Linking Characteristics of the Polycyclic Aromatic Hydrocarbon Population with Galaxy Properties: A Quantitative Approach Using the NASA Ames PAH IR Spectroscopic Database

TL;DR

This study uses the NASA Ames PAH IR Spectroscopic Database to analyze PAH features in over 900 galaxy spectra, revealing correlations with galaxy properties and providing a spectral template library for future galaxy modeling.

Contribution

It introduces a quantitative method linking PAH characteristics with galaxy properties and offers a library of parameterized PAH spectra for galaxy spectral analysis.

Findings

PAH population in galaxies averages 55 carbon atoms.

Approximately 40% of PAHs are ionized.

Correlations found between PAH ratios and galaxy parameters.

Abstract

Utilizing the data and tools provided through the NASA Ames PAH IR Spectroscopic Database (PAHdb), we study the PAH component of over 900 Spitzer-IRS galaxy spectra. Employing a database-fitting approach, the average PAH size, the PAH size distribution, and PAH ionization fraction are deduced. In turn, we examine their connection with the properties of the host galaxy. We found that PAH population within galaxies consists of middle-sized PAHs with an average number of carbon atoms of $\bar{N_{C}}$ = 55, and a charge state distribution of $\sim$40% ionized - 60% neutral. We describe a correlation between the 6.2/11.2 $\mu$m PAH ratio with the ionization parameter ($\gamma\equiv(G_{0}/n_{\rm e})(T_{\rm gas} / 1\ \mathrm{K})^{0.5}$), a moderate correlation between the 8.6/11.2 $\mu$m PAH ratio and specific star-formation rate, and a weak anti-correlation between $\gamma$ and M$_{*}$. From the PAHdb decomposition we provide estimates for the 3.3 $\mu$m PAH band, not covered by Spitzer observations, and establish a correlation between the 3.3/11.2 $\mu$m PAH ratio with N$_{\mathrm{C}}$. We further deliver a library of mid-IR PAH template spectra parameterized on PAH size and ionization fraction, which can be used in galaxy spectral energy distribution fitting codes for the modeling of the mid-IR PAH emission component in galaxies.