Abstract. This brief paper is an extended abstract adapted from the introduction to the JWST Physics at High Angular Resolution in Nearby Galaxies (PHANGS) JWST February 2023 Focus Issue for ApJ Letters[1] and the PHANGS-JWST survey paper.[2]
JWST is revolutionizing fields across astrophysics, and chief among them is the study of the earliest stages of star formation and the dusty interstellar medium (ISM). Mid-infrared observations from space observatories, including IRAS, ISO, Spitzer and WISE, have been key to building our understanding of star formation during the phase when it is concealed beneath a shroud of dust that blocks the passage of visible light. JWST’s order of magnitude leap in sensitivity and resolution are finally allowing us to step beyond the Local Group, and observe young embedded stellar populations and the ISM at 10-100 parsec scales across environments characterized by physical conditions not found locally. Combined with observations across the electromagnetic spectrum that capture all major stages of the star formation cycle, we can follow the progression of star formation – from molecular clouds to embedded and unembedded stellar populations – to provide new constraints on the process of star formation to inform theoretical models.
In the first year of science observations with JWST, we have carried out a Treasury survey to obtain 8-band imaging from 2 to 21 μm of a diverse sample of 19 nearby (D < 20 Mpc) massive star-forming galaxies. This survey is conducted in the larger context of the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) program, which aims to study the multiscale processes of galaxy evolution, star formation, and stellar feedback by combining high-resolution panchromatic observations with the latest theoretical models. A rich set of data has already been obtained for each PHANGS-JWST target including CO(2–1) mapping (molecular gas), optical integral field spectroscopy (ionized gas and stellar populations), and high-resolution UV-optical imaging (star clusters and associations) through large programs with ALMA, VLT/MUSE, and Hubble, respectively. Our goals are to measure the timescales and efficiencies of the earliest phases of star formation and feedback, build an empirical model of the dependence of small dust grain properties on local ISM conditions, and test our understanding of how dust-reprocessed starlight traces star formation activity and gas, all across a diversity of galactic environments. As a Treasury program, the data have no exclusive access period, and we are providing higher level science products[3] for the full PHANGS JWST-ALMA-MUSE-HST datasets to maximize community science with the survey.
The JWST images of the galaxies in the PHANGS survey are extraordinary, and mind-blowing even for researchers who have studied the same galaxies for decades. The data have been on broad display, and the level of attention that the data have received took many of us by surprise. The STScI Office of Public Outreach reported that four days after the January 29 2024 press release,[4] media coverage was just over 1,000 articles with a potential reach of 4 billion. The images are not only aesthetically stunning, they also vividly illustrate the physics of star formation, stellar feedback, and the ISM. The composite mid-IR images (Figure 1) reveal how gas and dust in spiral galaxies are shaped by large-scale gas flows, star formation, and stellar feedback. Just as striking is where the dust is not seen; shells and bubble-like features, likely carved out by stellar feedback in the form of stellar winds, radiation, and supernova explosions, are ubiquitous. Star-forming regions appear as bright compact IR sources, due to heating by dust-enshrouded young clusters, within a larger diffuse filamentary network of dust. The images and data seem have taken a life of their own.
An initial set of 21 “First Results” papers based on the four PHANGS galaxies observed in the first two months of JWST science operations (NGC 628, NGC 1365, NGC 7496, and IC 5332) was published for a Focus Issue for the Astrophysical Journal Letters. The papers presented a broad range of results, ranging from the discovery of embedded star cluster populations and filamentary dust structures down to the smallest scales probed; a census of feedback-driven bubbles and shells; calibration of the dust emission as a gas tracer; and new insights into how the properties of small dust grains (polycyclic aromatic hydrocarbons, PAHs) depend on local ISM conditions.
In particular, work with JWST to study the prevalence of the youngest invisible star clusters (Figure 3) builds on the PHANGS-Hubble effort which has recently released the largest census to-date of ~100,000 star clusters and stellar associations.[5] To search for embedded clusters, we have taken advantage of NIRCam imaging at 3.3 μm using the F335M filter, which is the highest resolution, dust emission dominated imaging filter aboard JWST. The filter captures polycyclic emission from small neutral aromatic hydrocarbons (PAH), and we have demonstrated that 3.3 μm PAH emission is an effective tracer of the youngest dusty clusters (Rodriguez et al. 2023).
Through a visual search for bright compact sources in the F335M image of NGC 7496 which had little to no emission in the Hubble visible images, a small set of 12 dusty clusters was first identified to determine selection criteria which could be used to select a larger sample.
All 12 sources show a F300M − F335M color excess, which indicates the presence of PAH emission (Figure 4). Application of a 300M − F335M color excess criterion results in a total of 67 candidate embedded clusters in NGC 7496. Cross-correlation with the PHANGS-Hubble cluster census shows that only eight of these are found in the Hubble catalog, and all are young (six have SED fit ages of ∼1 Myr). This suggests that studies with JWST 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 shown to be preferentially located in dust lanes and are coincident with the peaks in the PHANGS-ALMA CO (2–1) maps.
Initial results from expansion of the study to all 19 galaxies in Figure 1 appears to confirm these results. Constraints on the timescale of the dust embedded phase (and the 3.3 μm PAH emission) can be inferred and appears to be less than 2-3 Myr. If confirmed, such a timescale implies that pre-supernova feedback plays a central role to clear natal gas and dust from embedded populations. Next steps include investigation of the completeness of embedded cluster samples based on 3.3 μm PAH emission, which will also provide insight into the processes that heat and destroy small neutral PAHs.
References
Hassani, H. et al. 2023, ApJ, L944, 21.
Lee, J.C. et al. 2023, ApJ, 944, L17.
Rodriguez, M.J. et al. 2023, ApJ, 944, L26.
Thilker, D. et al. 2023, ApJ, 944, L13.
Watkins, E. et al. 2023, ApJ, 944, L24.
Williams, T. et al. 2024, arXiv:2401.15142.
[1] https://iopscience.iop.org/collections/2041-8205_PHANGS-JWST-First-Results
[2] https://ui.adsabs.harvard.edu/abs/2023ApJ...944L..17L/abstract
[3] https://archive.stsci.edu/hlsp/phangs
[4] https://webbtelescope.org/contents/news-releases/2024/news-2024-105
[5] https://archive.stsci.edu/hlsp/phangs/phangs-cat
[6] https://webbtelescope.org/contents/media/images/2024/105/01HM9KGGP1EWFFSRRSKR8NZGWZ