Evolution of grain size distribution with enhanced abundance of small carbonaceous grains in galactic environments

Abstract

We propose an updated dust evolution model that focuses on the grain size distribution in a galaxy. We treat the galaxy as a one-zone object and include five main processes (stellar dust production, dust destruction in supernova shocks, grain growth by accretion and coagulation, and grain disruption by shattering). In this paper, we improve the predictions related to small carbonaceous grains, which are responsible for the 2175 Å bump in the extinction curve and the polycyclic aromatic hydrocarbon (PAH) emission features in the dust emission spectral energy distribution (SED), both of which were underpredicted in our previous model. In the new model, we hypothesize that small carbonaceous grains are not involved in interstellar processing. This avoids small carbonaceous grains being lost by coagulation. We find that this hypothetical model shows a much better match to the Milky Way (MW) extinction curve and dust emission SED than the previous one. The following two additional modifications further make the fit to the MW dust emission SED better: (i) The chemical enrichment model is adjusted to give a nearly solar metallicity in the present epoch, and the fraction of metals available for dust growth is limited to half. (ii) Aromatization for small carbonaceous grains is efficient, so that the aromatic fraction is unity at grain radii ≲ 20 Å. As a consequence of our modelling, we succeed in obtaining a dust evolution model that explains the MW extinction curve and dust emission SED at the same time.

A Framework for Modeling Polycyclic Aromatic Hydrocarbon Emission in Galaxy Evolution Simulations

“…Major differences between the models are instead confined to the actual star formation history, metal enrichment (and stellar yields), and gas flows surrounding galaxies. Differences in the results between our model and those of Seok et al (2014), , Hirashita (2023) can therefore be ascribed to the galaxy formation model itself. This underscores the potential importance of bona fide cosmological simulations with PAH physics included.…”

“…This two-size model, or extensions that include a full-size distribution, has been implemented by a number of groups either on the fly in galaxy evolution evolution simulations, or applied in postprocessing (Aoyama et al 2018(Aoyama et al , 2020Gjergo et al 2018;McKinnon et al 2018;Hou et al 2019;Huang et al 2021;Li et al 2021;Romano et al 2022). Seok et al (2014), , Hirashita (2023) expanded on the aforementioned works, and developed a formulation for modeling silicates, aliphatics, and aromatic carbon species in models with multiple grain-size distributions. These authors assumed that the smallest aromatic grains were PAHs, and studied the q PAH -Z gas relationship in one-zone galaxy models, as well as the time evolution of PAH abundances.…”