A Multiscale Study of Polycyclic Aromatic Hydrocarbon Properties in Galaxies

TL;DR

This study analyzes mid-IR spectra of galaxies and Galactic regions to understand PAH properties, revealing their universality and the influence of ionization, with implications for star formation and interstellar medium modeling.

Contribution

It provides a comprehensive analysis of PAH feature variations across different spatial scales and environments, highlighting the role of ionization over size distribution or extinction effects.

Findings

PAH properties are remarkably universal across the sample.

Variations in band ratios are mainly controlled by PAH ionization.

Empirical relation between I(6.2)/I(11.3) ratio and G0/ne ratio.

Abstract

In the present contribution, I summarize a systematic study of ISO and Spitzer mid-IR spectra of Galactic regions and star forming galaxies. This study quantifies the relative variations of the main aromatic features inside spatially resolved objects as well as among the integrated spectra of 50 objects. Our analysis implies that the properties of the PAHs are remarkably universal throughout our sample and at different spatial scales. In addition, the relative variations of the band ratios, as large as one order of magnitude, are mainly controled by the fraction of ionized PAHs. In particular, I show that we can rule out both the modification of the PAH size distribution and the mid-IR extinction, as an explanation of these variations. High values of the I(6.2)/I(11.3) ratio are found to be associated with the far-UV illuminated surface of PDRs, at the scale of an interstellar cloud, and associated with star formation activity, at the scale of a galaxy. Using a few well-studied Galactic regions, we provide an empirical relation between the I(6.2)/I(11.3) ratio and the ionization/recombination ratio G0/ne. Finally, I show that these trends are consistent with the detailed modeling of the PAH emission within photodissociation regions, taking into account the radiative transfer, the stochastic heating and the charge exchange between gas and dust.