JWST observations of photodissociation regions

Context. The northwest photodissociation region (PDR) of the NGC 7023 reflection nebula has been observed with JWST spectroscopy, revealing the carbonaceous emission bands at a high level of detail. Compared to other PDRs (Horsehead and Orion Bar) the softer radiation field driving NGC 7023 results in an extended atomic hydrogen region and more pronounced spatial variations in the band profiles. Its weaker thermal dust continuum makes NGC 7023 an ideal target for studying the 16–18 µm emission and its relation to the main emission bands at 3.3, 3.4, 5.2, 5.7, 6.2, 7.7, 8.6, 11.3, and 12.7 µm. Aims. We performed a spatially resolved study to reveal which emission bands originate from the same or cospatial populations of small carbonaceous emitters. Methods. We applied a spectral decomposition with PAHFIT to around 500 extracted spectra and produced maps of the individual subcomponents of the carbonaceous emission bands. The spatial resolution of around 0.7 mpc effectively resolved the dissociation front. Results. Nearly all feature maps peak at the dissociation front (DF1), while the emission in the atomic PDR region (ATM) varies strongly between the bands and their subcomponents. We organized the spatial distributions into three categories based on the intensity ratio in ATM relative to DF1. Most bands are of type I (low ATM/DF1; 3.3, 3.4, 5.2, 5.7, 11.3 µm) or II (medium ATM/DF1; 16.2, 7.7, 8.6, 12.7, 16.4 µm), while only few are of type III (high ATM/DF1; 11.0, 17.4 µm). The decompositions and maps of the 5.7, 7.7, 11.3, and 12.7 µm bands reveal that their bluer subcomponents are of type III, while their redder subcomponents are of type I or II. The 17.4 µm band correlates strongly with these blue subcomponents and the 8.6 and 11.0 µm features, which trace charged carriers. Conclusions. The wide range of 17.4 and 16.4 µm spatial distributions indicate at least two different populations contributing to the 16–18 µm range. The strongly differing spatial distribution types of carbonaceous band subcomponents reveal a connection between the 11.0 and 17.4 µm bands and the spectral profiles of the 5.7, 7.7, 11.3, and 12.7 µm bands. The maps indicate that these profiles will continue evolving as the central cavity is approached. Since C60 emission was previously detected in the cavity, we speculate that the population of emission carriers could be in an intermediate photochemical evolution stage of fullerene formation.