PHANGS–JWST First Results: The Influence of Stellar Clusters on Polycyclic Aromatic Hydrocarbons in Nearby Galaxies

Abstract

We present a comparison of theoretical predictions of dust continuum and polycyclic aromatic hydrocarbon (PAH) emission with new JWST observations in three nearby galaxies: NGC 628, NGC 1365, and NGC 7496. Our analysis focuses on a total of 1063 compact stellar clusters and 2654 stellar associations previously characterized by the Hubble Space Telescope in the three galaxies. We find that the distributions and trends in the observed PAH-focused infrared colors generally agree with theoretical expectations, and that the bulk of the observations is more aligned with models of larger, ionized PAHs. These JWST data usher in a new era of probing interstellar dust and studying how the intense radiation fields near stellar clusters and associations play a role in shaping the physical properties of PAHs.

PHANGS–JWST First Results: Mapping the 3.3 μm Polycyclic Aromatic Hydrocarbon Vibrational Band in Nearby Galaxies with NIRCam Medium Bands

“…Our observed ratios are generally consistent with expectations from the Draine et al (2021) models for PAH size distributions shifted toward larger grains and charge distributions with either "high" or "standard" ionization. This is in agreement with the results from Dale et al (2023) who also find the best alignment with the "large" and "high" ionization results from the Draine et al (2021) model grid, although they use much younger stellar population ages to represent the environments of embedded clusters.…”

“…Our results show variations of the ratio between 0.02 and 0.08, with an average value of ∼0.05 across the three targets. These ratios are investigated in more detail in Chastenet et al (2023) and Dale et al (2023). For comparison, we plot two ratios from the Draine et al (2021) models.…”

“…For comparison, we plot two ratios from the Draine et al (2021) models. These values are derived by filter convolutions of the models as described in Dale et al (2023). We use the results for a stellar population age of 1 Gyr and representative size ("small," "standard," and "large") and charge distributions ("low," "standard," and "high").…”

“…These targets have deep, high-resolution, ancillary information from the Atacama Large Millimeter/submillimeter Array (Leroy et al 2021), Very Large Telescope-MUSE (Emsellem et al 2022), Hubble (Lee et al 2022), AstroSat (H. Hassani et al 2023, in preparation), and more. Combining NIRCam imaging of the 3.3 μm PAH feature with MIRI imaging of the 7.7 and 11.3 μm features enables one of the first studies of the variation of both PAH size and charge over large regions of nearby galaxies (Chastenet et al 2023), in HII regions (Egorov et al 2023), and near young star clusters and associations (Dale et al 2023;Rodriguez et al 2023). As part of this effort, we found it was necessary to adjust the current best NIRCam medium-band continuum removal prescription in the literature from Lai et al (2020) to account for PAHemission (or related dust-emission) contamination of F360M filter (Section 3).…”

Spectral Energy Distributions for 258 Local Volume Galaxies

“…Previous studies of PAH abundances have shown that the strength of PAH emission is sensitive to the hardness of the interstellar radiation field, the metal abundance, molecular gas surface density, star-formation activity level (star-formation rate, dust temperature, etc. ), or some combination thereof (e.g., Engelbracht et al 2005;Madden et al 2006;Wu et al 2006;Draine et al 2007;Engelbracht et al 2008;Dale et al 2009;Sandstrom et al 2010;Wu et al 2010;Rémy-Ruyer et al 2015;Chastenet et al 2019;Galliano et al 2021;Dale et al 2023). The LVL sample is a useful testbed for examining how dwarf galaxies and their generally lower metallicities and harder radiation fields affect PAHs.…”