Contribution of Polycyclic Aromatic Hydrocarbon Ionization To Neutral Gas Heating In Galaxies

By Keith Cowing
Press Release
August 19, 2022
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Contribution of Polycyclic Aromatic Hydrocarbon Ionization To Neutral Gas Heating In Galaxies
Emission maps of gas cooling. Maps on the upper row show the [C ii] line at 158 µm, as well as the [O i] lines at 145 µm and 63 µm, which were observed with Herschel-PACS at a resolution of 11”, 8.8”, and 4.5” respectively (Bernard-Salas et al. 2015). Maps on the lower row show the emission from the H2 rotational S(1), S(2) et S(3) lines at 17.035, 12.278, and 9.7 µm, respectively. These maps were extracted from the Spitzer archival data using CUBISM Smith et al. (2007a). The S(2) and S(3) maps are from the SL module of IRS (resolution of 3.6”), and the S(1) map is from the IRS LL module (resolution of 10”). Emission from the H2 v=1-0 S(1) line at 2.121 µm is presented in contours as a spatial reference for the dissociation front, but it is not included in the cooling budget, since this line is pumped by UV photons and hence does not cool the gas.

The ionization of polycyclic aromatic hydrocarbons (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.

ronments. The results of our photoelectric heating model by PAHs can thus be used to assess the contribution of UV radiative heating in galaxies (vs shocks, for instance). We provide the empirical formulas fitted to the model results, and the full python code itself, to calculate the heating rates and heating efficiencies for PAHs.

O. Berné, S. Foschino, F. Jalabert, C. Joblin

Comments: Accepted for publication in Astronomy and Astrophysics
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2208.08762 [astro-ph.GA] (or arXiv:2208.08762v1 [astro-ph.GA] for this version)
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Submission history
From: Olivier Berne
[v1] Thu, 18 Aug 2022 10:45:00 UTC (2,255 KB)
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Astrobiology, Astrochemistry,

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