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Abstract We investigate the emergence phase of young star clusters in the nearby spiral galaxy NGC 628. We use JWST NIRCam and MIRI observations to create spatially resolved maps of the Pa α 1.87 μ m and Br α 4.05 μ m hydrogen recombination lines, as well as 3.3 and 7.7 μ m emission from polycyclic aromatic hydrocarbons (PAHs). We extract 953 compact H ii regions and analyze the PAH emission and morphology at ∼10 pc scales in the associated photodissociation regions. While H ii regions remain compact, radial profiles help us to define three PAH morphological classes: compact (∼42%), extended (∼34%), and open (∼24%). The majority of compact and extended PAH morphologies are associated with very young star clusters (<5 Myr), while open PAH morphologies are mainly associated with star clusters older than 3 Myr. We observe a general decrease in the 3.3 and 7.7 μ m PAH band emission as a function of cluster age, while their ratio remains constant with age out to 10 Myr and morphological class. The recovered PAH 3.3 μ m /PAH 7.7 μ m ratio is lower than values reported in the literature for reference models that consider neutral and ionized PAH populations and analyses conducted at galactic physical scales. The 3.3 and 7.7 μ m bands are typically associated with neutral and ionized PAHs, respectively. While we expected neutral PAHs to be suppressed in proximity to an ionizing source, the constant PAH 3.3 μ m /PAH 7.7 μ m ratio would indicate that both families of molecules disrupt at similar rates in proximity to H ii regions.
Pedrini et al. (Thu,) studied this question.