[Paper Review] PAHs in Galaxies: their Properties and Evolution

F. Galliano|arXiv (Cornell University)|Oct 27, 2006

Astrophysics and Star Formation Studies1 references3 citations

TL;DR

This paper demonstrates that the ratios of key 6.2, 7.7, 8.6, and 11.3 μm PAH emission bands are primarily governed by the ionization state of PAHs, making them robust diagnostics of interstellar physical conditions. It further shows that the low PAH emission in low-metallicity galaxies results from delayed injection of PAHs by long-lived AGB stars, not destruction or depletion, with PAH mass fractions tightly correlated to AGB carbon dust production rates.

ABSTRACT

I summarize the results of two recent studies, based on ISO and Spitzer mid-IR spectra of galaxies and Galactic regions, aimed at understanding the origins of the variations of the aromatic features among and inside galaxies. I show that the ratios between the most intense bands (6.2, 7.7, 8.6 and 11.3 microns) are principally sensitive to the charge of the molecules, and therefore represent a powerful diagnostic tool of the physical conditions inside the region where the emission is originating. Then, I show that the weakness of the aromatic bands, in low-metallicity environments, is a consequence of the delayed injection of their carriers, the Polycyclic Aromatic Hydrocarbons (PAHs), into the interstellar medium (ISM) of galaxies. Indeed, PAHs are believed to form in the envelopes of post-AGB stars, several hundreds of million years after the beginning of the star formation, when the system is already chemically evolved.

Motivation & Objective

• To understand the origin of variations in mid-IR aromatic emission features across galaxies and Galactic regions.
• To determine whether PAH band ratios trace physical conditions such as ionization or electron density.
• To resolve the long-standing puzzle of why PAHs are underabundant in low-metallicity galaxies.
• To test competing hypotheses—PAH destruction, depletion, or delayed injection—by modeling dust-to-gas mass ratios in galaxies with diverse metallicities.
• To establish a physical link between PAH emission properties and the evolutionary state of star-forming regions via AGB stellar contributions.

Proposed method

• Spectral decomposition of ISO and Spitzer mid-IR data using two conservative methods: one underestimating band wings and one allowing free band width variation.
• Band intensity measurements for 6.2, 7.7, 8.6, and 11.3 μm features in 35 nearby galaxies and Galactic regions (H II regions, PDRs, planetary nebulae).
• Use of stochastic heating models with realistic optical properties to simulate PAH emission and test hypotheses about band ratio variations.
• Correlation of observed band ratios with physical parameters such as $G_0/n_e \times \sqrt{T_e}$, a proxy for ionization balance in PDRs.
• SED modeling of UV-to-millimeter spectral energy distributions to derive dust-to-gas mass ratios for PAHs ($Z_{\text{PAH}}$) and far-IR dust ($Z_{\text{FIR dust}}$) as functions of ISM metallicity ($Z_{\text{ISM}}$).
• Comparison of observed $Z_{\text{PAH}}$ trends with theoretical dust production rates from AGB stars and massive stars to infer injection timescales.

Experimental results

Research questions

• RQ1 What physical conditions within interstellar regions are encoded in the relative intensities of the 6.2, 7.7, 8.6, and 11.3 μm PAH emission bands?
• RQ2 Why are PAH emission features significantly weaker in low-metallicity galaxies compared to solar-metallicity systems?
• RQ3 Is the low PAH abundance in low-metallicity galaxies due to destruction by hard radiation, depletion by shocks, or delayed injection from long-lived stars?
• RQ4 To what extent do the observed PAH band ratios correlate with the ionization state of PAHs rather than molecular size or extinction effects?
• RQ5 How does the evolution of PAH mass fractions in galaxies relate to the production rate of carbon dust by asymptotic giant branch (AGB) stars?

Key findings

• The ratios between the 6.2, 7.7, and 8.6 μm bands are remarkably constant across diverse galactic environments, indicating a universal PAH population with minimal size dependence.
• Variations in the 11.3 μm band ratio (especially 6.2/11.3) are primarily driven by the fraction of ionized PAHs, not by molecular size or extinction effects.
• The 6.2/11.3 band ratio correlates strongly with the physical parameter $G_0/n_e \times \sqrt{T_e}$, enabling it to serve as a diagnostic of ionization conditions in PDRs.
• The PAH-to-gas mass ratio ($Z_{\text{PAH}}$) is tightly correlated with the rate of carbon dust production by AGB stars, indicating that PAHs are injected into the ISM with a delay of several hundred million years after star formation begins.
• The observed low PAH abundance in low-metallicity galaxies ($Z_{\text{ISM}} < 0.1\,Z_\odot$) is best explained by delayed injection from AGB stars rather than by destruction or depletion mechanisms.
• The far-IR dust-to-gas mass ratio ($Z_{\text{FIR dust}}$) tracks the dust production rate from massive stars down to $0.1\,Z_\odot$, with discrepancies below this metallicity possibly due to cold dust or non-uniform star formation histories.