Contribution of polycyclic aromatic hydrocarbon ionization to neutral gas heating in galaxies: model versus observations

Abstract

The ionization of large carbonaceous molecules, i.e., polycyclic aromatic hydocarbons (PAHs), by ultraviolet (UV) photons from massive stars is expected to account for a large fraction of the heating of neutral gas in galaxies. Evaluation of this proposal, however, has been limited by our ability to directly compare observational diagnostics to the results of a molecular model describing PAH ionization. The objective of this article is to take advantage of the most recent values of molecular parameters derived from laboratory experiments and quantum chemical calculations on PAHs and provide a detailed comparison between modeled values and observational diagnostics for the PAH charge state and the heating efficiency for PAHs. Despite the use of a simple analytical model, we obtain a good agreement between model results and observational diagnostics over a wide range of radiation fields and physical conditions, in environments such as star-forming regions, galaxies, and protoplanetary disks. In addition, we found that the modeled photoelectric heating rates by PAHs are close to the observed cooling rates given by the gas emission. These results show that PAH ionization is the main source of neutral gas heating in these environments. The results of our photoelectric heating model by PAHs can thus be used to assess the contribution of UV radiative heating in galaxies (vs e.g. shock). We conclude on the importance of implementing the physics of PAH ionization in astrophysical codes, which are developed, for example, for the evaporating surfaces of protoplanetary disks, the diffuse interstellar medium, and the photodissociation regions associated with star-forming regions in the local and distant universe. We provide the empirical formulas to calculate the heating rates and heating efficiencies for PAHs in this article. The python code for the full model is available at https://gitlab.irap.omp.eu/COSMIC-PAH/photoelectric-heating/tree/master.

Photoionization of Nitrile-substituted Naphthalene and Benzene: Cation Spectroscopy, Photostability, and Implications for Photoelectric Gas Heating

“…Bakes & Tielens (1994) proposed theoretical models to estimate the contribution of nanograins and PAHs to the heating of interstellar gases depending on their sizes, typically up to 100 nm. Berné et al (2022) used a recent model considering new data from laboratory measurements on PAHs, and comparison of the model results with observational diagnostics yielded good agreement over a wide range of radiation fields, concluding that PAH ionization is the main source of neutral gas heating in regions where they are present abundantly and subjected to VUV photons, such as near starforming regions.…”

“…It is noteworthy to mention that the present 2D matrix can be exploited to obtain the KE of the photoelectrons emitted from PAHs due to any incoming ISRF and, in turn, can be used to estimate the heating effect in the ISM. So far, the majority of studies on the heating of gases have been based on theoretical predictions, and such experimental measurements can serve as important input for models like that of Berné et al (2022), providing accurate information for the contribution of PAHs in maintaining the temperature balance in the ISM.…”

Effect of protonation on the UV/VUV photostability of cyano-substituted anthracene and phenanthrene

“…The PAH-VUV interaction is a crucial process that partially governs the composition of the PAH population in space and thus the ensuing chemistry. Furthermore, the ionization of PAHs by VUV photons from massive stars is expected to account for a large fraction of the heating of neutral gas in galaxies and thus laboratory data on PAHs and any of their derivatives that are abundant in space are essential to test photoelectric heating models against observational diagnostics (Berné et al 2022). The substitution of PAHs by a CN functional group does not significantly alter the photoionization yield, considering that for similar and larger sizes, the bare radical cation PAHs presented photoionization yields ranging from 0.2 to 0.5 at 13.6 eV (Zhen et al 2016).…”

Evolution of the ionisation energy with the stepwise growth of chiral clusters of [4]helicene

“…PAHs, and hence helicenes, are strong UV-absorbers thus causing the heating of gas in the ISM, especially via their photoemission 20 . It has been suggested that through analysis of the relative intensities of specific mid-IR spectral features, it becomes feasible to trace the ionisation state of PAHs and thereby explore their radiative environment 9 .…”