Victorine A. Buiten

Context . Galaxy mergers drive intense radiation environments that fundamentally alter interstellar dust properties through photo-destruction and thermal processing. The luminous infrared galaxy II Zw 096 presents several distinctive regions, including starburst clumps and a proposed buried Active galactic nucleus (AGN) in region D1, making it an ideal laboratory for studying dust evolution under extreme conditions. Aims . This work aims to determine the carbonaceous dust properties across II Zw 096 by characterizing the spatial variations in carbonaceous feature ratios and to investigate how intense radiation fields process carbonaceous dust grains, using the relationship between dust processing signatures and traditional radiation field diagnostics. Methods . Using JWST NIRSpec IFU and MIRI MRS, carbonaceous features were observed and analyzed using the Continuum and Feature Extraction tool (CAFE) with 61 carbonaceous features from the PDRs4all project and new absorption features for ice and crystalline silicates. We analyze spatial variations in carbonaceous feature ratios at multiple resolutions (0.281″, 0.787″, and 0.961″) to trace dust composition and grain-size distribution. The 3.4/3.3 µm ratio probes aliphatic versus aromatic content, while the 11.3/3.3 µm ratio diagnoses grain-size distribution. Results . The 3.4/3.3 µm ratio spans between 0.056–0.148, with the lowest values near the proposed AGN (D1) and starburst clumps (C0, D0), indicating preferential destruction of C–H bonds connected to aliphatic structures in highly irradiated environments. The 11.3/3.3 µm ratio varies from 0.334 to ∼2.348 in D1. Both polycyclic aromatic hydrocarbon (PAH) ratios show no clear correlation with [Ne III]/[Ne II], suggesting ionization diagnostics break down either in extreme merger environments or as an effect of tracing different interstellar medium (ISM) phases and radiation-field regimes in this highly obscure merger environment. We detect water ice (3.1 µm), CO ₂ ice (4.27 µm), and crystalline silicates (11.1 µm) with spatial distributions anti-correlating with carbonaceous processing signatures. Conclusions . Spatial resolution critically affects dust diagnostics: the 3.4/3.3 µm dynamic range doubles when increasing the resolution from 0.961″ to 0.281″, demonstrating that apparent uniformity at low resolution masks significant variations in compact regions undergoing intense dust processing. Multiple coherent diagnostics point to intense dust processing, such as photo-destruction of the smallest nanograins and aromatisation, consistent with a buried AGN in D1, although definitive confirmation remains elusive.

Context. Supermassive black holes power active galactic nuclei (AGNs), injecting energy that may regulate accretion and shapes host galaxies. High-resolution observations of circumnuclear gas and dust are essential to understanding these processes. Aims. We aim to investigate the morphology, excitation, and kinematics of warm molecular hydrogen (H_2) in the inner circumnuclear disk of Centaurus A, the nearest radio galaxy. Methods. We present JWST/MIRI MRS integral-field spectroscopy of the central 170 Warm H_2 shows a complex morphology, dominating the central region where CO emission is weak or undetected. Low-excitation ̋mol lines trace an inhomogeneous ring with a 20-pc radius cavity aligned with the jet's near side, suggesting that the jet affects the morphology of the molecular disk. Higher excitation lines form a filamentary structure around the AGN. Kinematics are primarily rotational with an S-shaped distortion, indicating noncircular motions or a warped disk. A coherent, low-dispersion (∼70 100 pc2$ at 0.3"--0.7" (5--12 pc) resolution, focusing on pure rotational H_2 lines. The spectra exhibit a strong nuclear continuum, and bright H$_2 lines from S(1) to S(8), including the first S(8) detection in Centaurus A. The lines are optically thin in the nucleus, enabling maps of temperature, column density, and ortho-to-para ratio from spaxel-level excitation diagram fitting. Results. -1 $) streamer spirals inward. A power-law temperature distribution yields a warm (100--2000 K) H_2 mass of $(5.6 ± 1.4) , and sub-equilibrium ortho-to-para ratios (1.6--2.4). M_⊙ and a dynamical mass of 5 / M_⊙ within 100 pc. Shock excitation is supported by enhanced H_2/continuum and H$_2/PAH ratios, elevated Ne iii Ne ii Conclusions. Turbulent dissipation can balance the observed H_2 cooling and likely dominates heating beyond 30 pc. In the inner 100 pc of Centaurus A, AGN feeding and feedback are linked: shocks excite H_2, regulate the gas temperature, and prevent cooling below 100 K, explaining the weak CO emission and lack of a massive outflow. These shocks may drive angular-momentum loss and help fuel the nucleus.

Context. Polycyclic aromatic hydrocarbons (PAHs) constitute essential components of dust in galaxies and have a fundamental role in the physics of the interstellar medium (ISM). The impact of AGN feedback on these molecules is still under debate. Aims. We aim to present a detailed analysis of the spatially resolved properties of PAHs in the central 7″ × 12″ (∼100 × 200 pc ² ) of Centaurus A (Cen A). We used the JWST/MIRI-MRS observations at λ ∼ 5 − 28 μm taken as part of the MIRI European consortium’s Guaranteed Time Observation (GTO) program MICONIC, with angular resolution between 0.35″ and 1″ (∼6 − 17 pc). Methods. We derived PAH moment-zero maps through local continuum subtraction and extracted one-dimensional spectra in five regions of interest, including the nucleus, the circumnuclear disc, and a region characterised by a prominent deficiency in PAH emission. We decomposed the spectra into continuum, emission lines, and PAHs, from which we extracted PAH intensities and equivalent widths (EWs). Results. Polycyclic aromatic hydrocarbon emission is predominantly distributed in a ring-like structure with localised intensity enhancements, at a radius of ∼40 pc from the active nucleus. Towards the north-west we observe a distinct PAH-deficient area, roughly perpendicular to the jet axis and coincident with enhanced ionised-gas velocity dispersion as well as inflowing warm and cold molecular streamers. The PAH 11.3/7.7 μm and 6.2/7.7 μm intensity ratios exceed model predictions for peri-condensed PAHs, suggesting heavily reprocessed populations characterised by more open and irregular molecular structures. PAH 11.3/12.7 μm ratios point to a prevalence of solo over duo or trio hydrogen sites, consistent with a non-negligible degree of dehydrogenation, particularly within the PAH-deficient region, where shock-driven erosion may play a major role. We measured the largest EWs in the PAH ring, whereas reduced values in the PAH-deficient region likely reflect partial destruction by shocks; in the nucleus, the small EWs are largely attributable to continuum dilution.

We investigated the properties of the interstellar medium (ISM) at giant molecular cloud (GMC) scales (∼100 pc) in a sample of 27 nearby luminous infrared galaxies (LIRGs) spanning all interacting stages along the merger sequence, i.e. from isolated systems to late-stage mergers. In particular, we study the relations between star-formation (SF) and molecular gas surface density as a function of the interaction stage by (1) defining beam-sized (unresolved, line-of-sight) regions and (2) identifying actual gas clumps and physical structures within the galaxies. In total, we identify more than 4000 beam-sized CO-emitting regions defined on scales of ∼100 pc and more than 1000 molecular gas clumps in the sample. To map the distribution of molecular gas we used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the J = 2–1 CO transition, and to map the distribution of star formation we used the Hubble Space Telescope (HST) observations of the Pa α or Pa β hydrogen recombination lines. We derived spatially resolved Kennicutt–Schmidt (KS) relations for each LIRG in the sample. When using beam-sized regions, we find that 67% of galaxies follow a single relation between Σ _SFR and Σ _H2 . However, in the remaining galaxies, the relation splits into two branches – one characterised by higher Σ _SFR and Σ _H2 , the other by lower value – indicating the presence of a duality in this relation. In contrast, when using physical gas clumps, the duality disappears and all galaxies show a single trend. These results provide two complementary perspectives when studying the star formation process. The first maximises the number statistics (beam-sized regions), and the second focuses on actual structures associated with gas clumps in which the measured sizes have a physical meaning. We also studied other ISM and clump properties as a function of the merger stage of the LIRG systems. We find that isolated galaxies and systems in early stages of interaction exhibit smaller amounts of gas and lower star formation rates (SFRs). As the merger progresses, however, the amount of gas in the central kiloparsecs of the galaxy undergoing the merger increases, along with the SFR, and the slope of the KS relation becomes steeper, indicating an increase in the SF efficiency of the molecular gas clumps. Clumps in late-stage mergers are predominantly located at small distances from the nucleus, confirming that most of the activity is concentrated in the central regions. Interestingly, the relation between the star formation efficiency and the boundedness parameter (which measures the effects of gravity against velocity dispersion) evolves from being roughly flat in the early stages of the merger to becoming positive in the final phases, indicating that clump self-gravity only starts to regulate the star formation process between the early and mid merger stages.

We present full 3–28 μm JWST MIRI/MRS and NIRSpec/IFU spectra of the western nucleus of Arp 220, the nearest ultraluminous infrared galaxy. This nucleus has long been suggested to possibly host an embedded Compton-thick AGN. Millimetre observations of the dust continuum suggest the presence of a distinct 20 pc core with a dust temperature of T_d≳500 K, in addition to a 100 pc circumnuclear starburst disc. However, unambiguously identifying the nature of this core is challenging, due to the immense obscuration, the nuclear starburst activity, and the nearby eastern nucleus. With the JWST integral field spectrographs, for the first time we can separate the two nuclei across this full wavelength range, revealing a wealth of molecular absorption features towards the western nucleus. We analysed the rovibrational bands detected at 4–22 μm, deriving column densities and rotational temperatures for ten distinct species. Optically thick features of C₂H₂, HCN, and HNC suggest that this molecular gas is hidden behind a curtain of cooler dust and indicate that the column densities of C₂H₂ and HCN are an order of magnitude higher than previously derived from Spitzer observations. We identified a warm HCN component with a rotational temperature of T_rot = 330 K, which we associate with radiative excitation by the hot inner nucleus. We propose a geometry where the detected molecular gas is located in the inner regions of the starburst disc, directly surrounding the hot 20 pc core. The chemical footprint of the western nucleus is reminiscent of that of hot cores, with additional evidence of shocks. Despite the molecular material's close proximity to the central source, no evidence for the presence of an AGN in the form of X-ray-driven chemistry or extreme excitation was found.

We present a kinematical study of the ionized and molecular gas in the central region (∼7–14″∼100–200 pc) of the nearby radio galaxy Centaurus A (Cen A). We used JWST/MIRI MRS ∼5−28 μm observations taken as part of the Mid-Infrared Characterization of Nearby Iconic galaxy Centers (MICONIC) of the MIRI European Consortium. The two gas phases present contrasting morphologies and kinematics in Cen A. The brightest emission from the ionized gas, traced with a range of ionization potential (IP) lines analyzed here (from [Fe II] to [Ne VI]), is extended along the direction of the radio jet. We also detected emission from low IP emission lines and H₂ transitions in the galaxy disk region mapped with MRS. Both gas phases present rotational motions in the disk but also complex kinematics. The MRS observations reveal several ionized gas kinematical features that are consistent with simulation predictions of a jet-driven bubble and outflow interacting with the galaxy interstellar medium. These include broad components in the nuclear line profiles (σ∼600 km s⁻¹ in [Ar II] and [Ne III]), high velocities (reaching approximately +1000, −1400 km s⁻¹) confined within the nuclear region, velocities of hundreds of kilometers per second in several directions in the central 2″, and enhanced velocity dispersions perpendicular to the radio jet. Moreover, we find evidence of shock excitation in the nuclear region of Cen A based on mid-infrared line ratios. We compared the ionized gas mass outflow rate with Cen A's active galactic nucleus (AGN) luminosity and radio jet power and demonstrate that both mechanisms provide sufficient energy to launch the outflow. The noncircular motions observed in the mid-infrared H₂ lines can be reproduced with either a warped rotating disk model or a radial component. The latter might be to related to gas streamers detected in cold molecular gas. Notably, there is no clear indication of a fast nuclear H₂ outflow in Cen A, only a weak blueshifted component in the line profiles. This could be due to a relatively low nuclear warm H₂ column density and/or the limited geometrical coupling of Cen A's inner radio jet with the circumnuclear disk of the galaxy.

We explore the accretion rates of supermassive black holes (SMBHs) in late-stage galaxy mergers by observing three ultraluminous infrared galaxies (ULIRGs), IRAS 14378-3651, IRAS 17208-0014 and IRAS 23365+3604, using the JWST/MIRI Medium Resolution Spectrometer and JWST/NIRSpec integral field unit. In all three cases, we fail to detect [Ne vi] λ7.65 μm, a robust active galactic nuclei (AGN) tracer lying in a low-opacity interstellar window, nor do we detect any other lines that might indicate AGNs. The only detected high-excitation emission line, [Mg iv] (λ4.488 μm), arises from shocks associated with supernovae. Our new, deep flux limits on AGN tracers in the near- and mid-infrared indicate that the nuclear obscuration of any purported AGNs in our sample is isotropic, i.e., the far-infrared luminosities of these galaxies are unlikely to be driven by escaping AGN power. This allows us to show that the Eddington ratios of their SMBHs are low. We then assemble an unbiased sample of 19 ULIRGs (from the IRAS Bright Galaxy Sample with L(TIR) ≥ 10¹² L _⊙) in late-stage mergers and show that their dynamically measured black hole masses are consistent with the values from scaling from their stellar masses. On this basis, we show that the Eddington ratios of any AGNs in 15 of these galaxies are also very low, ≲10%. This indicates that any black holes are in a relatively quiescent state. That is, high levels of accretion are found in only a minority of late-merger-phase ULIRGs.

We introduce a prescription for estimating the flux of the 7.7 μm and 11.3 μm polycyclic aromatic hydrocarbon (PAH) features from broadband JWST/MIRI images. Probing PAH flux with MIRI imaging data has advantages in field of view, spatial resolution, and sensitivity compared with MIRI spectral maps, but comparisons with spectra are needed to calibrate these flux estimations over a wide variety of environments. For 267 MIRI/MRS spectra from independent regions in the four luminous infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey Early Release Science program, we derive synthetic filter photometry and directly compare estimated PAH fluxes to those measured from detailed spectral fits. We find that for probing PAH 7.7 μm, the best combination of filters is F560W, F770W, and either F1500W or F2100W, and the best for PAH 11.3 μm is F560W, F1000W, F1130W, and F1500W. The prescription with these combinations yields predicted flux densities that typically agree with values from spectral decomposition within ∼7% and ∼5% for PAH 7.7 and 11.3 μm, respectively.

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.