# Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps II: A direct link between band profiles and the radiation field strength

## TL;DR

This study analyzes over 7000 mid-infrared spectra to link PAH emission band profiles with the local radiation field, revealing environmental effects on spectral features and a linear relation between specific band ratios and UV field strength.

## Contribution

It introduces a stable four-Gaussian component decomposition of the 7.7 μm PAH complex and establishes a direct link between band ratios and radiation field intensity across different interstellar environments.

## Key findings

- Four Gaussian components are stable across spectra.
- Band flux relationships depend on environment (HII regions vs. reflection nebulae).
- The ratio of two PAH components correlates linearly with UV field strength.

## Abstract

We decompose the observed 7.7 $\mu$m polycyclic aromatic hydrocarbon (PAH) emission complexes in a large sample of over 7000 mid-infrared spectra of the interstellar medium (ISM) using spectral cubes observed with the Spitzer/IRS-SL instrument. In order to fit the 7.7 $\mu$m PAH emission complex we invoke four Gaussian components which are found to be very stable in terms of their peak positions and widths across all of our spectra, and subsequently define a decomposition with fixed parameters which gives an acceptable fit for all the spectra. We see a strong environmental dependence on the inter-relationships between our band fluxes - in the HII regions all four components are inter-correlated, while in the reflection nebulae (RNe) the inner and outer pairs of bands correlate in the same manner as previously seen for NGC~2023. We show that this effect arises because the RNe maps are dominated by strongly irradiated PDR emission, while the much larger HII region maps are dominated by emission from regions much more distant from the exciting stars, leading to subtly different spectral behavior. Further investigation of this dichotomy reveals that the ratio of two of these components (centered at 7.6 and 7.8 $\mu$m) is linearly related to the UV field intensity (log G$_0$). We find that this relationship does not hold for sources consisting of circumstellar material, which are known to have variable 7.7 $\mu$m spectral profiles.