Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps I: Catalog and simple diagnostics

TL;DR

This paper catalogs PAH emission features in galactic HII regions and PDRs using Spitzer/IRS spectral maps, revealing distinct source groups and unique cases influenced by local radiation fields and environmental conditions.

Contribution

It provides detailed spectral mapping data and analysis of PAH features, highlighting differences between HII regions and reflection nebulae, and identifying unique cases with environmental influences.

Findings

Two main source groups: HII regions and reflection nebulae.

Distinct PAH emission correlation patterns for different sources.

Unique spectral behaviors in NGC 7023 and Horsehead PDR due to environmental factors.

Abstract

We present a sample of resolved galactic HII regions and photodissociation regions (PDRs) observed with the Spitzer infrared spectrograph (IRS) in spectral mapping mode between the wavelengths of 5--15 $\mu$m. For each object we have spectral maps at a spatial resolution of $\sim$4" in which we have measured all of the mid-infrared emission and absorption features. These include the PAH emission bands, primarily at 6.2, 7.7, 8.6, 11.2 and 12.7 $\mu$m, as well as the spectral emission lines of neon and sulfur and the absorption band caused by silicate dust at around 9.8 $\mu$m. In this work we describe the data in detail, including the data reduction and measurement strategies, and subsequently present the PAH emission band intensity correlations for each of the objects and the sample as a whole. We find that there are distinct differences between the sources in the sample, with two main groups, the first comprising the HII regions and the second the reflection nebulae (RNe). Three sources, the reflection nebula NGC~7023, the Horsehead nebula PDR (an interface between the HII region IC~434 and the Orion B molecular cloud) and M 17, resist this categorization, with the Horsehead PDR points mimicking the RNe and the NGC~7023 fluxes displaying unique bifurcated appearance in our correlation plots. These discrepancies seem to be due to the very low radiation field experienced by the Horsehead PDR and the very clean separation between the PDR environment and a diffuse environment in the NGC~7023 observations.