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.

Context. The chemical composition of warm gas giant exoplanet atmospheres (with T_eq < 1000 K) is not well known due to the lack of observational constraints.

Aims. HAT-P-12 b is a warm, sub-Saturn-mass transiting exoplanet that is ideal for transmission spectroscopy. We aim to characterise its atmosphere and probe the presence of carbonaceous species using near-infrared observations.

Methods. One transit of HAT-P-12 b was observed in spectroscopy with JWST NIRSpec in the 2.87–5.10 µm range with a resolving power of ~1000. The JWST data are combined with archival observations from HST WFC3 covering the 1.1–1.7 µm range. The data were analysed using two data reduction pipelines and two atmospheric retrieval tools. Atmospheric simulations using chemical forward models were performed to interpret the spectra.

Results. CO₂, CO, and H₂O are detected at 12.2, 4.1, and 6.0 σ confidence, respectively. Their volume mixing ratios are consistent with an atmosphere of ~10× solar metallicity and production of CO₂ by photochemistry. CH₄ is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO₂ is also not detected and its production seems limited by low upper atmosphere temperatures (~500 K at P ≲ 10⁻³ bar derived from one-dimensional retrievals), insufficient to produce it in detectable quantities (≳ 800 K required according to photochemical models). H₂S is marginally detected using one data analysis method, but not by the other. Retrievals indicate the presence of clouds between 2 and 11 mbar using one data analysis method, and between 5 and 269 mbar using the other. The derived C/O ratio is below unity, but is not well constrained.

Conclusions. This study points towards an atmosphere for HAT-P-12 b that could be enriched in carbon and oxygen with respect to its host star, a possibly cold upper atmosphere that may explain the non-detection of SO₂, and a CH₄ depletion that is yet to be fully understood. When including the production of CO₂ via photochemistry, an atmospheric metallicity that is close to Saturn’s can explain the observations. Metallicities inferred for other gas giant exoplanets based on their CO₂ mixing ratios may need to account for its photochemical production pathways. This may impact studies on mass-metallicity trends and links between exoplanet atmospheres, interiors, and formation history.

We present JWST NIRCam observations of the emerging young star clusters (eYSCs) detected in the nearby spiral galaxy M83. The NIRcam mosaic encompasses the nuclear starburst, the bar, and the inner spiral arms. The eYSCs, detected in Paα and Brα maps, have been largely missed in previous optical campaigns of young star clusters (YSCs). We distinguish between eYSCI, if they also have compact 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission associated with them, and eYSCII, if they only appear as compact Paα emitters. We find that the variations in the 3.3 μm PAH feature are consistent with an evolutionary sequence where eYSCI evolve into eYSCII and then optical YSCs. This sequence is clear in the F300M​​​​​​−F335M (tracing the excess in the 3.3 μm PAH feature) and the F115W−F187N (tracing the excess in Paα) colors, which become increasingly bluer as clusters emerge. The central starburst stands out as the region where the most massive eYSCs are currently forming in the galaxy. We estimate that only about 20% of eYSCs will remain detectable as compact YSCs. Combining eYSCs and YSCs (≤10 Myr), we recover an average clearing timescale of 6 Myr in which clusters transition from embedded to fully exposed. We see evidence of shorter emergence timescales (∼5 Myr) for more massive (>5 × 10³ M_⊙) clusters, while star clusters of ∼10³ M_⊙ about 7 Myr. We estimate that eYSCs remain associated with the 3.3 μm PAH emission for 3–4 Myr. Larger samples of eYSC and YSC populations will provide stronger statistics to further test environmental and cluster mass dependencies on the emergence timescale.

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.