We present a comparative study of the molecular gas in two galaxies from the Legacy ExtraGalactic UV Survey (LEGUS) sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793, especially young massive clusters (<10 Myr, >10⁴M_⊙). Using Atacama Large Millimeter/submillimeter Array (ALMA) CO(2–1) observations of the two galaxies with the same sensitivity and resolution (13 pc), we directly compare the molecular gas in these two similar galaxies to determine the physical conditions responsible for their large disparity in cluster formation. By fitting size–line width relations for the clouds in each galaxy, we find that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kinetic energies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which may be connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show a stronger correlation with young clusters than with the general cloud population. We find surprisingly small differences between the distributions of molecular cloud populations in the two galaxies, though the largest of those differences is that NGC 1313 has higher surface densities and lower freefall times.

We compare the molecular cloud properties in subgalactic regions of two galaxies, barred spiral NGC 1313, which is forming many massive clusters, and flocculent spiral NGC 7793, which is forming significantly fewer massive clusters despite having a similar star formation rate to NGC 1313. We find that there are larger variations in cloud properties between different regions within each galaxy than there are between the galaxies on a global scale, especially for NGC 1313. There are higher masses, line widths, pressures, and virial parameters in the arms of NGC 1313 and the center of NGC 7793 than in the interarm and outer regions of the galaxies. The massive cluster formation of NGC 1313 may be driven by its greater variation in environment, allowing more clouds with the necessary conditions to emerge, although no one parameter seems primarily responsible for the difference in star formation. Meanwhile NGC 7793 has clouds that are as massive and have as much kinetic energy as the clouds in the arms of NGC 1313, but have densities and pressures more similar to those in the interarm regions and so are less inclined to collapse and form stars. The cloud properties in NGC 1313 and NGC 7793 suggest that spiral arms, bars, interarm regions, and flocculent spirals each represent distinct environments with regard to molecular cloud populations. We see surprisingly little difference in surface density between the regions, suggesting that the differences in surface densities frequently seen between arm and interarm regions in lower-resolution studies are indicative of the sparsity of molecular clouds, rather than differences in their true surface density.

We analyze the far-ultraviolet (1130−1770 Å rest frame) spectroscopy of 20 young (<50 Myr) and massive (>10⁴M_⊙) star clusters (YSCs) in 11 nearby star-forming galaxies. We probe the interstellar gas intervening along the line of sight, detecting several metal absorption lines of a wide range of ionization potentials, from 6.0 to 77.5 eV. Multiple-component Voigt fits to the absorption lines are used to study the kinematics of the gas. We find that nearly all targets in the sample feature gas outflowing from 30 up to 190 km s⁻¹, often in both the neutral and ionized phases. The outflow velocities correlate with the underlying stellar population properties directly linked to the feedback: the mass of the YSCs, the photon production rate, and the instantaneous mechanical luminosity produced by stellar winds and supernovae. We detect a neutral inflow in four targets, which we interpret as likely not associated with the star cluster but tracing larger-scale gas kinematics. A comparison between the outflows' energy and that produced by the associated young stellar populations suggests an average coupling efficiency of 10% with a broad scatter. Our results extend the relation found in previous works between galactic outflows and the host galaxy star formation rate to smaller scales, pointing toward the key role that clustered star formation and feedback play in regulating galaxy growth.

Context. Recent James Webb Space Telescope (JWST) spectroscopic observations of the z = 10.6 galaxy GN-z11 have revealed a very peculiar UV spectrum exhibiting intense emission lines of nitrogen, which are not typically detected in galaxy spectra. This observation indicates a super-solar N/O abundance ratio at low metallicity, which only resembles the abundances seen in globular cluster (GC) stars. This discovery suggests that we might be seeing proto-GCs in formation or possibly even signatures of supermassive stars.

Aims. To examine whether other objects with strong N IV and/or N III emission lines (N-emitters, hereafter) exist and to better understand their origin and nature, we have examined the available JWST spectra and data from the literature.

Methods. Using the NIRSpec/JWST observations from CEERS, we found an extreme N-emitter, CEERS-1019 at z = 8.6782, showing intense N IV] λ1486 and N III] λ1750 emission. From the observed rest-UV and optical lines, we conclude that it is compatible with photoionization from stars and we have determined accurate abundances for C, N, O, and Ne, relative to H. We also (re-)analyzed other N-emitters from the literature, including three lensed objects at z = 2.3 − 3.5 (Sunburst cluster, SMACS2031, and Lynx arc) and a low-redshift compact galaxy, Mrk 996. We carried out a comparison among the observed abundance ratios to observations from normal star-forming galaxies, predicted wind yields from massive stars, and predictions from supermassive stars (SMS with ∼10⁴ − 10⁵M_⊙).

Results. For CEERS-1019, we find a highly supersolar ratio log(N/O)= − 0.18 ± 0.11, and abundances of log(C/O)= − 0.75 ± 0.11 and log(Ne/O)= − 0.63 ± 0.07, which are normal compared to other galaxies at the low metallicity (12 + log(O/H) = 7.70 ± 0.18) of this galaxy. The three lensed N-emitters also show strongly enhanced N/O ratios and two of them normal C/O. The high N/O abundances can be reproduced by massive star winds assuming a special timing and essentially no dilution with the ambient interstellar medium (ISM). Alternatively, these N/O ratios can be explained by mixing the ejecta of SMS with comparable amounts of unenriched ISM. Massive star ejecta (from WR stars) are needed to explain the galaxies with enhanced C/O (Lynx arc, Mrk 996). On the other hand, a SMS in the “conveyer-belt model” (put forward to explain globular clusters) would predict a high N/O and small changes in C/O, compatible with CEERS-1019, the Sunburst cluster, SMACS2031, and GN-z11. Based on the chemical abundances, possible enrichment scenarios, and other properties (e.g., their compactness and high ISM density), we discuss which objects could contain proto-GCs. We suggest that this is the case for CEERS-1019, SMACS2031, and the Sunburst cluster. Enrichment in the Lynx arc and Mrk 996 is likely due to normal massive stars (WR), which implies that the star-forming regions in these objects cannot become GCs. Finally, we propose that some N-emitters enriched by SMS could also have formed intermediate mass black holes and we suggest that this might be the case for GN-z11.

Conclusions. Our observations and analysis reinforce the suggested link between some N-emitters and proto-GC formation, which is supported both by empirical evidence and quantitative models. Furthermore, the observations provide possible evidence for the presence of supermassive stars in the early Universe (z >  8) and at z ∼ 2 − 3. Our analysis also suggests that the origin and nature of the N-emitters is diverse, including objects such as GN-z11, which may possibly host an active galactic nucleus (AGN).

We study the populations of stellar clumps in three high-redshift galaxies, at z = 4.92, 4.88, and 4.03, gravitationally lensed by the foreground galaxy clusters MS1358, RCS0224, and MACS0940, respectively. The lensed galaxies consist of multiple counter-images with large magnifications, mostly above 𝜇>5 and in some cases reaching 𝜇>20⁠. We use rest-frame UV observations from the HST to extract and analyse their clump populations, counting 10, 3, and 11 unique sources, respectively. Most of the clumps have derived effective radii in the range 𝑅_eff=10−100 pc, with the smallest one down to 6 pc, i.e. consistent with the sizes of individual stellar clusters. Their UV magnitudes correspond to SFR_UV mostly in the range 0.1−1 M_⊙yr⁻¹⁠; the most extreme ones, reaching SFR_UV=5 M_⊙yr⁻¹ are among the UV-brightest compact (⁠𝑅_eff<100 pc) star-forming regions observed at any redshift. Clump masses span a broad range from 10⁶ to 10⁹M_⊙⁠; stellar mass surface densities are comparable and in many cases larger than the ones of local stellar clusters, while being typically 10 times larger in size. By compiling published properties of clump populations at similar spatial resolution between redshifts 0 and 5, we find a tentative evolution of Σ_SFR and Σ_𝑀⋆ with redshift, especially when very compact clumps (⁠𝑅_eff⩽20 pc) are considered. We suggest that these trends with redshift reflect the changes in the host galaxy environments where clumps form. Comparisons with the local universe clumps/star clusters shows that, although rare, conditions for elevated clump Σ_SFR and Σ_𝑀⋆ can be found.

We present JWST/Near Infrared Spectrograph (NIRSpec) integral field spectroscopy (IFS) of a lensed Population III candidate stellar complex (dubbed Lensed And Pristine 1, LAP1), with a lensing-corrected stellar mass of ≲10⁴ M_⊙ and an absolute luminosity of M_UV > −11.2 (m_UV > 35.6), confirmed at redshift 6.639 ± 0.004. The system is strongly amplified (μ ≳ 100) by straddling a critical line of the Hubble Frontier Field galaxy cluster MACS J0416. Although the stellar continuum is currently not detected in the Hubble and JWST/Near Infrared Camera (NIRCam) and Near Infrared Imager and Slitless Spectrograph (NIRISS) imaging, arclet-like shapes of Lyman and Balmer lines, Lyα, Hγ, Hβ and Hα are detected with NIRSpec IFS with signal-to-noise ratios (S/N) of approximately 5 − 13 and large equivalent widths (> 300 − 2000 Å), along with a remarkably weak [O III]λλ4959, 5007 at S/N ≃ 4. LAP1 shows a large ionizing photon production efficiency, log(ξ_ion[erg Hz⁻¹]) > 26. From the metallicity indexes R23 = ([O III] + [O II])/Hβ ≲ 0.74 and R3 = ([O III]/Hβ) = 0.55 ± 0.14, we derive an oxygen abundance of 12 + log(O/H)≲6.3. Intriguingly, the Hα emission is also measured in mirrored subcomponents where no [O III] is detected, providing even more stringent upper limits on the metallicity if in situ star formation is ongoing in this region (12 + log(O/H) < 6). The formal stellar mass limit of the subcomponents would correspond to ∼10³ M_⊙ or M_UV fainter than −10. Alternatively, this metal-free, pure line-emitting region could be the first case of a fluorescing H I gas region induced by transverse escaping ionizing radiation from a nearby star complex. The presence of large equivalent-width hydrogen lines and the deficiency of metal lines in such a small region make LAP1 the most metal-poor star-forming region currently known in the reionization era and a promising site that may host isolated, pristine stars.

The ionizing radiation of young and massive stars is a crucial form of stellar feedback. Most ionizing (Lyman-continuum; LyC, λ < 912Å) photons are absorbed close to the stars that produce them, forming compact H ii regions, but some escape into the wider galaxy. Quantifying the fraction of LyC photons that escape is an open problem. In this work, we present a seminovel method to estimate the escape fraction by combining broadband photometry of star clusters from the Legacy ExtraGalactic UV Survey (LEGUS) with H ii regions observed by the Star formation, Ionized gas, and Nebular Abundances Legacy Survey (SIGNALS) in the nearby spiral galaxy NGC 628. We first assess the completeness of the combined catalogue, and find that 49  per cent of H ii regions lack corresponding star clusters as a result of a difference in the sensitivities of the LEGUS and SIGNALS surveys. For H ii regions that do have matching clusters, we infer the escape fraction from the difference between the ionizing power required to produce the observed H ii luminosity and the predicted ionizing photon output of their host star clusters; the latter is computed using a combination of LEGUS photometric observations and a stochastic stellar population synthesis code slug (Stochastically Lighting Up Galaxies). Overall, we find an escape fraction of f_esc= across our sample of 42 H ii regions; in particular, we find H ii regions with high f_esc are predominantly regions with low Hα-luminosity. We also report possible correlation between f_esc and the emission lines [OII]/[NII] and [OII]/Hβ⁠.

Multiwavelength images from the Hubble Space Telescope covering the wavelength range 0.27–1.6 μm show that the central area of the nearby dwarf galaxy NGC 4449 contains several tens of compact sources that are emitting in the hydrogen recombination line Paβ (1.2818 μm) but are only marginally detected in Hα (0.6563 μm) and undetected at wavelengths λ ≤ 0.55 μm. An analysis of the spectral energy distributions (SEDs) of these sources indicates that they are likely relatively young stellar clusters heavily attenuated by dust. The selection function used to identify the sources prevents meaningful statistical analyses of their age, mass, and dust extinction distributions. However, these cluster candidates have ages ∼5–6 Myr and A_V > 6 mag, according to their SED fits, and are extremely compact, with typical deconvolved radii of 1 pc. The dusty clusters are located at the periphery of the dark clouds within the galaxy and appear to be partially embedded. Density and pressure considerations indicate that the H ii regions surrounding these clusters may be stalled, and that pre-supernova (pre-SN) feedback has not been able to clear the clusters of their natal cocoons. These findings are in potential tension with existing models that regulate star formation with pre-SN feedback, since pre-SN feedback acts on short timescales, ≲4 Myr, for a standard stellar initial mass function. The existence of a population of dusty stellar clusters with ages >4 Myr, if confirmed by future observations, paints a more complex picture for the role of stellar feedback in controlling star formation.

We study the young star cluster populations in 23 dwarf and irregular galaxies observed by the Hubble Space Telescope (HST) Legacy ExtraGalactic Ultraviolet Survey (LEGUS), and examine relationships between the ensemble properties of the cluster populations and those of their host galaxies: star formation rate (SFR) density (Σ_SFR). A strength of this analysis is the availability of SFRs measured from temporally resolved star formation histories that provide the means to match cluster and host galaxy properties on several time-scales (1–10, 1–100, and 10–100 Myr). Nevertheless, studies of this kind are challenging for dwarf galaxies due to the small numbers of clusters in each system. We mitigate these issues by combining the clusters across different galaxies with similar Σ_SFR properties. We find good agreement with a well-established relationship (⁠M_V^brightest–SFR), but find no significant correlations between Σ_SFR and the slopes of the cluster luminosity function, mass function, nor the age distribution. We also find no significant trend between the fraction of stars in bound clusters at different age ranges (Γ_1–10, Γ_10–100, and Γ_1–100) and Σ_SFR of the host galaxy. Our data show a decrease in Γ over time (from 1–10 to 10–100 Myr) suggesting early cluster dissolution, though the presence of unbound clusters in the youngest time bin makes it difficult to quantify the degree of dissolution. While our data do not exhibit strong correlations between Σ_SFR and ensemble cluster properties, we cannot rule out that a weak trend might exist given the relatively large uncertainties due to low number statistics and the limited Σ_SFR range probed.

Current observations favour that the massive ultraviolet-bright clumps with a median stellar mass of ∼10⁷M_⊙, ubiquitously observed in z ∼ 1–3 galaxies, are star-forming regions formed in situ in galaxies. It has been proposed that they result from gas fragmentation due to gravitational instability of gas-rich, turbulent, and high-redshift discs. We bring support to this scenario by reporting the new discovery of giant molecular clouds (GMCs) in the strongly lensed, clumpy, main-sequence galaxy, A521-sys1, at z = 1.043. Its CO(4–3) emission was mapped with the Atacama Large Millimetre/submillimetre Array (ALMA) at an angular resolution of 0.19 × 0.16 arcsec², reading down to 30 pc, thanks to gravitational lensing. We identified 14 GMCs, most being virialized, with 10^5.9−10^7.9M_⊙ masses and a median 800M_⊙ pc⁻² molecular gas mass surface density, that are, respectively, 100 and 10 times higher than for nearby GMCs. They are also characterized by 10 times higher supersonic turbulence with a median Mach number of 60. They end up to fall above the Larson scaling relations, similarly to the GMCs in another clumpy z ≃ 1 galaxy, the Cosmic Snake, although differences between the two sets of high-redshift GMCs exist. Altogether they support that GMCs form with properties that adjust to the ambient interstellar medium conditions prevalent in the host galaxy whatever its redshift. The detected A521-sys1 GMCs are massive enough to be the parent gas clouds of stellar clumps, with a relatively high star formation efficiency per free-fall time of ∼11 per cent.