PHANGS–JWST First Results: Dust-embedded Star Clusters in NGC 7496 Selected via 3.3 μm PAH Emission

Abstract

The earliest stages of star formation occur enshrouded in dust and are not observable in the optical. Here we leverage the extraordinary new high-resolution infrared imaging from JWST to begin the study of dust-embedded star clusters in nearby galaxies throughout the Local Volume. We present a technique for identifying dust-embedded clusters in NGC 7496 (18.7 Mpc), the first galaxy to be observed by the PHANGS–JWST Cycle 1 Treasury Survey. We select sources that have strong 3.3 μm PAH emission based on a F300M − F335M color excess and identify 67 candidate embedded clusters. Only eight of these are found in the PHANGS-HST optically selected cluster catalog, and all are young (six have SED fit ages of ∼1 Myr). We find that this sample of embedded cluster candidates may significantly increase the census of young clusters in NGC 7496 from the PHANGS-HST catalog; the number of clusters younger than ∼2 Myr could be increased by a factor of 2. Candidates are preferentially located in dust lanes and are coincident with the peaks in the PHANGS-ALMA CO (2–1) maps. We take a first look at concentration indices, luminosity functions, SEDs spanning from 2700 Å to 21 μm, and stellar masses (estimated to be between ∼104 and 105 M ⊙). The methods tested here provide a basis for future work to derive accurate constraints on the physical properties of embedded clusters, characterize the completeness of cluster samples, and expand analysis to all 19 galaxies in the PHANGS–JWST sample, which will enable basic unsolved problems in star formation and cluster evolution to be addressed.

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

“…In addition, the 3.3 μm feature can be mapped with NIRCam at 2-3 times finer angular resolution than the 7.7 μm or 11.3 μm bands, yielding 5-10 pc resolution in our targets. This allows measurements of the sizes of H II regions and bubbles (see Barnes et al 2023;Watkins et al 2023), the identification of filamentary structure (Meidt et al 2023;Thilker et al 2023), the identification of embedded clusters (Rodriguez et al 2023), and potentially tracing the gas column at higher resolution than is routinely possible with any millimeter or radio facilities (Leroy et al 2023;Sandstrom et al 2023).…”

“…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).…”

PHANGS–JWST First Results: The 21 μm Compact Source Population

“…Here we focus on identifying lines of sight with bright PAH emission by contrasting the F335M band, which covers the 3.35 μm PAH feature, against the more continuum-dominated nearby F330M and F360M. For r 3.0 ≡ f F335M /f F300M and r 3.6 ≡ f F335M /f F360M , the regions with a significant PAH feature show (2023b; see also Rodriguez et al 2023).…”

“…We aim to understand the nature of the compact 21 μm sources seen in the JWST imaging data, extending the studies from the Local Group to a more diverse set of star-forming environments and determining if these sources include a set of truly embedded star-forming regions. But even with the 0 67 FWHM of the 21 μm filter with JWST, these maps are not resolving individual stars or even stellar clusters (e.g., Rodriguez et al 2023). We thus face the challenge of how to identify and extract a uniform set of regions.…”

PHANGS–JWST First Results: A Statistical View on Bubble Evolution in NGC 628

“…MIR bands, such as the Mid-Infrared Instrument's (MIRI) F770W, trace hot dust heated by young stars and polycyclic aromatic hydrocarbons (PAHs), where PAHs are vibrationally excited in the presence of starlight (Sandstrom et al 2023), especially when illuminated by UV photons. Therefore MIR observations allow us to identify new, young embedded clusters obscured at optical wavelengths (Rodriguez et al 2023), large-scale filamentary structures containing dense, cold gas expected to host future star formation (Thilker et al 2023), and hot dust emission shining in the presence of UV radiation emitted by OB stars (Leroy et al 2023). Piecing these results together provides the observations needed to trace recent star formation histories within these galaxies (Kim et al 2023).…”