We examine the spatial distribution of star clusters in NGC628 using the statistical tool INDICATE to quantify clustering tendencies. Our sample, based on Hubble Space Telescope and James Webb Space Telescope observations, is the most complete to date, spanning ages from 1 to >100 Myr. We find cluster spatial behaviour varies with galactic position, age, and mass. Most emerging young clusters are tightly spatially associated with each other, while fully emerged clusters are in 1.5 times looser spatial associations, irrespective of age. Young Massive Clusters (YMCs ≥ 10⁴ M⁠_⨀) tend to associate with lower-mass clusters but not strongly with other YMCs, implying that intense star formation regions produce a few YMCs alongside many lower-mass clusters rather than multiple YMCs together. Young concentrated clusters show a wide radial distribution in the galactic disc, which narrows with age; with concentrated clusters >100 Myr mostly residing between 2 and 6 kpc. This pattern may reflect either faster dispersal of isolated tight cluster spatial ‘structure’ in a lower gas density outer disc or gradual inside-out growth, with the formation of this structure shifting outwards over time. We also detect distinct spatial behaviours for clusters within 2 kpc, linked to the inner Lindblad resonance (⁠≤1 kpc), nuclear ring (∼⁠0.5–1 kpc), and the start of spiral arms (∼⁠1.25–2 kpc), suggesting these regions exhibit strong radial motions that could hinder clusters from forming and remaining in tight concentrations. Our results highlight how spatially resolved studies of clusters can reveal the influence of galactic dynamics on star formation and cluster evolution.

We present JWST NIRCam observations of the emerging young star clusters (eYSCs) detected in the nearby spiral galaxy M83. The NIRcam mosaic encompasses the nuclear starburst, the bar, and the inner spiral arms. The eYSCs, detected in Paα and Brα maps, have been largely missed in previous optical campaigns of young star clusters (YSCs). We distinguish between eYSCI, if they also have compact 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission associated with them, and eYSCII, if they only appear as compact Paα emitters. We find that the variations in the 3.3 μm PAH feature are consistent with an evolutionary sequence where eYSCI evolve into eYSCII and then optical YSCs. This sequence is clear in the F300M​​​​​​−F335M (tracing the excess in the 3.3 μm PAH feature) and the F115W−F187N (tracing the excess in Paα) colors, which become increasingly bluer as clusters emerge. The central starburst stands out as the region where the most massive eYSCs are currently forming in the galaxy. We estimate that only about 20% of eYSCs will remain detectable as compact YSCs. Combining eYSCs and YSCs (≤10 Myr), we recover an average clearing timescale of 6 Myr in which clusters transition from embedded to fully exposed. We see evidence of shorter emergence timescales (∼5 Myr) for more massive (>5 × 10³ M_⊙) clusters, while star clusters of ∼10³ M_⊙ about 7 Myr. We estimate that eYSCs remain associated with the 3.3 μm PAH emission for 3–4 Myr. Larger samples of eYSC and YSC populations will provide stronger statistics to further test environmental and cluster mass dependencies on the emergence timescale.

We present new JWST/NIRCam observations of the starburst irregular galaxy NGC 4449, obtained in Cycle 1 as part of the Feedback in Emerging extrAgalactic Star clusTers program, which we use to investigate its resolved stellar populations and their spatial distributions. NGC 4449 near-IR color-magnitude diagrams reveal a broad range of stellar populations, spanning different evolutionary phases, from young main sequence stars, to old red giant branch stars and asymptotic giant branch (AGB) stars. The analysis of their spatial distributions shows that younger (≤10 Myr) populations form an S-shaped distribution aligned with the galaxy’s north-south axis, while stars aged 10-60 Myr show shifting concentrations from the north to the south, consistent with the possibility that external interactions or tidal effects may have triggered star formation in spatially distinct bursts. Clusters of comparable ages generally follow these distributions, suggesting that cluster and field stars form at the same pace in each galaxy region. Thanks to the unprecedented high-spatial resolution and sensitivity of the JWST data, we recover a clear gap between oxygen-rich and the carbon star branch of the AGB population, as well as the presence of a massive AGB star “finger.” The analysis of these stars can provide constraints on AGB evolution models and dust production in this galaxy. These results confirm NGC 4449's status as a compelling example of a local dwarf starburst galaxy undergoing complex and possibly externally driven star formation and underscore the power of JWST in probing the full lifecycle of stars in nearby starburst systems.

We combine James Webb Space Telescope images of the nearby galaxy NGC 5194 in the hydrogen recombination line Paα (1.8756 μm) from the Cycle 1 program JWST-FEAST with 21 μm dust continuum images from the Cycle 2 Treasury program JWGT to quantify the difference in the calibration of mid-infrared star formation rates (SFRs) between H II regions and galaxies. We use archival Hubble Space Telescope Hα imaging to correct the Paα emission for the effects of dust attenuation. Our data confirm previous results that the dust-corrected Paα flux is tightly correlated with the 21 μm emission at the scales of H II regions. When combined with published JWST data for the H II regions of the galaxy NGC 628 and Spitzer Space Telescope 24 μm data for whole galaxies and for kiloparsec-size galaxy regions, we show that the L(24)–L(Paα) relation has exponent > 1 across six decades in luminosity. In addition, the hybrid 24 μm + Hα SFR indicator has a scaling constant about 4.4 times higher for H II regions than for whole galaxies, also in agreement with previous results. Models of stellar populations with a range of star formation histories reveal that the observed trends can be entirely ascribed to and quantified with the contribution to the infrared emission by stellar populations older than ∼5–6 Myr. Based on the models’ results, we provide (1) a calibration for the infrared SFR across 6 orders of magnitude in L(24), from H II regions to luminous galaxies, and (2) a prescription for the scaling constant of the hybrid infrared SFR indicators as a function of the star formation timescale.

We present a combined Hubble Space Telescope (HST) and JWST 0.2–to–5 μm analysis of the spectral energy distributions (SED) of emerging young star clusters (eYSCs) in four nearby galaxies from the Feedback in Emerging Extrgalactic Star Clusters survey: M51, M83, NGC 628, and NGC 4449. These clusters, selected for their bright Paα and 3.3 μm polycyclic aromatic hydrocarbon emission, are still associated with their natal gas cloud and have been largely missed in previous HST optical campaigns. We modeled their SEDs using the CIGALE fitting code and identified (i) a systematic flux excess at 1.5–2.5 μm that is not accounted for by current stellar population models and (ii) the preference for a set of dust model parameters that is not aligned with expectations from self-consistent analyses of star-forming regions, suggesting model shortcomings also in the 3–5 μm. The near-infrared excess is most prominent in low-mass (≤3000 M ⊙) and young (≤6 Myr) clusters. Additionally, we see that the SED fitting analysis wrongly assigns ages ≥6 Myr to a fraction of strong Paα emitters with equivalent widths suggestive of significantly younger ages. A parallel analysis with the slug code suggests that stochastic initial mass function (IMF) sampling of pre-main-sequence stars combined with extinction might partially reduce the gap. We conclude that the inclusion of young stellar object SEDs, along with more realistic sampling of the cluster IMF, might be needed to fully account for the stellar population and dust properties of eYSCs.

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 Paα 1.87 μm and Brα 4.05 μm hydrogen recombination lines, as well as 3.3 and 7.7 μm 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 μm 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 PAH3.3μm/PAH7.7μm 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 μm 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 PAH3.3μm/PAH7.7μm ratio would indicate that both families of molecules disrupt at similar rates in proximity to H ii regions.

We present maps of ionized gas (traced by Paα and Brα) and 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission in the nearby spiral galaxy NGC 628, derived from new JWST/NIRCam data from the Feedback in Emerging extrAgalactic Star clusTers (FEAST) survey. With this data, we investigate and calibrate the relation between 3.3 μm PAH emission and star formation rate (SFR) in and around emerging young star clusters (eYSCs) on a scale of ∼40 pc. We find a tight (correlation coefficient ρ ∼ 0.9) sublinear (power-law exponent α ∼ 0.75) relation between the 3.3 μm PAH luminosity surface density and SFR traced by Brα for compact, cospatial (within 0.″16 or ∼7 pc) peaks in Paα, Brα, and 3.3 μm (eYSC-I). The scatter in the relationship does not correlate well with variations in local interstellar medium metallicity, due to a radial metallicity gradient, but rather is likely due to stochastic sampling of the stellar initial mass function (IMF) and variations in the PAH heating and age of our sources. The deviation from a linear relation may be explained by PAH destruction in more intense ionizing environments, variations in age, and IMF stochasticity at intermediate to low luminosities. We test our results with various continuum subtraction techniques using combinations of NIRCam bands and find that they remain robust with only minor differences in the derived slope and intercept. An unexpected discrepancy is identified between the relations of hydrogen recombination lines (Paα versus Brα; Hα versus Brα).

Context. The blue compact dwarf (BCD) galaxy I Zw 18 is one of the most metal-poor (Z ∼ 3% Z⊙) star-forming galaxies known in the local Universe. Since its discovery, the evolutionary status of this system has been at the center of numerous debates within the astronomical community. Aims. We aim to probe and resolve the stellar populations of I Zw 18 in the near-IR using JWST/NIRCam's unprecedented imaging spatial resolution and sensitivity. Additionally, our goal is to derive the spatially resolved star formation history (SFH) of the galaxy within the last 1 Gyr, and to provide constraints for older epochs. Methods. We used the point spread function fitting photometry package DOLPHOT to measure positions and fluxes of point sources in the F115W and F200W filters' images of I Zw 18, acquired as part of the JWST GTO ID 1233 (PI: Meixner). Furthermore, to derive I Zw 18's SFH, we applied a state-of-the-art color-magnitude diagram (CMD) fitting technique (SFERA 2.0), using two independent sets of stellar models: PARSEC-COLIBRI and MIST. Results. Our analysis of I Zw 18's CMD reveal three main stellar populations: one younger than ∼30 Myr, mainly associated with the northwest star-forming (SF) region; an intermediate-age population (∼100-800 Myr), associated with the southeast SF region; and a red and faint population, linked to the underlying halo of the galaxy, older than 1 Gyr and possibly as old as 13.8 Gyr. The main body of the galaxy shows a very low star formation rate (SFR) of ∼10-4 M⊙ yr-1 between 1 and 13.8 Gyr ago. In the last billion years, I Zw 18 shows an increasing trend, culminating in two strong bursts of SF around ∼10 and ∼100 Myr ago. Notably, I Zw 18 Component C mimics the evolution of the main body, but with lower SFRs on average. Conclusions. Our results confirm that I Zw 18 is populated by stars of all ages, without any major gaps. Thus, I Zw 18 is not a truly young galaxy, but rather a system characterized by an old underlying stellar halo, in agreement with what has been found in other BCDs by similar studies. The low SF activity exhibited at epochs older than 1 Gyr is in agreement with the "slow cooking"dwarf scenario proposed in the literature, and could have contributed to its low metal content. The galaxy is now experiencing its strongest episode of star formation (∼0.6 M⊙ yr-1 ) mainly located in the northwest region. A recent gravitational interaction between the main body and Component C is the most likely explanation for this starburst.

We present a JWST imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming (SF), blue compact dwarf galaxy. With an oxygen abundance of only ∼3% Z☉, it is among the lowest-metallicity systems known in the local Universe, and is, therefore, an excellent accessible analog for the galactic building blocks which existed at early epochs of ionization and star formation. These JWST data provide a comprehensive infrared (IR) view of I Zw 18 with eight filters utilizing both Near Infrared Camera (F115W, F200W, F356W, and F444W) and Mid-Infrared Instrument (F770W, F1000W, F1500W, and F1800W) photometry, which we have used to identify key stellar populations that are bright in the near- and mid-IR. These data allow for a better understanding of the origins of dust and dust-production mechanisms in metal-poor environments by characterizing the population of massive, evolved stars in the red supergiant (RSG) and asymptotic giant branch (AGB) phases. In addition, it enables the identification of the brightest dust-enshrouded young stellar objects (YSOs), which provide insight into the formation of massive stars at extremely low metallicities typical of the very early Universe. This paper provides an overview of the observational strategy and data processing, and presents first science results, including identifications of dusty AGB, RSG, and bright YSO candidates. These first results assess the scientific quality of JWST data and provide a guide for obtaining and interpreting future observations of the dusty and evolved stars inhabiting compact dwarf SF galaxies in the local Universe.

New, deep HST photometry allowed us to identify and study eight compact and bright (M_V ≤ −5.8) star clusters in the outskirts of the star-forming isolated dwarf galaxy NGC 5238 (M_* ≃ 10⁸ M_⊙). Five of these clusters are new discoveries, and six appear projected onto and/or aligned with the tidal tails recently discovered around this galaxy. The clusters are partially resolved into stars, and their colour magnitude diagrams reveal a well-developed red giant branch, implying ages older than 1–2 Gyr. Their integrated luminosity and structural parameters are typical of classical globular clusters, and one of them, with M_V = −10.56 ± 0.07, is as bright as ω Cen, the brightest globular cluster in the Milky Way. Since the properties of this cluster are in the range spanned by those of nuclear star clusters we suggest that it may be the nuclear remnant of the disrupted satellite of NGC 5238 that produced the observed tidal tails.