙ because of coagulation. Also, coagulation decreases the PAH mass fraction in regions with a high hydrogen surface density. Finally, we compared the above indicators in the NGC 628-like simulation with those observed in NGC 628 by Herschel, Spitzer, and JWST. Consequently, we find that our simulation underestimates the PAH mass fraction throughout the entire galaxy by a factor of ~8 on average. This could be due to the efficient loss of PAHs by coagulation in our model, suggesting that our treatment of PAHs in dense regions needs to be improved.