We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally lensed galaxy (μ = 17 - 21) at a redshift of 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is a typical compact and UV-faint (MUV = - 17.8) Lyman-α emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical Hα, Hβ, and [OIII]λλ4959, 5007 lines are spatially resolved with the high-spectral-resolution grating (G395H, R 2700), with large equivalent widths, EW(Hβ+OIII) ≳ 1000 Å, and elevated ionizing photon production efficiencies, log(ξion/erg- 1Hz) = 25.2-25.7. NIRCam deep imaging reveals the presence of compact rest-UV-bright regions along with individual star clusters of Reff = 3 - 8 pc in size and M ~ 2 · 105 - 5 · 106 M⊙ in mass. These clusters are characterized by steep UV slopes, βUV ≲ - 2.5, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-α emission peaking at ~100 km s- 1 from the systemic redshift. While the entire system is characterized by low metallicity, ~0.1 Z⊙, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with Z ≲ 2% Z⊙, which is barely detected in NIRCam imaging; this region is displaced by > 200 pc from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

Context. Recent James Webb Space Telescope observations detected a system of stellar clumps around the z ≃ 1.4 gravitationally lensed Sparkler galaxy (of stellar mass M_* ∼ 10⁹ M_⊙) with ages and metallicities compatible with globular cluster (GC) progenitors. However, most of their masses (> 10⁶ M_⊙) and sizes (> 30 pc) are about ten times those of GCs in the local Universe.

Aims. To assess whether these clumps can evolve into GC-like objects, we performed N-body simulations of their dynamical evolution from z ≃ 1.4 to z = 0 (∼9.23 Gyr) under the effect of dynamical friction and tidal stripping.

Methods. We studied dynamical friction by performing multiple runs of a system of clumps in a Sparkler-like spherical halo of mass M₂₀₀ ≃ 5 × 10¹¹ M_⊙, that was inferred from the stellar-to-halo mass relation. For the tidal stripping, we simulated resolved clumps orbiting in an external static gravitational potential including the same halo as in the dynamical friction simulations and a Sparkler-like stellar disc.

Results. Dynamical friction causes the clumps with a mass greater than 10⁷ M_⊙ to sink into the central galaxy regions, possibly contributing to the bulge growth. In absence of tidal stripping, the mass distribution of the surviving clumps (≈40%) peaks at ≈5 × 10⁶ M_⊙, implying the presence of uncommonly over-massive clumps at z = 0. Tidal shocks from the stellar disc strip considerable mass from low-mass clumps, but their sizes remain larger than those of present-day GCs. When the surviving clump masses are corrected for tidal stripping, their distribution peak shifts to ∼2 × 10⁶ M_⊙, that is compatible with very massive GCs.

Conclusions. Our simulations suggest that a fraction of the Sparkler clumps is expected to fall into the central regions, where they might become bulge fossil fragments or contribute to the formation of a nuclear star cluster. The remaining clumps are too large in size to be progenitors of GCs.

We present a sample of 1956 individual stellar clumps at redshift 0.7<z<10⁠, detected with JWST/NIRCam in 476 galaxies lensed by the galaxy cluster Abell2744. The lensed clumps present magnifications ranging between μ = 1.8 and μ = 300. We perform simultaneous size-photometry estimates in 20 JWST/NIRCam median and broad-band filters from 0.7 to 5 μm. Spectral energy distribution (SED) fitting analyses enable us to recover the physical properties of the clumps. The majority of the clumps are spatially resolved and have effective radii in the range R_eff=10−700 pc⁠. We restrict this first study to the 1751 post-reionization era clumps with redshift <5.5⁠. We find a significant evolution of the average clump ages, star formation rates (SFRs), SFR surface densities, and metallicity with increasing redshift, while median stellar mass and stellar mass surface densities are similar in the probed redshift range. We observe a strong correlation between the clump properties and the properties of their host galaxies, with more massive galaxies hosting more massive and older clumps. We find that clumps closer to their host galactic centre are on average more massive, while their ages do not show clear sign of migration. We find that clumps at cosmic noon sample the upper-mass end of the mass function to higher masses than at z>3⁠, reflecting the rapid increase towards the peak of the cosmic star formation history. We conclude that the results achieved over the studied redshift range are in agreement with expectation of in situ clump formation scenario from large-scale disc fragmentation.

The Cosmic Gems arc is among the brightest and highly magnified galaxies observed at redshift z ≈ 10.2 (ref. 1). However, it is an intrinsically ultraviolet faint galaxy, in the range of those now thought to drive the reionization of the Universe2–4. Hitherto the smallest features resolved in a galaxy at a comparable redshift are between a few hundreds and a few tens of parsecs (pc)5,6. Here we report JWST observations of the Cosmic Gems. The light of the galaxy is resolved into five star clusters located in a region smaller than 70 pc. They exhibit minimal dust attenuation and low metallicity, ages younger than 50 Myr and intrinsic masses of about 106M⊙. Their lensing-corrected sizes are approximately 1 pc, resulting in stellar surface densities near 105M⊙ pc−2, three orders of magnitude higher than typical young star clusters in the local Universe7. Despite the uncertainties inherent to the lensing model, they are consistent with being gravitationally bound stellar systems, that is, proto-globular clusters. We conclude that star cluster formation and feedback likely contributed to shaping the properties of galaxies during the epoch of reionization.

Context. Faint, star-forming galaxies are likely to play a dominant role in cosmic reionisation. Great strides have been made in recent years to characterise these populations at high redshifts (z > 3). Now, for the first time, with JWST photometry beyond 1 μm in the rest frame, we can derive accurate stellar masses and position these galaxies on the galaxy main sequence.

Aims. We seek to assess the place of 96 individual Lyman-α emitters (LAEs) selected behind the A2744 lensing cluster with MUSE IFU spectroscopy on the galaxy main sequence. We also compare the derived stellar masses to Lyman-α luminosities and equivalent widths to better quantify the relationship between the Lyman-α emission and the host galaxy.

Methods. These 96 LAEs lie in the redshift range of 2.9 < z < 6.7, with their range of masses extending down to 10⁶ M_⊙ (over half with M_⋆ < 10⁸ M_⊙). We used the JWST/NIRCam and HST photometric catalogues from the UNCOVER project, giving us excellent wavelength coverage from 450 nm to 4.5 μm. We also performed an SED fitting using CIGALE, fixing the redshift of the LAEs to the secure, spectroscopic value. This combination of photometric coverage with spectroscopic redshifts allows us to robustly derive stellar masses for these galaxies.

Results. We found a main sequence relation for these low-mass LAEs of log SFR = (0.88 ± 0.07 − 0.030 ± 0.027 × t) log M_⋆ − (6.31 ± 0.41 − 0.08 ± 0.37 × t). This is in relative agreement with the best-fit results of prior collated studies; however, here we see a steeper slope and a higher normalisation. This indicates that low-mass LAEs towards the epoch of reionisation lie above the typical literature main sequence relations derived at lower redshift and higher masses. In addition, by comparing our results to UV-selected samples, we can see that while low-mass LAEs lie above these typical main sequence relations, they are likely not singular in this respect at these particular masses and redshifts. While low-mass galaxies have been shown to play a significant role in cosmic reionisation, our results point to the likelihood that LAEs hold no special position in this regard.

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.

BlueMUSE is a blue-optimised, medium spectral resolution, panoramic integral field spectrograph under development for the Very Large Telescope (VLT).With an optimised transmission down to 350 nm, spectral resolution of R∼3500 on average across the wavelength range, and a large FoV (1 arcmin2), BlueMUSE will open up a new range of galactic and extragalactic science cases facilitated by its specific capabilities.The BlueMUSE consortium includes 9 institutes located in 7 countries and is led by the Centre de Recherche Astrophysique de Lyon (CRAL).The BlueMUSE project development is currently in Phase A, with an expected first light at the VLT in 2031.We introduce here the Top Level Requirements (TLRs) derived from the main science cases, and then present an overview of the BlueMUSE system and its subsystems fulfilling these TLRs.We specifically emphasize the tradeoffs that are made and the key distinctions compared to the MUSE instrument, upon which the system architecture is built.

Context. The circumgalactic medium (CGM) serves as a baryon reservoir that connects galaxies to the intergalactic medium and fuels star formation. The spatial distribution of the metal-enriched cool CGM has not yet been directly revealed at cosmic noon (z ≃ 2–4), as bright emission lines at these redshifts are not covered by optical integral field units.

Aims. To remedy this situation, we performed the first-ever detections and exploration of extended Si II* emission in the low-ionization state (LIS), referred to as Si II* halos, at redshifts ranging from z = 2 to 4 as a way to trace the metal-enriched cool CGM.

Methods. We used a sample of 39 galaxies with systemic redshifts of z = 2.1–3.9 measured with the [C III] doublet in the MUSE Hubble Ultra Deep Field catalog, whose integration times span from ≃30 to 140 hours. We searched for extended Si II* λ1265, 1309, 1533 emission (fluorescent lines) around individual galaxies. We also stacked a subsample of 14 UV-bright galaxies.

Results. We report five individual detections of Si II* λ1533 halos. We also confirm the presence of Si II* λ1533 halos in stacks for the subsample containing UV-bright sources. The other lines do not show secure detections of extended emission in individual or in stacking analyses. These detections may imply that the presence of metal-enriched CGM is a common characteristic for UV-bright galaxies. To investigate whether the origin of Si II* is continuum pumping, as suggested in previous studies, we checked the consistency of the equivalent width (EW) of Si II* emission and the EW of Si II absorption for the individual halo object with the most reliable detection. We confirm the equivalence, suggesting that photon conservation works for this object and points toward continuum pumping as the source of Si II*. We also investigated Si II* lines in a RAMSES-RT zoom-in simulation including continuum pumping, and find the ubiquitous presence of extended halos.

We present a new method to simultaneously and self-consistently model the mass distribution of galaxy clusters that combines constraints from strong lensing features, X-ray emission, and galaxy kinematics measurements. We are able to successfully decompose clusters into their collisionless and collisional mass components thanks to the X-ray surface brightness, as well as use the dynamics of cluster members, to obtain more accurate masses exploiting the fundamental plane of elliptical galaxies. Knowledge from all observables is included through a consistent Bayesian approach in the likelihood or in physically motivated priors. We apply this method to the galaxy cluster Abell S1063 and produce a mass model that we publicly release with this paper. The resulting mass distribution presents different ellipticities for the intra-cluster gas and the other large-scale mass components as well as deviation from elliptical symmetry in the main halo. We assess the ability of our method to recover the masses of the different elements of the cluster using a mock cluster based on a simplified version of our Abell S1063 model. Thanks to the wealth of mutliwavelength information provided by the mass model and the detected X-ray emission, we also found evidence for an ongoing merger event with gas sloshing from a smaller infalling structure into the main cluster. In agreement with previous findings, the total mass, gas profile, and gas mass fraction are all consistent with small deviations from the hydrostatic equilibrium. This new mass model for Abell S1063 is publicly available, as the LENSTOOL extension used to construct it.

Context. Faint galaxies are theorised to have played a major role, perhaps the dominant role, in reionising the Universe. Their properties, as well as the Lyman-α emitter (LAE) fraction, X_LAE, could provide useful insights into this epoch.

Aims. We used four clusters of galaxies from the Lensed Lyman-alpha MUSE Arcs Sample (LLAMAS) that also have deep HST photometry to select a population of intrinsically faint Lyman break galaxies (LBGs) and LAEs. We study the interrelation between these two populations, their properties, and the fraction of LBGs that display Lyman-α emission.

Methods. The use of lensing clusters allows us to access an intrinsically faint population of galaxies, the largest such sample collected for this purpose: 263 LAEs and 972 LBGs with redshifts between 2.9 and 6.7, Lyman-α luminosities in the range 39.5 ≲ log(L_Lyα)(erg s⁻¹)≲42, and absolute UV magnitudes in the range −22 ≲ M₁₅₀₀ ≲ −12. In addition to matching LAEs and LBGs, we define an LAE+continuum sample for the LAEs that match with a continuum object that is not selected as an LBG. Additionally, with the use of MUSE integral field spectroscopy, we detect a population of LAEs completely undetected in the continuum.

Results. We find a redshift evolution of X_LAE in line with literature results, with diminished values above z = 6. In line with past studies, we take this as signifying an increasingly neutral intervening intergalactic medium. When inspecting this redshift evolution with different limits on EW_Lyα and M₁₅₀₀, we find that the X_LAE for the UV-brighter half of our sample is higher than the X_LAE for the UV-fainter half, a difference that increases at higher redshifts. This is a surprising result and can be interpreted as the presence of a population of low Lyman-α equivalent width (EW_Lyα), UV-bright galaxies situated in reionised bubbles and overdensities. This result is especially interesting in the context of similar, UV-bright, low EW_Lyα objects recently detected during and around the epoch of reionisation. For intrinsically fainter objects, we confirm the previously observed trend of LAEs among LBGs as galaxies with high star formation rates and low dust content, as well as the trend of the strongest LAEs having, in general, fainter M₁₅₀₀ and steeper UV slopes.