JWST can pierce through dusty molecular clouds to study the early stages of star formation, where young star clusters are actively driving stellar feedback and still emerging from their natal cloud. We present a first look of the JWST/NIRSpec multiplex spectroscopy observations acquired by the Feedback in Emerging extrAgalactic Star clusTers program for the nearby spiral galaxy NGC 628. We showcase JWST’s ability to resolve the spectral properties of emerging young star clusters (eYSCs) and their immediate interstellar medium by focusing on a bright star-forming complex (0.5 × 0.5 kpc²) in the northern spiral arm as a science proof-of-concept. The eYSC spectra are rich in ionized gas (from H II regions), as well as warm H₂ and polycyclic aromatic hydrocarbon (PAH) emission from photodissociation regions (PDRs), consistent with young star formation. Paα equivalent widths and H/He ionizing photon fluxes both indicate the presence of hot, young massive stars (O8.5V–O8V), consistent with photometry spectral energy distribution estimates. The ionized gas is highly correlated with H₂ and PAH emission, suggesting that the PDR morphology evolves as clusters emerge from their natal cloud. We find a photoionization-dominated regime from independent line diagnostics, with little contribution from supernovae-driven shocks, highlighting the importance of presupernovae feedback when massive stars are present. This pilot study showcases how JWST’s multiplex spectroscopy mode can disentangle the mechanisms present in the youngest stages of star formation for the first time outside the Local Group.

We apply the angular two-point correlation function (TPCF) to the spatial distributions of young star clusters (YSCs) in four nearby star forming galaxies (NGC 628, NGC 4449, M51, and M83) in order to investigate their underlying hierarchical structuring. Using newly constructed catalogs of YSCs in the emerging phase (eYSCs), identified in the infrared with JWST, and optical YSCs detected in archival Hubble Space Telescope data, we compute the TPCFs for various cluster samples and age bins across the four galaxies, as part of the Feedback in Emerging extrAgalactic Star ClusTers (FEAST) program. We find clear evidence of hierarchical structuring, especially in eYSCs and YSCs with ages <10 Myr (referred to as oYSCs), which show similar TPCFs within each galaxy. NGC 628 exhibits a clear distinction between the TPCFs of eYSCs and oYSCs, implying a shorter randomization timescale. In contrast, clusters aged 10–300 Myr exhibit progressively more random spatial distributions, becoming effectively random after ∼100 Myr, consistent with earlier studies. The two-dimensional fractal index D₂ values of the YSCs’ underlying distributions are calculated from model fits to the TPCFs. Our values of D₂ derived from the youngest YSC populations align better with the expected value of D₂ ∼ 1.3 for a universal star formation process compared to previous findings.

We present new MIRI F560W, F770W, and F1000W imaging of the galaxy GN-z11 at a redshift of 10.603. We report a significant detection (14σ) in the F560W and F770W images, and a marginal detection (3.2σ) in the F1000W filter. The new MIRI observations cover the optical-red spectral range and significantly extend previous NIRCam wavelength coverage from rest-frame 0.38 μm up to 0.86 μm. In this work, we analyse the spectral energy distribution (SED) combining this new MIRI imaging data with archival NIRSpec/Prism and MRS spectroscopy, and NIRCam imaging, i.e. covering the rest-frame 0.12–0.86 μm. New constraints such as the equivalent widths of the strong optical lines ([O III]λ5008, Hβ and Hα) and the continuum emission at rest-frame 0.48 μm, 0.66 μm, and 0.86 μm, free of emission line contributions, are presented. The continuum emission shows a flat energy distribution, in f_ν, up to 0.5 μm, compatible with the presence of a mixed stellar population of young (4 ± 1 Myr) and mature (63 ± 23 Myr) stars that also account for the [O III], Hβ, and Hα emission lines. The continuum at rest-frame 0.66 μm shows a 36 ± 3% flux excess above the predicted flux for a mixed stellar population, pointing to the presence of an additional source contributing at these wavelengths. This excess increases to 91 ± 28% at rest-frame 0.86 μm, although with a large uncertainty due to the marginal detection in the F1000W filter. We consider that hot dust emission in the dusty torus around a type 2 active galactic nucleus (AGN) could be responsible for the observed excess. Alternatively, this excess could be due to hot dust emission or a photoluminiscence dust process (Extended Red Emission, ERE) under the extreme UV radiation field, as is observed in local metal-poor galaxies and in young compact starbursts. The presence of a type 1 AGN is not supported by the observed SED as the hot dust emission in luminous high-z quasi-stellar objects (QSOs) contributes at wavelengths above rest-frame 1 μm, and an additional ad hoc red source would be required to explain the observed flux excess at 0.66 and 0.86 μm. Additional deep MIRI imaging covering the rest-frame near-IR is needed to confirm the flux detection at 10 μm, and to discriminate between the different hot dust emission in the extreme starburst and AGN scenarios.

We present high-resolution (0 .'' 13-0 .'' 23) NOEMA observations of the dust continuum emission at 1.1 mm (rest frame 220 mu m) and JWST/NIRCam and MIRI imaging of the z = 4.055 starburst galaxy GN20. The sensitive NOEMA imaging at 1.6 kpc resolution reveals extended dust emission, approximate to 14 kpc in diameter (re approximate to 2.5 kpc, b/a = 0.5), which is centrally asymmetric and clumpy. The dust emission is as extended as the stellar emission and molecular gas traced by 12CO(2-1), with a common center, and is brightest in the strongly obscured nuclear part of the galaxy. Approximately one-third of the total dust emission emerges from the nucleus and the most prominent clump to the south, with (only) 60% from the central 3.5 x 1.5 kpc (0 .'' 5-0 .'' 2), implying that the starburst is very extended. The combined JWST and NOEMA morphology suggests GN20 experienced a recent interaction or merger, likely invigorating the starburst. The radial surface brightness profiles of the molecular gas and near-IR stellar emission are similar, while, in contrast, the dust emission appears significantly more concentrated. Through self-consistent radiative-transfer modeling of the integrated and resolved 12CO and dust emission, we derive Mmol=2.9-0.3+0.4x1011 f M circle dot, with alpha CO=2.8-0.3+0.5 . We find the extended dust implies a lower global dust optical depth than previously reported but a high dust mass of Mdust=5.7-0.6+0.8x109 M circle dot and gas-to-dust ratio of approximate to 50. Furthermore, we show that the distinct apparent radial profiles of the gas and dust can be explained purely by radiative-transfer effects (differences in the radial optical depths and temperatures), and that the observations are consistent with the gas and dust masses being similarly distributed throughout the starburst. The latter highlights the importance of accounting for radiative-transfer effects when comparing molecular gas and dust distributions from different tracers.

Context. The shape of the initial mass function (IMF) remains a fundamental yet contentious topic in the study of stellar formation and evolution. It is imperative to understand the potential variability of the IMF across different young regions. This study examines the IMF within the young massive cluster RCW 36 situated in the Vela Molecular Ridge, comparable with the Orion Nebula Cluster in terms of stellar surface density.

Aims. The primary objective of this research is to construct the most comprehensive census of the stellar population in RCW 36 to date and determine the first ever IMF and star to brown dwarf (BD) ratio for the cluster.

Methods. We used state-of-the art observational techniques, drawing on new GLAO observations conducted with HAWK-I/VLT in addition to archival data from 2MASS, SOFI/NTT, and new kinematics from Gaia DR3. To enhance photometric accuracy and source extraction, we employed DENEB, an advanced deep learning algorithm capable of removing the complex filamentary nebula in our images. Statistical comparisons of color-magnitude diagrams were performed between RCW 36 and a control field, also obtained using HAWK-I under the same mode, to assign membership weights for the sources in our field. Mass estimates to individual sources were also derived through comparison with model isochrones in order to determine the IMF using the membership weights.

Results. We found a new distance of 954 ± 40 pc. We determined the IMF for RCW 36 down to ~0.03 M_⊙, characterized by a broken power law (dN/d M ∝ M^−α) with α = 1.62 ± 0.03 (0.20 M_⊙ −20 M_⊙) and α = 0.46 ± 0.14 (0.03 M_⊙–0.20 M_⊙). We also determined the star-BD ratio to be 2–5, in agreement with other Galactic clusters. Lastly, through a study of the differences in the IMF within and outside 0.2 pc and the cumulative mass distributions for low-mass and intermediate to high-mass sources, we also detected signs of possible mass segregation within RCW 36, which should be primordial.

Conclusions. RCW 36 shares many characteristics with other young massive clusters, such as a shallower than Salpeter high-mass slope and the possibility of mass segregation. The flatter lower-mass regime of the IMF is similar to most Galactic clusters. The star-BD ratio is also in line with the observed values in other clusters, independent of their inherent properties.

We use JWST/NIRCam and MIRI imaging acquired by the Feedback in Emerging extrAgalactic Star clusTers (or FEAST) program along with archival Hubble Space Telescope imaging to map ionized gas (Paα, Brα, and Hα) and polycyclic aromatic hydrocarbon (PAH) emission (3.3 and 7.7 μm) across a sample of four nearby galaxies (NGC 5194, 5236, 628, and 4449). These maps are utilized to calibrate the PAH features as star formation rate (SFR) indicators in 40 pc-size regions around massive emerging young star clusters. We find a tight, sublinear (power-law exponent α  ∼ 0.8) relation between the PAH luminosities (3.3 and 7.7 μm) and SFR (extinction-corrected Paα) in near-solar-metallicity environments. PAH destruction in more intense ionizing environments and/or variations in the age of our sources may drive the deviation from a linear relation. In the metal-poor environment of NGC 4449 (∼1/3 Z_⊙), we see substantial deficits in the PAH feature strengths at fixed SFR and significantly higher scatter in the PAH–SFR relations. We determine that the 3.3/7.7 μm PAH luminosity ratio increases towards lower-metallicity environments. This is interpreted as a result of a shift in the size distribution towards smaller PAHs at lower metallicities, possibly due to inhibited grain growth. Focusing on the regions in NGC 4449, we observe a decreasing 3.3/7.7 μm ratio towards higher SFR, which could indicate that small PAHs are preferentially destroyed relative to larger PAHs in significantly subsolar-metallicity conditions. We estimate that ∼two-thirds of the PAH emission in typical local star-forming galaxies is excited by older stars and unrelated to recent (<10 Myr) star formation.

Quantifying the timescales of star cluster emergence from their natal clouds remains one of the main challenges in understanding the star formation process. These timescales are fundamental measurements of the star formation cycle within galaxies, yet are difficult to constrain due to the complex interplay between stellar feedback and star formation across multiple physical scales. Here we present Hubble Space Telescope and James Webb Space Telescope observations of thousands of young star clusters in four nearby galaxies (M51, M83, NGC 628 and NGC 4449). A substantial fraction of these clusters are still embedded within their natal gas and remain invisible at optical wavelengths. We constrain their emergence process by measuring the timescales required to disperse the surrounding material. We find a strong correlation between dispersal timescale and cluster stellar mass, with massive clusters emerging faster than their lower-mass counterparts. This is a critical constraint on star formation and stellar feedback simulations, which struggle to fully reproduce star clusters formation and emergence. Our results emphasize the central role of massive clusters in driving the escape of ionizing radiation into the galactic medium. Finally, they impose time limitations for planet formation in massive cluster environments where disks get exposed to ultraviolet irradiation and further gas infall is halted.

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.

The recent discovery of five massive stellar clusters at z = 9.625 in the Cosmic Gems galaxy has raised the question about the formation mechanism of star clusters in the first 500 Myr after the Big Bang. We inferred the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM = n₀M^β), assuming three different slopes (β = −1.5, −2.0, and −2.5) and different lower-mass limits between 10² and 10⁵ M_⊙. We compared the total integrated cluster stellar mass to the stellar mass inferred from the counter image of the Cosmic Gems, which provides the best modestly magnified (μ = 1.84 ± 0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy was estimated as × 10⁷ M_⊙, with an effective radius of R_eff =  parsecs and a stellar surface mass density of Σ_mass =  M_⊙ pc⁻². Accounting for normalization uncertainties – including different lensing magnification scenarios for the arc – a modified SCMF, combined with a significantly high star cluster formation efficiency (approaching 100%), appears to be a necessary condition to explain the relatively short formation timescale of both the star clusters and the counter image, without exceeding the galaxy’s stellar mass. By extrapolating the physical properties at the peak of the burst, we found that in its recent past (≲30 Myr) the Cosmic Gems galaxy likely experienced a specific star formation rate exceeding 25 Gyr⁻¹ and luminosity approaching the “blue monster” regime (M_UV < −20). Our study provides insights into the extreme clustered nature of star formation in early galaxies and sheds light on the formation of bound star clusters that might survive to z = 0 as globular clusters older than 13 Gyr.

Context. The X-ray output of low-metallicity starburst galaxies is a key component of stellar feedback, tracing the processes responsible for gas ionization and chemical enrichment. The integrated X-ray luminosity (L_X) from high-mass X-ray binaries in star-forming galaxies scales with the star formation rate (SFR) and the host galaxy metallicity Z. Due to the inverse correlation between L_X/SFR and Z, the contribution of X-ray binaries to the ionizing photon budget is expected to be enhanced in metal-poor systems. Their radiation can potentially ionize He II in the surrounding interstellar medium, powering nebular He IIλ4686 Å emission. However, detailed studies of the X-ray emission in individual low-Z starburst galaxies are rare, and their X-ray properties are not well explored.

Aims. The blue compact dwarf galaxy ESO 338-IG04 (ESO 338-4 hereafter) serves as a nearby template for studying stellar feedback and X-ray emission in low-metallicity starbursts. It combines vigorous recent star formation, a low metallicity (12 + log(O/H)≈7.9, or 12% solar), and a rich population of massive stellar clusters. Extensively observed in optical and UV wavelengths with HST and VLT MUSE, ESO 338-4 is ideally suited for multiwavelength feedback studies. We characterize the X-ray emission of ESO 338-4 and its galactic halo using new deep observations obtained with the Chandra X-ray Observatory (Chandra) and XMM-Newton.

Methods. We analyzed X-ray spectra, light curves, and images of ESO 338-4 to constrain the nature of its X-ray sources. Additionally, we employed photoionization modeling to assess the significance of X-ray sources to the observed nebular He IIλ4686 Å emission.

Results. We identified five ultraluminous X-ray sources (ULXs) and diffuse hot gas surrounding ESO 338-4. Two of the ULXs are spatially associated with stellar clusters. The total galactic X-ray luminosity exceeds 10⁴¹ erg s⁻¹. The brightest point source, ULX1, is variable on timescales of days and is not associated with a stellar cluster. Last, our modeling demonstrates that X-ray sources significantly affect the ionizing photon budget of the galaxy. Photoionization modeling with ULX1 as the ionizing source predicts a high nebular He IIλ4686 Å line luminosity of approximately 10³⁹ erg s⁻¹.