Power spectra (PS) of high-resolution images of M51 (NGC 5194) taken with the Hubble Space Telescope and the James Webb Space Telescope (JWST) have been examined for evidence of disk thickness in the form of a change in slope between large scales, which map two-dimensional correlated structures, and small scales, which map three-dimensional correlated structures. Such a slope change is observed here in Hα, and possibly Paα, using average PS of azimuthal intensity scans that avoid bright peaks. The physical scale of the slope change occurs at ∼120 pc and ∼170 pc for these two transitions, respectively. A radial dependence in the shape of the Hα PS also suggests that the length scale drops from ∼180 pc at 5 kpc, to ∼90 pc at 2 kpc, to ∼25 pc in the central ∼kpc. We interpret these lengths as comparable to the thicknesses of the star-forming disk traced by H ii regions. The corresponding emission measure is ∼100 times larger than what is expected from the diffuse ionized gas. The PS of JWST Mid-IR Instrument images in eight passbands have more gradual changes in slope, making it difficult to determine a specific value of the thickness for this emission.

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.

We present new JWST/NIRCam observations of the starburst irregular galaxy NGC 4449, obtained in Cycle 1 as part of the Feedback in Emerging extrAgalactic Star clusTers program, which we use to investigate its resolved stellar populations and their spatial distributions. NGC 4449 near-IR color-magnitude diagrams reveal a broad range of stellar populations, spanning different evolutionary phases, from young main sequence stars, to old red giant branch stars and asymptotic giant branch (AGB) stars. The analysis of their spatial distributions shows that younger (≤10 Myr) populations form an S-shaped distribution aligned with the galaxy’s north-south axis, while stars aged 10-60 Myr show shifting concentrations from the north to the south, consistent with the possibility that external interactions or tidal effects may have triggered star formation in spatially distinct bursts. Clusters of comparable ages generally follow these distributions, suggesting that cluster and field stars form at the same pace in each galaxy region. Thanks to the unprecedented high-spatial resolution and sensitivity of the JWST data, we recover a clear gap between oxygen-rich and the carbon star branch of the AGB population, as well as the presence of a massive AGB star “finger.” The analysis of these stars can provide constraints on AGB evolution models and dust production in this galaxy. These results confirm NGC 4449's status as a compelling example of a local dwarf starburst galaxy undergoing complex and possibly externally driven star formation and underscore the power of JWST in probing the full lifecycle of stars in nearby starburst systems.

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 a metal-poor, starburst galaxy believed to be the result of an ongoing merger, which is shaping the properties of the galaxy. 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 reproduces key, observed features of Haro 11, including its morphology, gas kinematics, star formation history, and stellar population ages and masses. In particular, we present a fiducial Haro 11 model that produces the single observed tidal tail, three stellar knots, and inner gas morphology and kinematics. The resulting orbit and galactic morphology are robust against small variations of the initial parameters. 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 morphology as our simulations.

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. The embedded phase of star formation is a crucial period in the development of a young star when the system still accretes matter, emerges from its natal cloud with assistance from powerful jets and outflows, and forms a disk, thus setting the stage for the birth of a planetary system. The mid-infrared spectral line observations now possible with unprecedented sensitivity, spectral resolution, and sharpness from the James Webb Space Telescope (JWST) are key for probing many of the physical and chemical processes on subarcsecond scales that occur in highly extincted regions. They provide unique diagnostics and complement millimeter observations.

Aims. The aim of the JWST Observations of Young protoStars (JOYS) program is to address a wide variety of topics ranging from protostellar accretion and the nature of primeval jets, winds, and outflows to the chemistry of gas and ice in hot cores and cold dense protostellar environments to the characteristics of the embedded disks. We introduce the JOYS program and show representative results.

Methods. The JWST Mid-InfraRed Instrument (MIRI) Medium Resolution Spectrometer (MRS) Integral Field Unit (IFU) 5-28 μm maps of 17 low-mass targets (23 if binary components are counted individually) and six high-mass protostellar sources were taken with resolving powers R = λ/Δλ = 1500-4000. We used small mosaics ranging from 1 × 1 to 3 × 3 MRS tiles to cover ~4″ to 20″ fields of view, providing spectral imaging on spatial scales down to ~30 au (low mass) and ~600 au (high mass). For HH 211, the complete ~1′ blue outflow lobe was mapped with the MRS. Atomic lines were interpreted with published shock models, whereas molecular lines were analyzed with simple rotation diagrams and local thermodynamic equilibrium slab models. We stress the importance of taking infrared pumping into account. Inferred abundance ratios were compared with detailed hot core chemical models including X-rays, whereas ice spectra were fit through comparison with laboratory spectra.

Results. The JWST MIRI-MRS spectra show a wide variety of features, with their spatial distribution providing key insight into their physical origin. The atomic line maps differ among refractory (e.g., Fe), semi-refractory (e.g., S), and volatile elements (e.g., Ne) and are linked to their different levels of depletion and local (shock) conditions. Jets are prominently seen in lines of [Fe II] and other refractory elements, whereas the pure rotational H₂ lines probe hot (~1000 K) and warm (few ×10² K) gas inside the cavity, as well as gas associated with jets, entrained outflows, and cavity walls for both low- and high-mass sources. Wide-angle winds are found in low-J H₂ lines. Nested stratified jet structures containing an inner ionized core with an outer molecular layer are commonly seen in the youngest sources. While [S I] follows the jet as seen in [Fe II] in the youngest protostars, it is different in more evolved sources, where it is concentrated on source. Noble gas lines such as [Ne II] 12.8 μm reveal a mix of jet shock and photoionized emission. The H I recombination lines serve as a measure of protostellar accretion rates but are also associated with more extended jets. Gaseous molecular emission (CO₂, C₂H₂, HCN, H₂O, CH₄, SO₂, SiO) is seen toward several sources, but it is cool compared with what is found in more evolved disks, with excitation temperatures of only 100-250 K, and likely associated with the warm inner envelopes (“hot cores”). Along the outflow, CO₂ is often extended, thus contrasting with C₂H₂ , which is usually centered on source. Water emission is commonly detected on source, even if relatively weak. Off source, it is seen only in the highest density shocks, such as those associated with NGC 1333 IRAS4B. Some sources show gaseous molecular lines in absorption, including NH₃ in one case. Deep ice features are seen toward the protostars, revealing not just the major ice components but also ions (as part of salts) and complex organic molecules, with comparable abundances from low- to high-mass sources. The relative abundances of some gas and ice species are similar, which is consistent with ice sublimation in hot cores. We present a second detection of HDO ice in a solar-mass source, with an HDO/H₂O ice ratio of ~0.4%, thus providing a link with HDO/H₂O in disks and comets. A deep search for solid O₂ suggests that it is not a significant oxygen reservoir. Only a few embedded Class I disks show the same forest of water lines as Class II disks. This may be due to significant dust extinction of the upper layers in young disks caused by less settling of small dust as well as radial drift bringing in fresh dust.

Conclusions. This paper illustrates the wide range of science questions that a single MIRI-MRS IFU data set can address. Our data suggest many similarities between low- and high-mass sources. Large source samples across evolutionary stages and luminosities are needed to further develop these diagnostics of the physics and chemistry of protostellar systems.

The origin of Lyman continuum (LyC) photons responsible for reionizing the Universe remains largely unknown, with the fraction of escaping LyC photons from galaxies at z ∼ 6 to 12 still uncertain. Direct detection of LyC photons from this epoch is challenging due to intergalactic medium absorption, making lower-redshift analogs valuable for studying LyC leakage. In this study, we present Hubble Space Telescope Cosmic Origins Spectrograph observations of five low-redshift (z ∼ 0.3) massive starburst galaxies, selected for high stellar mass and weak [S II] nebular emission, an indirect tracer of LyC escape. LyC leakage is detected in three of the five galaxies, highlighting weak [S II] as a reliable tracer—a finding supported by recent JWST discoveries of z > 5 galaxies with similarly weak [S II] emission. The dust-corrected LyC escape fractions (fesc,H I), representing LyC photons that would escape without dust, range from 33% to 84%. However, the absolute escape fractions (fesc,tot), accounting for both neutral hydrogen absorption and dust attenuation, are substantially lower, between 1% and 3%. This indicates that, although these galaxies are nearly optically thin to H I, their significant dust content restricts LyC escape. These [S II]-weak, massive leakers differ from typical low-redshift LyC emitters, exhibiting higher metallicity, lower ionization states, greater dust extinction, and higher star formation surface densities. We suggest that feedback-driven winds in these compact starbursts generate ionized channels, allowing LyC escape in line with a “picket-fence” model, indicating a distinct mechanism for LyC leakage.