Characterizing Spatial Variations of PAH Emission in the Reflection Nebula NGC 1333

TL;DR

This study investigates how polycyclic aromatic hydrocarbons (PAHs) emit infrared radiation in the reflection nebula NGC 1333, revealing their spatial variation and dependence on environmental conditions using multi-instrument observations and modeling.

Contribution

It provides a detailed analysis of PAH spectral variations and their relation to physical conditions in a PDR, using combined observations from SOFIA, Herschel, and Spitzer.

Findings

Distinct spatial characteristics for PAH spectral components.

PAH emission varies between irradiated PDR and diffuse regions.

Relationships identified between PAH ratios and FUV radiation field strength.

Abstract

Infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 $\mu$m, attributed to polycyclic aromatic hydrocarbons, show variations in relative intensity, shape, and peak position. These variations depend on the physical conditions of the photodissociation region (PDR) in which strong PAH emission arises, but their relationship has yet to be fully quantified. We aim to better calibrate the response of PAH species to their environment using observations with matching apertures and spatial resolution. We present observations from the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) of the gas cooling lines [OI] 63, 146 $\mu$m and [CII] 158 $\mu$m in the reflection nebula NGC 1333 and use archival dust continuum observations from the Photodetector Array Camera and Spectrometer (PACS) on board Herschel. We employ PDR modelling to derive the physical conditions and compare these with the characteristics of the PAH emission as observed with the Infrared Spectrometer (IRS) on board Spitzer. We find distinct spatial characteristics for the various PAH spectral components. We conclude that the ionic bands (6.2, 7.7, 8.6, and 11.0) and the 7-9 $\mu$m emission are due to multiple PAH sub-populations and that the plateaus are distinct from the features perched on top. The 6-9 $\mu$m PAH emission exhibit a significant change in behaviour between the irradiated PDR and diffuse outskirts, confirming these bands arise from multiple PAH sub-populations with different underlying molecular properties. We find multiple promising relationships between PAH ratios and the FUV radiation field strength but no clear correlations with the PAH ionization parameter.