The abundance and emission of PAHs from the local to early Universe

Abstract

We present the first large-volume cosmological simulation of galaxy evolution with a model for the abundance and luminous emission of Polycyclic Aromatic Hydrocarbons (PAHs) across a wide range of cosmic time ($0 < z \lesssim 6$). The simulation is built on a semi-analytic model (SAM) that includes a treatment of dust size distribution, with PAHs being identified with the smallest carbonaceous grains ($< 15$ A) residing in the diffuse ISM of galaxies. PAH emission is computed by post-processing the simulation output with the radiative transfer code GRASIL. In our model, where stars only inject large grains, the PAH abundance emerges from the growth and shattering of dust grains in the ISM, naturally reproducing the observed correlation between the PAH mass fraction ($q_{\rm PAH}$) and both metallicity and specific star formation rate (sSFR). We derive a relation to infer $q_{\rm PAH}$ from $L_{\rm PAH}$, $L_{\rm IR}$, and sSFR, with a relatively small scatter (a factor of $\approx 1.3$). Simulated PAH emission correlates well with key galaxy properties such as SFR and molecular gas content, and reproduces observational constraints. Taken together, these results support the viability of a top-down formation scenario for PAHs. We extend our model predictions to higher redshifts, which can be exploited for future missions such as PRIMA. This work is a significant step toward leveraging PAH observations as a tool for studying galaxy evolution.