arXiv subjects

Janice C. Lee

Publications and source records attributed to Janice C. Lee.

At least 19 recordsLinked to original sources

arXiv2026

The PHANGS-AstroSat Atlas of Nearby Star Forming Galaxies

We present the Physics at High Angular resolution in Nearby GalaxieS (PHANGS)-AstroSat atlas, which contains ultraviolet imaging of 31 nearby star-forming galaxies captured by the Ultraviolet Imaging Telescope (UVIT) on the AstroSat satellite. The atlas provides a homogeneous data set of far- and near-ultraviolet maps of galaxies within a distance of 22 Mpc and a median angular resolution of 1.4 arcseconds (corresponding to a physical scale between 25 and 160 pc). After subtracting a uniform ultraviolet background and accounting for Milky Way extinction, we compare our estimated flux densities to GALEX observations, finding good agreement. We find candidate extended UV disks around the galaxies NGC 6744 and IC 5332. We present the first statistical measurements of the clumping of the UV emission and compare it to the clumping of molecular gas traced with ALMA. We find that bars and spiral arms exhibit the highest degree of clumping, and the molecular gas is even more clumped than the FUV emission in galaxies. We investigate the variation of the ratio of observed FUV to H$α$ in different galactic environments and kpc-sized apertures. We report that $\sim 65$% variation of the $\log_{10}$(FUV/H$α$) can be described through a combination of dust attenuation with star formation history parameters. The PHANGS-AstroSat atlas enhances the multi-wavelength coverage of our sample, offering a detailed perspective on star formation. When integrated with PHANGS data sets from ALMA, VLT-MUSE, HST and JWST, it develops our comprehensive understanding of attenuation curves and dust attenuation in star-forming galaxies.

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arXiv2026

Dust-Embedded Star Formation: Bridging Magellanic Cloud Studies of Massive Young Stellar Objects to Nearby Spiral Galaxies

We use JWST NIRCam and MIRI imaging at 2, 4, 10, and 21 um to study young, dusty compact sources in four nearby galaxies at distances of ~ 1-5Mpc (M33, NGC300, NGC7793, and NGC5068). This work bridges well-characterized massive young stellar objects (MYSOs) in the Magellanic Clouds from the Spitzer SAGE survey to new studies of embedded clusters in more distant galaxies with JWST. Guided by the SAGE-LMC catalog, we define JWST color-magnitude selection criteria (F1000W versus F1000W-F2100W) and test them using resolution-degradation experiments. We identify 216, 32, 80 and 139 dusty young objects in the four galaxies, respectively. The selected population spans sources from systems dominated by a single MYSO to compact marginally resolved sources hosting multiple MYSOs. The color selection remains stable across 1-5 Mpc, and the 10um luminosity function retains a slope of alpha~ -2. However, blending and surface-brightness dilution remove fainter sources, leading to incompleteness of up to ~ 50% at 5.2 Mpc and biasing the sample toward brighter objects (F1000W < 19 mag). The sample spans approximate stellar masses of ~10-2 X 10^5 Mo. Spatial resolution affects the interpretation of mid-infrared emission: clustering increases the fraction of emission attributed to compact sources in active regions, while blending into diffuse emission dominates in quiescent environments. Comparisons with PAH-selected young clusters in the PHANGS galaxy NGC5068 show that our selection recovers ~ 80% of the PAH-selected sources. We show that the practical limit for studying individual MYSOs with JWST is ~3 Mpc. The resulting catalog provides a foundation for future resolved studies of star formation rates and early cluster evolution.

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arXiv2026

The Hidden Life of Stars: Embedded Beginnings to AGB Endings in the PHANGS-JWST Sample. I. Catalog of Mid-IR Sources

We present a multiwavelength catalog of mid-infrared-selected compact sources in 19 nearby galaxies, combining JWST NIRCam/MIRI, HST UV-optical broadband, H$α$ narrow-band, and ALMA CO observations. We detect 24,945 compact sources at 21 $μ$m and 55,581 at 10 $μ$m. Artificial star tests show 50% completeness limits of $\sim$5 $μ$Jy for the 10 $μ$m catalog, and $\sim$24 $μ$Jy for the 21 $μ$m catalog. We find that 21 $μ$m compact sources contribute $\sim$20% of the total galaxy emission in that band, but only contribute $5%$ at 10 $μ$m. We classify sources using stellar evolution and population synthesis models combined with empirical classifications derived from the literature. Our classifications include H$α$-bright and dust-embedded optically faint clusters, red supergiants (RSGs), oxygen-rich and carbon-rich AGB stars, and a range of rarer stellar types. In sampling a broad range of star forming environments with a uniform, well-characterized selection, this catalog enables enables analyses of infrared-bright stellar populations. We find that H$α$-faint sources account for only 10% of dusty (likely young) clusters, implying that the infrared-bright, optically-faint phase of cluster evolution is short compared to the H$α$-bright stage. The luminosity functions of 10 and 21 $μ$m sources follow power-law distributions, with the 21 $μ$m slope ($-1.7 \pm 0.1$) similar to that of giant molecular cloud mass functions and ultraviolet bright star-forming complexes, while the 10 $μ$m slope ($-2.0 \pm 0.1$) is closer to that of young stellar clusters.

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arXiv2026

To What Extent Are Star Cluster Ages Encoded in Their Environments? Exploring the Spatial Distribution of Age-Related Information with PHANGS-HST Imaging and Convolutional Neural Networks

The environments around star clusters evolve as stellar feedback reshapes the interstellar medium and dynamical processes reorganize the structure of the surrounding stellar field. As approximately single-age populations, star clusters can serve as clocks to trace these environmental changes. In this exploratory study, we test whether convolutional neural networks (CNNs) can identify age-dependent changes in cluster environments. We take cluster ages as given from basic SED fitting of five-band UV-optical aperture photometry from the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) HST survey. We first show that CNNs can be trained on image cutouts centered on clusters to recover ages directly from imaging. This demonstration provides the foundation for this study, which examines whether the information used by CNNs to predict age is coherent and physically meaningful. We perform controlled image occlusion experiments as an explainable AI method. These show that the CNNs extract age-predictive environmental cues in the absence of cluster light and when information on SED shape is removed by combining the five filters into one image. We find that reliance on environmental information increases at the youngest (<10 Myr) and oldest (>1 Gyr) ages, where clusters can exhibit similarly red colors. Our results are consistent with the long-recognized picture that cluster environments evolve systematically with age. We demonstrate that this information is encoded at a level detectable by machine-learning and recoverable from broadband imaging. This establishes a path for using new techniques to connect image-based age inference to the physical evolution of cluster environments.

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arXiv2026

Stellar associations powering HII regions $\unicode{x2013}$ II. Escape fraction of ionizing photons

Newly formed stars have a profound impact on their environment by depositing energy and momentum into the surrounding gas. However, only a fraction of the stellar feedback is retained in the cloud and observational constraints are needed to further our understanding of this process. In a sample of 19 nearby galaxies, we match HII regions from PHANGS$\unicode{x2013}$MUSE to their ionizing stellar source from PHANGS$\unicode{x2013}$HST and measure the percentage of ionizing radiation that is leaking into the surrounding diffuse ionized gas (DIG). Based on a catalogue, where each HII region is powered by a single young and massive stellar association, we measure a photon escape fraction of $f_\mathrm{esc}=82^{+12}_{-24}$ per cent. Comparable results are obtained when different procedures are used to match the ionized gas to its source. All samples we study contain a substantial fraction of objects (up to 20 per cent), where the stellar source is not sufficient to produce the H$α$ flux observed from the nebula. Many of them are probably related to uncertain age estimates, but we also find numerous regions, where a significant fraction of the ionizing photon budget is contributed by stars that reside outside the boundaries of the HII region. This motivates the use of an alternative galaxy-wide approach, in which we include all HII regions and stellar sources, not just the ones that show a clear overlap. When summing up the ionization budget over entire galaxies, we measure slightly lower, but consistent values.

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arXiv2025

Evolved Supergiants in PHANGS I: Red Supergiants in 19 Galaxies between 5-20 Mpc with HST and JWST

Red supergiants (RSGs) are important for our understanding of supernova progenitors, stellar populations, stellar evolution, mass loss and dust production. Extragalactic surveys of RSGs have a long history in the Local Group, but few studies exist beyond that due to the limited resolution and sensitivity of ground-based and previous space-based infrared observatories. Here we demonstrate the combined power of HST and JWST to push systematic searches of RSGs out to $\sim$20 Mpc. We introduce a catalog of 97057 RSGs -- the largest single-survey release of RSGs -- with masses $\gtrsim$10 M$_{\odot}$ in 19 galaxies from the PHANGS HST+JWST Treasury program. We use HST F814W and JWST F200W photometry to select stars as RSGs based on predicted colors and magnitudes from PARSEC isochrones. The spatial distribution of our recovered RSGs follow the familiar pattern of mostly being concentrated in active star-forming regions such as spiral arms and central starburst rings. The RSG number density on kpc-scales is strongly correlated ($r_s$$\sim$0.82) with local star-formation rate density ($Σ_{SFR}$) traced by extinction-corrected far-ultraviolet (FUV) from GALEX+WISE, and weakly correlated ($r_s$$\sim$0.57) with the total stellar mass density ($Σ_*$), traced by near-infrared emission from WISE+Spitzer. The number of RSGs per mass of stellar populations with ages 6-30 Myr (the likely age range of RSGs $>$10 M$_{\odot}$) is $\sim$1 per 10$^{3.77\pm0.27}$ M$_{\odot}$, assuming constant star-formation rates from FUV+W4. Our sample will be a useful resource for tracking progenitors and feedback sites of future supernovae in PHANGS, age-dating stellar populations, and more.

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arXiv2025

Ashes of Creation: JWST Uncovers Silicate Dust in Massive Star Clusters

Dust production is a fundamental aspect of the baryonic cycle of star formation. It is known that dust is injected into the interstellar medium during early star formation by supernovae and later on by evolved stars. From individual objects, these mechanisms are well understood, but the overall dust production in star clusters at different evolutionary stages is still challenging to quantify. We present 22 massive (> 105M$_{\odot}$) extra galactic star clusters with ages between 3 and 100 Myr exhibiting a compact dust morphology seen with JWST-MIRI. We only find PAH features associated with one star cluster and nineteen have already cleared themselves from their natal dust. Their main characteristic is a significant enhancement at 10$μ$m, which is likely due to silicate emission and cannot be explained by ionized gas. We discuss several possible explanations including dust production from evolved stars such as red super giants, more exotic star types like yellow hypergiants and luminous blue variable stars. Stochastic dust injection from supernovae or a single supernova in dense gas can also create significant silicate emission. However, for this scenario secondary tracers such as a X-ray signal are expected which we only observe in three star clusters. We find the most luminous 10$μ$m emitter to be the three most massive star clusters (> 106M$_{\odot}$) which is at least a magnitude stronger than any known stellar sources indicating a rare mechanism that only appears at extreme masses and a short lifetime.

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arXiv2025

PAH Marks the Spot: Digging for Buried Clusters in Nearby Star-forming Galaxies

The joint capabilities of the Hubble Space Telescope (HST) and JWST allow for an unparalleled look at the early lives of star clusters at near- and mid-infrared wavelengths. We present here a multiband analysis of embedded young stellar clusters in 11 nearby, star-forming galaxies, using the PHANGS-JWST and PHANGS-HST datasets. We use the Zooniverse citizen science platform to conduct an initial by-eye search for embedded clusters in near-UV/optical/near-infrared images that trace stellar continuum emission, the Paschen$α$ and H$α$ recombination lines, and the 3.3 $μ$m polycyclic aromatic hydrocarbon feature and its underlying continuum. With this approach, we identify 292 embedded cluster candidates for which we characterize their ages, masses, and levels of line-of-sight extinction by comparing the photometric data to predictions from stellar population models. The embedded cluster candidates have a median age of 4.5 Myr and an average line-of-sight extinction $\left< A_V \right> = 6.0$ mag. We determine lower limits on source stellar masses, resulting in a median stellar mass of $10^3$ $M_{\odot}$. We use this sample of embedded cluster candidates to train multiple convolutional neural network models to carry out deep transfer learning-based searches for embedded clusters. With the aim of optimizing models for future catalog production, we compare results for four variations of training data using two neural networks. Confusion matrices for all eight model configurations, as well as inter-model identification trends, are presented. With refinement of the training sample, we determine that optimized models could serve as a pathway for future embedded cluster identification beyond our 11 galaxy sample.

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arXiv2025

Echoes of the First Stars: Massive Star Evolution in Extremely Metal-Poor Environments with the Habitable Worlds Observatory

A remarkable span of frontier astrophysics, from gravitational-wave archaeology to the origin of the elements to interpreting snapshots of the earliest galaxies, depends sensitively on our understanding of massive star formation and evolution in near-pristine, relatively enriched gas. From the surprisingly massive black holes detected by LIGO/Virgo to highly ionized nebulae with peculiar enrichment patterns observed in galaxies at Cosmic Dawn, evidence is mounting that our understanding of massive-star populations at very low metallicity remains critically incomplete. The fundamental limitation is the hand nature has dealt us: only a few star-forming galaxies within $\lesssim$1 Mpc can currently be resolved into individual stars, and none reach the extreme metallicities and star-formation intensities that characterized the early Universe. With an ultraviolet integral-field spectrograph aboard the Habitable Worlds Observatory (HWO), this barrier will finally be broken. HWO will bring rare, actively star-forming, extremely metal-poor dwarf galaxies at $\sim$10-20 Mpc such as I Zw 18 within reach of resolved UV-optical spectroscopy, providing our first direct, statistical view of individual massive stars and the feedback they drive at $>$30 $M_\odot$ and $<$10% $Z_\odot$. This science is deeply synergistic with many next-generation facilities, yet requires the unique combination of spatial resolution and UV/optical sensitivity that only HWO can provide. The massive star science enabled by HWO within the Local Volume represents a transformational advance in our ability to probe the earliest stellar populations - those that seeded the Milky Way and other galaxies with the first heavy elements, and paved the way for life in the transparent, reionized Universe we inhabit today.

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arXiv2025

Polycyclic aromatic hydrocarbons destruction in star-forming regions across 42 nearby galaxies

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM) of Solar metallicity galaxies, where they play a critical role in ISM heating, cooling, and reprocessing stellar radiation. The PAH fraction, the abundance of PAHs relative to total dust mass, is a key parameter in ISM physics. Using JWST and MUSE observations of 42 galaxies from the PHANGS survey, we analyze the PAH fraction in over 17 000 H II regions spanning a gas-phase oxygen abundance of 12+log(O/H) = 8.0-8.8 (Z ~ 0.2-1.3 Zsun), and ~400 isolated supernova remnants (SNRs). We find a significantly lower PAH fraction toward H II regions compared to a reference sample of diffuse ISM areas at matched metallicity. At 12+log(O/H) > 8.2, the PAH fraction toward H II regions is strongly anti-correlated with the local ionization parameter, suggesting that PAH destruction is correlated with ionized gas and/or hydrogen-ionizing UV radiation. At lower metallicities, the PAH fraction declines steeply in both H II regions and the diffuse ISM, likely reflecting less efficient PAH formation in metal-poor environments. Carefully isolating dust emission from the vicinity of optically-identified supernova remnants, we see evidence for selective PAH destruction from measurements of lower PAH fractions, which is, however, indistinguishable at ~50 pc scales. Overall, our results point to ionizing radiation as the dominant agent of PAH destruction within H II regions, with metallicity playing a key role in their global abundance in galaxies.

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arXiv2025

Time-scales of polycyclic aromatic hydrocarbon and dust continuum emission from gas clouds compared to molecular gas cloud lifetimes in PHANGS-JWST galaxies

Recent JWST mid-infrared (mid-IR) images, tracing polycyclic aromatic hydrocarbons (PAHs) and dust continuum emission, provide detailed views of the interstellar medium (ISM) in nearby galaxies. Leveraging PHANGS-JWST Cycle 1 and PHANGS-MUSE data, we measure the PAH and dust continuum emission lifetimes of gas clouds across 17 nearby star-forming galaxies by analyzing the relative spatial distributions of mid-IR (7.7-11.3$μ$m) and H$α$ emission at various scales. We find that the mid-IR emitting time-scale of gas clouds in galaxy disks (excluding centers) ranges from 10 to 30Myr. After star formation is detected in H$α$, mid-IR emission persists for 3-7Myr during the stellar feedback phase, covering 70-80% of the H$α$ emission. This significant overlap is due to intense radiation from star-forming regions, illuminating the surrounding PAHs and dust grains. In most galaxies, the mid-IR time-scale closely matches the molecular cloud lifetime measured with CO. Although mid-IR emission is complex as influenced by ISM distribution, radiation, and abundances of dust and PAHs, the similarity between the two time-scales suggests that once gas clouds form with compact mid-IR emission, they quickly provide sufficient shielding for stable CO formation. This is likely due to our focus on molecular gas-rich regions of galaxies with near-solar metallicity. Finally, we find that the mid-IR emitting time-scale is longer in galaxies with well-defined HII regions and less structured backgrounds, allowing photons to more efficiently heat the ambient ISM surrounding the HII regions, rather than contributing to diffuse emission. This suggests that the shape of the ISM also influences mid-IR emission.

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arXiv2025

Empirical SED Templates for Star Clusters Observed with HST and JWST: No Strong PAH or IR Dust Emission after Five Myr

JWST observations, when combined with HST data, promise to improve age estimates of star clusters in nearby spiral galaxies. However, feedback from young cluster stars pushes out the natal gas and dust, making cluster formation and evolution a challenge to model. Here, we use JWST + HST observations of the nearby spiral galaxy NGC 628 to produce spectral energy distribution (SED) templates of compact star clusters spanning 275 nm through 21 μm. These preliminary SEDs capture the cluster stars and associated gas and dust within radii of 0.12" to 0.67" (corresponding to 6 to 33 pc at the distance of NGC 628). One important finding is that the SEDs of 1, 2, 3, and 4 Myr clusters can be differentiated in the infrared. Another is that in 80-90% of the cases we study, the PAH and H_alpha emission track one another, with the dust responsible for the 3.3 μm PAH emission largely removed by 4 Myr, consistent with pre-supernova stellar feedback acting quickly on the surrounding gas and dust. Nearly-embedded cluster candidates have infrared SEDs which are quite similar to optically visible 1 to 3 Myr clusters. In nearly all cases we find there is a young star cluster within a few tenths of an arcsec (10 - 30 pc) of the nearly embedded cluster, suggesting the formation of the cluster was triggered by its presence. The resulting age estimates from the empirical templates are compatible both with dynamical estimates based on CO superbubble expansion velocities, and the TODDLERS models which track spherical evolution of homogeneous gas clouds around young stellar clusters.

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arXiv2025

The PHANGS-HST-Halpha Survey: Warm Ionized Gas Physics at High Angular resolution in Nearby GalaxieS with the Hubble Space Telescope

The PHANGS project is assembling a comprehensive, multi-wavelength dataset of nearby (~5-20 Mpc), massive star-forming galaxies to enable multi-phase, multi-scale investigations into the processes that drive star formation and galaxy evolution. To date, large survey programs have provided molecular gas (CO) cubes with ALMA, optical IFU spectroscopy with VLT/MUSE, high-resolution NUV--optical imaging in five broad-band filters with HST, and infrared imaging in NIRCAM+MIRI filters with JWST. Here, we present PHANGS-HST-Halpha, which has obtained high-resolution (~2-10 pc), narrow-band imaging in the F658N or F657N filters with the HST/WFC3 camera of the warm ionized gas in the first 19 nearby galaxies observed in common by all four of the PHANGS large programs. We summarize our data reduction process, with a detailed discussion of the production of flux-calibrated, Milky Way extinction corrected, continuum-subtracted Halpha maps. PHANGS-MUSE IFU spectroscopy data are used to background subtract the HST-Halpha maps, and to determine the [NII] correction factors for each galaxy. We describe our public data products and highlight a few key science cases enabled by the PHANGS-HST-Halpha observations.

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arXiv2025

Polycyclic Aromatic Hydrocarbon and CO(2-1) Emission at 50-150 pc Scales in 70 Nearby Galaxies

Combining Atacama Large Millimeter/sub-millimeter Array CO(2-1) mapping and JWST near- and mid-infrared imaging, we characterize the relationship between CO(2-1) and polycyclic aromatic hydrocarbon (PAH) emission at ~100 pc resolution in 70 nearby star-forming galaxies. Leveraging a new Cycle 2 JWST treasury program targeting nearby galaxies, we expand the sample size by more than an order of magnitude compared to previous ~100 pc resolution CO-PAH comparisons. Focusing on regions of galaxies where most of the gas is likely to be molecular, we find strong correlations between CO(2-1) and 3.3 um, 7.7 um, and 11.3 um PAH emission, estimated from JWST's F335M, F770W, and F1130W filters. We derive power law relations between CO(2-1) and PAH emission, which have indices in the range 0.8-1.3, implying relatively weak variations in the observed CO-to-PAH ratios across the regions that we study. We find that CO-to-PAH ratios and scaling relationships near HII regions are similar to those in diffuse sight lines. The main difference between the two types of regions is that sight lines near HII regions show higher intensities in all tracers. Galaxy centers, on the other hand, show higher overall intensities and enhanced CO-to-PAH ratios compared to galaxy disks. Individual galaxies show 0.19 dex scatter in the normalization of CO at fixed I_PAH, and this normalization anti-correlates with specific star formation rate (sSFR) and correlates with stellar mass. We provide a prescription that accounts for these galaxy-to-galaxy variations and represents our best current empirical predictor to estimate CO(2-1) intensity from PAH emission, which allows one to take advantage of JWST's excellent sensitivity and resolution to trace cold gas.

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arXiv2025

PAH Feature Ratios Around Stellar Clusters and Associations in 19 Nearby Galaxies

We present a comparison of observed polycyclic aromatic hydrocarbon (PAH) feature ratios in 19 nearby galaxies with a grid of theoretical expectations for near- and mid-infrared dust emission. The PAH feature ratios are drawn from Cycle 1 JWST observations and are measured for 7224 stellar clusters and 29176 stellar associations for which we have robust ages and mass estimates from HST five-band photometry. Though there are galaxy-to-galaxy variations, the observed PAH feature ratios largely agree with the theoretical models, particularly those that are skewed toward more ionized and larger PAH size distributions. For each galaxy we also extract PAH feature ratios for 200 pc-wide circular regions in the diffuse interstellar medium, which serve as a non-cluster/association control sample. Compared to what we find for stellar clusters and associations, the 3.3um/7.7um and 3.3um/11.3um ratios from the diffuse interstellar medium are $\sim 0.10-0.15$ dex smaller. When the observed PAH feature ratios are compared to the radiation field hardness as probed by the [OIII]/H$β$ ratio, we find anti-correlations for nearly all galaxies in the sample. These results together suggest that the PAH feature ratios are driven by the shape intensity of the radiation field, and that the smallest PAHs -- observed via JWST F335M imaging -- are increasingly 'processed' or destroyed in regions with the most intense and hard radiation fields.

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arXiv2024

Tracing the earliest stages of star and cluster formation in 19 nearby galaxies with PHANGS-JWST and HST: compact 3.3 $μ$m PAH emitters and their relation to the optical census of star clusters

The earliest stages of star and cluster formation are hidden within dense cocoons of gas and dust, limiting their detection at optical wavelengths. With the unprecedented infrared capabilities of JWST, we can now observe dust-enshrouded star formation with $\sim$10 pc resolution out to $\sim$20 Mpc. Early findings from PHANGS-JWST suggest that 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) emission can identify star clusters in their dust-embedded phases. Here, we extend this analysis to 19 galaxies from the PHANGS-JWST Cycle 1 Treasury Survey, providing the first characterization of compact sources exhibiting 3.3$μ$m PAH emission across a diverse sample of nearby star-forming galaxies. We establish selection criteria, a median color threshold of F300M-F335M=0.67 at F335M=20, and identify of 1816 sources. These sources are predominantly located in dust lanes, spiral arms, rings, and galaxy centers, with $\sim$87% showing concentration indices similar to optically detected star clusters. Comparison with the PHANGS-HST catalogs suggests that PAH emission fades within $\sim$3 Myr. The H$α$ equivalent width of PAH emitters is 1-2.8 times higher than that of young PHANGS-HST clusters, providing evidence that PAH emitters are on average younger. Analysis of the bright portions of luminosity functions (which should not suffer from incompleteness) shows that young dusty clusters may increase the number of optically visible $\leq$ 3 Myr-old clusters in PHANGS-HST by a factor between $\sim$1.8x-8.5x.

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arXiv2024

PHANGS-JWST: Data Processing Pipeline and First Full Public Data Release

The exquisite angular resolution and sensitivity of JWST is opening a new window for our understanding of the Universe. In nearby galaxies, JWST observations are revolutionizing our understanding of the first phases of star formation and the dusty interstellar medium. Nineteen local galaxies spanning a range of properties and morphologies across the star-forming main sequence have been observed as part of the PHANGS-JWST Cycle 1 Treasury program at spatial scales of $\sim$5-50pc. Here, we describe pjpipe, an image processing pipeline developed for the PHANGS-JWST program that wraps around and extends the official JWST pipeline. We release this pipeline to the community as it contains a number of tools generally useful for JWST NIRCam and MIRI observations. Particularly for extended sources, pjpipe products provide significant improvements over mosaics from the MAST archive in terms of removing instrumental noise in NIRCam data, background flux matching, and calibration of relative and absolute astrometry. We show that slightly smoothing F2100W MIRI data to 0.9" (degrading the resolution by about 30 percent) reduces the noise by a factor of $\approx$3. We also present the first public release (DR1.1.0) of the pjpipe processed eight-band 2-21 $μ$m imaging for all nineteen galaxies in the PHANGS-JWST Cycle 1 Treasury program. An additional 55 galaxies will soon follow from a new PHANGS-JWST Cycle 2 Treasury program.

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arXiv2024

Hidden Gems on a Ring: Infant Massive Clusters and Their Formation Timeline Unveiled by ALMA, HST, and JWST in NGC 3351

We study young massive clusters (YMCs) in their embedded "infant" phase with $\sim0.\!^{\prime\prime}1$ ALMA, HST, and JWST observations targeting the central starburst ring in NGC 3351, a nearby Milky Way analog galaxy. Our new ALMA data reveal 18 bright and compact (sub-)millimeter continuum sources, of which 8 have counterparts in JWST images and only 6 have counterparts in HST images. Based on the ALMA continuum and molecular line data, as well as ancillary measurements for the HST and JWST counterparts, we identify 14 sources as infant star clusters with high stellar and/or gas masses (${\sim}10^5\;\mathrm{M_\odot}$), small radii (${\lesssim}\,5\;\mathrm{pc}$), large escape velocities ($6{-}10\;\mathrm{km/s}$), and short free-fall times ($0.5{-}1\;\mathrm{Myr}$). Their multiwavelength properties motivate us to divide them into four categories, likely corresponding to four evolutionary stages from starless clumps to exposed HII region-cluster complexes. Leveraging age estimates for HST-identified clusters in the same region, we infer an evolutionary timeline going from $\sim$1-2 Myr before cluster formation as starless clumps, to $\sim$4-6 Myr after as exposed HII region-cluster complexes. Finally, we show that the YMCs make up a substantial fraction of recent star formation across the ring, exhibit an non-uniform azimuthal distribution without a very coherent evolutionary trend along the ring, and are capable of driving large-scale gas outflows.

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