The first James Webb Space Telescope/MIRI photometric observations of TRAPPIST-1 b allowed for the detection of the thermal emission of the planet at 15 μm, suggesting that the planet could be a bare rock with a zero albedo and no redistribution of heat. These observations at 15 μm were acquired as part of Guaranteed Time Observer time that included a twin programme at 12.8 μm to obtain measurements inside and outside the CO₂ absorption band. Here we present five new occultations of TRAPPIST-1 b observed with MIRI in an additional photometric band at 12.8 μm. We perform a global fit of the ten eclipses and derive a planet-to-star flux ratio and 1σ error of 452 ± 86 ppm and 775 ± 90 ppm at 12.8 μm and 15 μm, respectively. We find that two main scenarios emerge. An airless planet model with an unweathered (fresh) ultramafic surface, that could be indicative of relatively recent geological processes, fits the data well. Alternatively, a thick, pure-CO₂ atmosphere with photochemical hazes that create a temperature inversion and result in the CO₂ feature being seen in emission also works, although with some caveats. Our results highlight the challenges in accurately determining a planet’s atmospheric or surface nature solely from broadband filter measurements of its emission, but also point towards two very interesting scenarios that will be further investigated with the forthcoming phase curve of TRAPPIST-1 b.

Context. The newly accessible mid-infrared (MIR) window offered by the James Webb Space Telescope (JWST) for exoplanet imaging is expected to provide valuable information to characterize their atmospheres. In particular, coronagraphs on board the JWST Mid-InfraRed instrument (MIRI) are capable of imaging the coldest directly imaged giant planets at the wavelengths where they emit most of their flux. The MIRI coronagraphs have been specially designed to detect the NH3 absorption around 10.5 μm, which has been predicted by atmospheric models and should be detectable for planets colder than 1200 K. Aims. We aim to assess the presence of NH3 while refining the atmospheric parameters of one of the coldest companions detected by directly imaging GJ 504 b. Its mass is still a matter of debate and depending on the host star age estimate, the companion could either be placed in the brown dwarf regime of ~20 MJup or in the young Jovian planet regime of ~4 MJup. Methods. We present an analysis of new MIRI observations, using the coronagraphic filters F1065C, F1140C, and F1550C of the GJ 504 system. We took advantage of previous observations of reference stars to build a library of images and to perform a more efficient subtraction of the stellar diffraction pattern. We used an atmospheric grid from the Exo-REM model to refine the atmospheric parameters by combining archival near-infrared (NIR) photometry with the MIR photometry. Results. We detected the presence of NH3 at 12.5 σ and measured its volume mixing ratio of 10- 5.3±0.07 in the atmosphere of GJ 504 b. These results are in line with atmospheric model expectations for a planetary-mass object and observed in brown dwarfs within a similar temperature range. The best-fit model with Exo-REM provides updated values of its atmospheric parameters, yielding a temperature of Teff = 512 ± 10 K and radius of R = 1.08- 0.03+0.04 RJup. Conclusions. These observations demonstrate the capability of MIRI coronagraphs to detect NH3 and to provide the first MIR observations of one of the coldest directly imaged companions. Overall, NH3 is a key molecule for characterizing the atmospheres of cold planets, offering valuable insights into their surface gravity. These observations provide valuable information for future spectroscopic observations planned with JWST, in particular, with the MIRI medium-resolution spectrometer (MRS), which will allow us to characterize the atmosphere of GJ 504 b in depth.

Haro 11 is the closest known Lyman continuum leaking galaxy and serves as an important laboratory for studying the escape of Lyman continuum radiation. The galaxy is a metal-poor, starburst galaxy believed to be undergoing a merger that might help facilitate the escape of radiation. In this study, we carry out a large suite of numerical simulations of a merger between two disc galaxies, to study possible origins of Haro 11 and understand under which conditions various features of the galaxy are formed. By varying galaxy parameters describing the orbital configurations, masses, and their inclination, we perform a total of ~500 simulations. We demonstrate that a two-disc galaxy merger is able to reproduce key, observed features of Haro 11, including its morphology, gas kinematics, star formation history, and stellar population ages and masses. We also find that small parameter variations have minimal impact on the orbits and resulting galaxy properties. In particular, we present a fiducial Haro 11 model that produces the single observed tidal tail, the presence of three stellar knots, and inner gas morphology and kinematics. By performing mock observations, we compare with the results of observational data and discuss possible origins for various features. Furthermore, we present newly gathered observational data that confirms the presence of a stellar tidal tail with similar length and direction as our simulations.

T-type brown dwarfs present an opportunity to explore atmospheres teeming with molecules such as H2O, CH4, and NH3, which exhibit a wealth of absorption features in the mid-infrared. With JWST, we can finally explore this chemistry in detail, including for the coldest brown dwarfs that were not yet discovered in the Spitzer era. This allows precise derivations of the molecular abundances, which in turn inform our understanding of vertical transport in these atmospheres and can provide clues about the formation of cold brown dwarfs and exoplanets. This study presents the first JWST/MRS mid-IR spectrum (R ∼ 1500-3000) of a T dwarf: the T8.5+T9 brown dwarf binary WISE J045853.90+643451.9. We fit the spectrum using a parameterized P-T profile and free molecular abundances (i.e., a retrieval analysis), treating the binary as unresolved. We find a good fit with a cloud-free atmosphere and identify H2O, CH4, and NH3 features. Moreover, we make the first detections of HCN and C2H2 (at 13.4σ and 9.5σ respectively) in any brown dwarf atmosphere. The detection of HCN suggests intense vertical mixing (Kzz ∼ 1011 cm2 s−1), challenging previous literature derivations of Kzz values for T-type brown dwarfs. Even more surprising is the C2H2 detection, which cannot be explained with existing atmospheric models for isolated objects. This result challenges model assumptions about vertical mixing and/or our understanding of the C2H2 chemical network, or might hint towards more complex atmospheric processes such as magnetic fields driving aurorae or lightning driving ionization. These findings open a new frontier in studying carbon chemistry within brown dwarf atmospheres.

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. Galaxy mergers are an important and complex phase during the evolution of galaxies. They may trigger nuclear activity and/or strong star forming episodes in galaxy centres that potentially alter the evolution of the system. Aims. As part of the guaranteed time observations program Mid-Infrared Characterization Of Nearby Iconic galaxy Centers (MICONIC), we used the medium-resolution spectrometer (MRS) of the Mid-Infrared Instrument on board the James Webb Space Telescope (JWST) to study NGC 6240. We aim to characterise the dual active galactic nuclei (AGN), the ionised gas outflows, and the main properties of the interstellar medium over a mapped area of 6.6″ × 7.7″. Aims. We obtained integral field spectroscopic mid-infrared data (wavelength from 4.9 to 28 μm) of NGC 6240. We modelled the emission lines through a kinematic decomposition that accounts for the possible existence of various components. Methods. We have resolved both nuclei of NGC 6240 for the first time in the full 5- 28 μm spectral range. The fine structure lines in the southern (S) nucleus are broader than for the northern (N) nucleus (full width at half maximum of ≥1500 versus ~700 km s- 1 on average, respectively). High excitation lines, such as [Ne V], [Ne VI], and [Mg V], are clearly detected in the N nucleus. In the S nucleus, the same lines can be detected but only after a decomposition of the polycyclic aromatic hydrocarbon features in the integrated spectrum, due to a combination of a strong mid-IR continuum, broad emission lines, and intense star formation (SF). The SF is distributed all over the mapped field of view of 3.5 kpc × 4.1 kpc (projected), with the maximum located around the S nucleus. Both nuclear regions appear to be connected by a bridge region that is detected with all the emission lines. Based on the observed MRS line ratios and the high velocity dispersion (σ ~ 600 km s- 1), shocks also dominate the emission in this system. We detected the presence of outflows as a bubble north-west from the N nucleus and at the S nucleus. We estimated an ionised mass outflow rate of 1.4 ± 0.3 M⊙ yr- 1 and 1.8 ± 0.2 M⊙ yr- 1, respectively. Given the derived kinetic power of these outflows, both the AGN and the starburst could have triggered them.

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.

Context. We report MIRI-JWST coronagraphic observations at 11.3 and 15.5 μm of the debris disk around the young star HD 106906. The wavelength range is sensitive to the thermal emission of the dust heated by the central star.

Aims. The observations were made to characterize the structure of the disk through the thermal emission, to search for clues to the presence of a central void of dust particles, and to derive the mass of the dust and the temperature distribution. Another goal was also to constrain the size distribution of the grains.

Methods. The data were reduced and calibrated using the JWST pipeline. The analysis was based on a forward-modeling of the images using a multiparameter radiative transfer model coupled to an optical code for coronagraphy processing.

Results. The disk is clearly detected at both wavelengths. The slight asymmetry is geometrically consistent with the asymmetry observed in the near-IR, but it is inconsistent the brightness distribution. The observed structure is well reproduced with a model of a disk (or belt) with a critical radius 70 au, a mildly inward-increasing density (index 2) and a steeper decrease outward (index −6). This indication of a filled disk inside the critical radius is inconsistent with sculpting from an inner massive planet. The size distribution of the grains that cause the mid-IR emission is well constrained by the flux ratio at the two wavelengths : 0.45–10 and 0.65–10 μm for silicate and graphite grains, respectively. The minimum size is consistent with predictions of blowout through radiative pressure.

Conclusions. We derive a mass of the dust that causes the mid-IR emission of 3.3–5.0 10⁻³ M_⊕. When the larger grains (up to 1 cm) that cause the millimeter emission are included, we extrapolate this mass to 0.10–0.16 M_⊕. We point out to that this is fully consistent with ALMA observations of the disk in terms of dust mass and of its millimeter flux. We estimate the average dust temperature in the planetesimal belt to be 74 K, but the temperature range within the whole disk is rather wide: from 40 to 130 K.

The study of the atmosphere of exoplanets orbiting white dwarfs is a largely unexplored field. With WD 0806-661 b, we present the first deep dive into the atmospheric physics and chemistry of a cold exoplanet around a white dwarf. We observed WD 0806-661 b using JWST's Mid-InfraRed Instrument Low-Resolution Spectrometer, covering the wavelength range from 5 to 12 μm, and the Imager, providing us with 12.8, 15, 18, and 21 μm photometric measurements. We carried the data reduction of those data sets, tackling second-order effects to ensure a reliable retrieval analysis. Using the TauREx retrieval code, we inferred the pressure–temperature structure, atmospheric chemistry, mass, and radius of the planet. The spectrum of WD 0806-661 b is shaped by molecular absorption of water, ammonia, and methane, consistent with a cold Jupiter atmosphere, allowing us to retrieve their abundances. From the mixing ratio of water, ammonia, and methane we derive C/O = 0.34 ± 0.06, C/N=14.4−1.8+2.5, and N/O = 0.023 ± 0.004 and the ratio of detected metals as a proxy for metallicity. We also derive upper limits for the abundance of CO and CO₂ (1.2 × 10⁻⁶ and 1.6 × 10⁻⁷, respectively), which were not detected by our retrieval models. While our interpretation of WD 0806-661 b's atmosphere is mostly consistent with our theoretical understanding, some results—such as the lack of evidence for water clouds, an apparent increase in the mixing ratio of ammonia at low pressure, or the retrieved mass at odds with the supposed age—remain surprising and require follow-up observational and theoretical studies to be confirmed.

JWST/MIRI images have been used to study the Fourier transform power spectra (PS) of two spiral galaxies, NGC 628 and NGC 5236, and two dwarfs, NGC 4449 and NGC 5068, at distances ranging from 4 to 10 Mpc. The PS slopes on scales larger than 200 pc range from −0.6 at 21µm to −1.2 at 5.6µm. These slopes for one-dimensional PS are consistent with the PS slopes observed elsewhere using HI and dust emission. They are likely related to turbulence, but they may also be viewed as a hierarchical distribution of objects having a size-luminosity relation and size distribution function. There is no evidence for a kink or steepening of the PS at some transition from two-dimensional to three-dimensional turbulence on the scale of the disk thickness. This lack of a kink could be from large positional variations in the PS depending on two opposite effects: local bright sources that make the slope shallower and exponential galaxy profiles that make the slope steeper. The sources could also be confined to a layer of molecular clouds that is thinner than the HI or cool dust layers where PS kinks have been observed before. If the star formation layers observed in the mid-infrared here are too thin, then the PS kink could also be hidden in the broad tail of the JWST point spread function.