This paper presents a deep MIRI/JWST medium-resolution spectroscopy (MRS) covering the rest-frame optical spectrum of the GN-z11 galaxy. The [O III] 5008 Å and Hα emission lines are detected and spectroscopically resolved. The line profiles are well modeled by a narrow Gaussian component with intrinsic full widths at half maximum of 189 ± 25 and 231 ± 52 km s-1, respectively. We do not find any evidence of a dominant broad Hα emission line component tracing a broad-line region in a type 1 active galactic nucleus (AGN). The existence of an accreting black hole dominating the optical continuum and emission lines of GN-z11 is not compatible with the measured Hα and [O III] 5008 Å luminosities. If the well-established relations for low-z AGNs apply in GN-z11, the [O III] 5008 Å and Hα luminosities would imply extremely high super-Eddington ratios (λE > 290), and bolometric luminosities ∼20 times those derived from the UV/optical continuum. However, a broad (∼430-470 km s-1) and weak (< 20-30%) Hα line component, tracing a minor AGN contribution in the optical, cannot be completely ruled out with the sensitivity of the current data. The physical and excitation properties of the ionized gas are consistent with a low-metallicity starburst with a star formation rate of 24 ± 3 M⊙ yr-1. The electron temperature of the ionized gas is Te (O++) = 14 000 ± 2100 K, while the direct-Te gas-phase metallicity is 12 + log(O/H) = 7.91 ± 0.07 (Z = 0.17 ± 0.03 Z⊙). The optical line ratios locate GN-z11 in the starburst or AGN region, but they are more consistent with those of local low-metallicity starbursts and high-z luminous galaxies detected at redshifts similar to GN-z11. We conclude that the MRS optical spectrum of GN-z11 is consistent with that of a massive, compact, and low-metallicity starburst galaxy. Its high star formation and stellar mass surface densities are close to those of the densest stellar clusters, and we therefore speculate that GN-z11 might undergo a feedback-free, highly efficient starburst phase. Additional JWST data are needed to validate this scenario and other recently proposed alternatives to explain the existence of bright compact galaxies in the early Universe.

Context. Thanks to decades of observations using the Hubble Space Telescope (HST), the structure of galaxies at redshift z>2 has been widely studied in the rest-frame ultraviolet regime, which traces recent star formation from young stellar populations. However, we still have little information about the spatial distribution of the older, more evolved stellar populations, constrained by the rest-frame infrared portion of the galaxies’ spectral energy distribution.

Aims. We present the morphological characterization of a sample of 49 massive galaxies (log(M_⋆/M_⊙)>9) at redshift 3<z<5. These galaxies are observed as part of the MIRI Deep Imaging Survey (MIDIS), the guaranteed time observations program with the MIRI instrument on board the James Webb Space Telescope (JWST). Deep MIRI 5.6 μm imaging (28.64 mag 5σ depth) allows us to characterize the rest-frame near-infrared structure of galaxies beyond cosmic noon, at higher redshifts than possible with NIRCam, tracing their older and dust-insensitive stellar populations.

Methods. We derived the nonparametric morphology of galaxies, focusing on the Gini, M₂₀, concentration, asymmetry, and deviation statistics. Furthermore, we modeled the light distribution of galaxies with a single Sérsic component and derived their parametric morphology (i.e., effective radius and Sérsic index).

Results. We find that at z>3 massive galaxies show a smooth distribution of their rest-infrared light, strongly supporting the increasing number of regular disk galaxies already in place at early epochs. These results are further reinforced by the analysis of JWST/NIRCam data at 4.4 μm. On the contrary, the ultraviolet structure obtained from HST/WFC3 and JWST/NIRCam observations at ∼1.5 μm is generally more irregular, catching the most recent episodes of star formation. Importantly, we find a segregation of morphologies across cosmic time, where galaxies at redshift z>3.75 show later-type morphologies compared to z∼3 galaxies. These findings suggest a transition phase in galaxy assembly and central mass build-up, which takes place already at z∼3−4.

Conclusions. The combined analysis of NIRCam and MIRI imaging datasets allows us to prove that the rest-frame near-infrared morphology of massive galaxies at cosmic noon is typical of compact disk galaxies with a smooth mass distribution.

Context. This paper investigates the star formation histories (SFHs) of a sample of massive galaxies (M_⋆ ≥ 10¹⁰ M_⊙) in the redshift range 1 < z < 4.5.

Methods. We analyzed spectro-photometric data, combining broadband photometry from HST and JWST with low-resolution grism spectroscopy from JWST/NIRISS, obtained as part of the MIRI Deep Imaging Survey program. SFHs were derived through spectral energy distribution fitting using two independent codes, BAGPIPES and synthesizer, under various SFH assumptions. This approach enables a comprehensive assessment of the biases introduced by different modeling choices.

Results. The inclusion of NIRISS spectroscopy, even with its low resolution, significantly improves constraints on key physical parameters, such as the mass-weighted stellar age (t_M) and formation redshift (z_form), by narrowing their posterior distributions. The massive galaxies in our sample exhibit rapid stellar mass assembly, forming 50% of their mass between 3 ≤ z ≤ 9. The highest inferred formation redshifts are compatible with elevated star formation efficiencies (ϵ) at early epochs. Nonparametric SFHs generally imply an earlier and slower mass assembly compared to parametric forms, highlighting the sensitivity of inferred formation timescales to the chosen SFH model–particularly for galaxies at z < 2. We find that quiescent galaxies are, on average, older (t_M ∼ 1.1 Gyr) and assembled more rapidly at earlier times than their star-forming counterparts. These findings support the “downsizing” scenario, in which more massive and passive systems form earlier and more efficiently.

Context. The recently launched James Webb Space Telescope (JWST) is opening new observing windows on the distant Universe. Among JWST’s instruments, the Mid Infrared Instrument (MIRI) offers the unique capability of imaging observations at wavelengths of λ > 5 μm. This enables unique access to the rest frame near-infrared (NIR, λ ≥ 1 μm) emission from galaxies at redshifts of z > 4 and the visual (λ ≳ 5000 Å) rest frame for z > 9. We report here on the guaranteed time observations (GTO), from the MIRI European Consortium, of the Hubble Ultra Deep Field (HUDF), forming the MIRI Deep Imaging Survey (MIDIS), consisting of an on source integration time of ∼41 hours in the MIRI/F560W (5.6 μm) filter. The F560W filter was selected since it would produce the deepest data in terms of AB magnitudes in a given time. To our knowledge, this constitutes the longest single filter exposure obtained with JWST of an extragalactic field as of yet.

Aims. The HUDF is one of the most observed extragalactic fields, with extensive multi-wavelength coverage, where (before JWST) galaxies up to z ∼ 7 have been confirmed, and at z > 10 suggested, from HST photometry. We aim to characterise the galaxy population in HUDF at 5.6 μm, enabling studies such as: the rest frame NIR morphologies for galaxies at z ≲ 4.6, probing mature stellar populations and emission lines in z > 6 sources, intrinsically red and dusty galaxies, and active galactic nuclei (AGNs) and their host galaxies at intermediate redshifts.

Methods. We reduced the MIRI data using the official JWST pipeline, augmented by in-house custom scripts. We measured the noise characteristics of the resulting image. Galaxy photometry was obtained, and photometric redshifts were estimated for sources with available multi-wavelength photometry (and compared to spectroscopic redshifts when available).

Results. Over the deepest part of our image, the 5σ point source limit is 28.65 mag AB (12.6 nJy), ∼0.35 mag better than predicted by the JWST exposure time calculator. We find ∼2500 sources, the overwhelming majority of which are distant galaxies, but we note that spurious sources likely remain at faint magnitudes due to imperfect cosmic ray rejection in the JWST pipeline. More than 500 galaxies with available spectroscopic redshifts, up to z ≈ 11, have been identified, the majority of which are at z < 6. More than 1000 galaxies have reliable photometric redshift estimates, of which ∼25 are at 6 < z < 12. The point spread function in the F560W filter has a full width at half maximum (FWHM) of ≈0.2″ (corresponding to 1.4 kpc at z = 4), allowing the NIR rest frame morphologies and stellar mass distributions to be resolved for z < 4.5. Moreover, > 100 objects with very red NIRCam vs MIRI (3.6–5.6 μm > 1 mag) colours have been found, suggestive of dusty or old stellar populations at high redshifts.

Conclusions. We conclude that MIDIS surpasses preflight expectations and that deep MIRI imaging has great potential to characterise the galaxy population from cosmic noon to dawn.

We present MIRI/JWST medium-resolution spectroscopy (MRS) and imaging (MIRIM) of B14-65666, a source identified as a Lyman-break and interacting galaxy at a redshift of z = 7.15. We detect the Hα line emission in this system, revealing a spatially resolved structure of the Hα-emitting gas, which consists of two distinct galaxies, E and W, at a projected distance of 0.4 arcsec apart (i.e., 2.2 kpc). One of the galaxies (E) is very compact (upper limit for the effective radius of 63 pc) in the rest-frame ultraviolet light, while the other galaxy (W) is more extended (effective radius of 348 pc), showing a clumpy structure reminiscent of a tidal tail. The total Hα luminosity implies that the system is forming stars at a rate of 76 ± 8 M_⊙ yr⁻¹ and 30 ± 4 M_⊙ yr⁻¹ for E and W galaxies, respectively. The ionizing photon production efficiency, log(ζ_ion), for galaxies E and W, has values of 25.1 ± 0.1 Hz erg⁻¹ and 25.5 ± 0.1 Hz erg⁻¹, which is within the range measured in galaxies at similar redshifts. The high values derived for the Hα equivalent widths (832 ± 100 and 536 ± 78 Å) and the distinct locations of the E and W galaxies in the log(ζ_ion) – equivalent width (Hα) plane indicate that the system is dominated by a young (under 10 Myr) stellar population. The overall spectral-energy distribution suggests that in addition to a young stellar population, the two galaxies may have mature (over 100 Myr) stellar populations and very different dust attenuations, with galaxy E showing a larger attenuation (A_V = 1.5 mag) compared to the almost dust-free (A_V = 0.1 mag) galaxy W. The derived star formation rate (SFR) and stellar masses identify the two galaxies as going through a starburst phase characterized by a specific SFR (sSFR) of 40–50 Gyr⁻¹. Galaxy E has an extreme stellar mass surface density (6 × 10⁴ M_⊙ pc⁻²), close to that of the nuclei of low-z galaxies, while galaxy W (10³ M_⊙ pc⁻²) is consistent with the surface densities measured in galaxies at these redshifts. The kinematics of the ionized gas traced by the Hα line show a velocity difference of 175 ± 28 km s⁻¹ between the two components of B14-65666 and a broader profile for galaxy W (312 ± 44 km s⁻¹) relative to galaxy E (243 ± 41 km s⁻¹). The detailed study of B14-65666 shows that the complex stellar and interstellar medium structure in merging galaxy systems was already in place by the Epoch of Reionization. The general properties of B14-65666 agree with those predicted for massive merging systems at redshifts of 7 and above in the FIRSTLIGHT cosmological simulations. The in-depth study of systems such as B14-65666 reveal how galaxy mergers in the early Universe drive intense star formation, shape the interstellar medium, and influence the buildup of stellar mass, just 700–800 Myr after the Big Bang.

Recent years have seen a surge of interest in the community studying the effect of ultraviolet radiation environment, predominantly set by OB stars, on protoplanetary disc evolution and planet formation. This is important because a significant fraction of planetary systems, potentially including our own, formed in close proximity to OB stars. This is a rapidly developing field, with a broad range of observations across many regions recently obtained or recently scheduled. In this paper, stimulated by a series of workshops on the topic, we take stock of the current and upcoming observations. We discuss how the community can build on this recent success with future observations to make progress in answering the big questions of the field, with the broad goal of disentangling how external photoevaporation contributes to shaping the observed (exo)planet population. Both existing and future instruments offer numerous opportunities to make progress towards this goal.

Context. Our knowledge of the initial conditions of terrestrial planet formation is mainly based on the study of protoplanetary disks around nearby isolated low-mass stars. However, most young stars and therefore planetary systems form in high-mass star-forming regions and are exposed to ultraviolet radiation, affecting the protoplanetary disk. These regions are located at large distances and only now with JWST has it become accessible to study the inner disks surrounding young stars.

Aims. We present the eXtreme UV Environments (XUE) program, which provides the first detailed characterization of the physical and chemical properties of the inner disks around young intermediate-mass (1–4 M_⊙) stars exposed to external irradiation from nearby massive stars. We present high-signal-to-noise MIRI-MRS spectroscopy of 12 disks located in three subclusters of the high-mass star-forming region NGC 6357 (d ~ 1690 pc).

Methods. Based on their mid-infrared spectral energy distribution, we classified the XUE sources into Group I and II based on the Meeus scheme. We analyzed their molecular emission features, and compared their spectral indices and 10 μm silicate emission profiles to the ones of nearby Herbig and intermediate T Tauri (IMTT) disks.

Results. The XUE program provides the first detailed characterization of the rich molecular inventory in IMTT disks, including water, CO, CO₂, HCN, and C₂H₂. In the XUE sample, the detected emission likely originates from within 10 au, although this inner disk origin may not be typical for all externally irradiated disks. Despite being more massive, the XUE stars host disks with a molecular richness comparable to isolated T Tauri systems. The spectral indices are also consistent with similar-mass stars in nearby regions. The 10 μm silicate features in the XUE sample exhibit lower F_11.3/F_9.8 ratios at a given F_peak, suggesting that the disk surfaces may be dominated by smaller grains compared to nearby disks. However, uncertainties in extinction prevent us from drawing firm conclusions about their inner disk properties. The majority of disks display water emission from the inner disk, suggesting that even in these extreme environments rocky planets can form in the presence of water. Only one object shows PAH emission, contrasting with the higher PAH detection rates in IMTT surveys from lower-UV environments.

Conclusions. The absence of strong line fluxes and other irradiation signatures suggests that the XUE disks have been truncated by external UV photons. However, this truncation does not appear to significantly impact the chemical richness of their inner regions. These findings indicate that even in extreme environments, IMTT disks can retain the ingredients necessary for rocky planet formation, comparable to the ones of lower-mass T Tauri disks in low-mass star-forming regions.

Unveiling the physical structure of protoplanetary disks is crucial for interpreting the diversity of the exoplanet population. Until recently, the census of the physical properties of protoplanetary disks probed by mid-infrared observations was limited to the solar neighborhood (d ≲ 250 pc). However, nearby star-forming regions (SFRs) such as Taurus—where no O-type stars reside—are not representative of the environments where the majority of the planet formation occurs in the Galaxy. The James Webb Space Telescope (JWST) now enables observations of disks in distant high-mass SFRs, where strong external far-ultraviolet radiation is expected to impact those disks. Nevertheless, a detailed characterization of the population of externally irradiated disks is still lacking. We use the thermochemical code ProDiMo to model JWST/MIRI spectroscopy and archival visual/near-infrared photometry aiming to constrain the physical structure of the irradiated disk around the solar-mass star XUE 1 in NGC 6357 (d ≈ 1690 pc). Our findings are as follows. (1) Mid-infrared dust emission features are explained by amorphous and crystalline silicates with compositions similar to nearby disks. (2) The molecular features detected with MIRI originate within the first ∼1 au, consistent with results from slab models. (3) Our model favors a disk truncated at 10 au with a gas-to-dust ratio of unity in the outskirts. (4) Comparing models of the same disk structure under different irradiation levels, we find that strong external irradiation raises gas temperature tenfold and boosts water abundance beyond 10 au by a factor of 100.

The rapid assembly of the first supermassive black holes is an enduring mystery. Until now, it was not known whether quasar ‘feeding’ structures (the ‘hot torus’) could assemble as fast as the smaller-scale quasar structures. We present JWST/MRS (rest-frame infrared) spectroscopic observations of the quasar J1120+0641 at z = 7.0848 (well within the epoch of reionization). The hot torus dust was clearly detected at λrest ≃ 1.3 μm, with a black-body temperature of Tdust=1,413.5−7.4+5.7 K, slightly elevated compared to similarly luminous quasars at lower redshifts. Importantly, the supermassive black hole mass of J1120+0641 based on the Hα line (accessible only with JWST), MBH = 1.52 ± 0.17 × 109 M⊙, is in good agreement with previous ground-based rest-frame ultraviolet Mg ii measurements. Comparing the ratios of the Hα, Paα and Paβ emission lines to predictions from a simple one-phase Cloudy model, we find that they are consistent with originating from a common broad-line region with physical parameters that are consistent with lower-redshift quasars. Together, this implies that J1120+0641’s accretion structures must have assembled very quickly, as they appear fully ‘mature’ less than 760 Myr after the Big Bang.

We report the discovery of Cerberus, an extremely red object detected with the MIRI Deep Imaging Survey (MIDIS) observations in the F1000W filter of the Hubble Ultra Deep Field. The object is detected at signal-to-noise ratio (S/N) ∼ 6, with F1000W ∼ 27 mag, and undetected in the NIRCam data gathered by the JWST Advanced Deep Extragalactic Survey (JADES), fainter than the 30.0-30.5 mag 5σ detection limits in individual bands, as well as in the MIDIS F560W ultradeep data (∼29 mag, 5σ). Analyzing the spectral energy distribution built with low-S/N (<5) measurements in individual optical-to-mid-infrared filters and higher-S/N (≳5) measurements in stacked NIRCam data, we discuss the possible nature of this red NIRCam-dark source using a battery of codes. We discard the possibility of Cerberus being a solar system body based on the <0.″016 proper motion in the 1 yr apart JADES and MIDIS observations. A substellar Galactic nature is deemed unlikely, given that the Cerberus’s relatively flat NIRCam-to-NIRCam and very red NIRCam-to-MIRI flux ratios are not consistent with any brown dwarf model. The extragalactic nature of Cerberus offers three possibilities: (1) a z ∼ 0.4 galaxy with strong emission from polycyclic aromatic hydrocarbons—the very low inferred stellar mass, M ⋆ = 105-106 M ⊙, makes this possibility highly improbable; (2) a dusty galaxy at z ∼ 4 with an inferred stellar mass M ⋆ ∼ 108 M ⊙; and (3) a galaxy with observational properties similar to those of the reddest little red dots discovered around z ∼ 7, but Cerberus lying at z ∼ 15, with the rest-frame optical dominated by emission from a dusty torus or a dusty starburst.