Susanne Aalto

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. Hubble Space Telescope (HST) images are used to study the structure of the central regions of the luminous infrared galaxy (LIRG) IC 860. IC 860 is of special interest as a system with an extreme central concentration of molecular gas cloaking its compact obscured nucleus (CON). The CON provides most of the 1.5 × 10 ¹¹ L _⊙ luminosity in IC 860 from an undetermined combination of stellar and active galactic nucleus power sources. Aims. We mapped and photometered the central molecular zone (CMZ) of IC 860 motivated by a previous detection of a luminous compact nuclear source. Our objective was to study the properties of the CMZ and its relationship to the CON, which we identified as an opaque central region in archival near-infrared (NIR) images. Methods. We measured the coordinates of the compact NIR source, IC860-a, from HST coordinates calibrated with Gaia positions. The photometry of the HST images yielded magnitudes, colors, and high V-band dust optical depths based on foreground screen dust models. Photometry corrected for dimming by dust yielded NIR luminosities of IC860-a and the CMZ. Results. IC 860 has distinct compact central luminosity sources. Most of the NIR luminosity is from the CMZ, while the LIRG-CON is an obscured region centered in the CMZ. IC860-a, on the northeastern side of the CMZ, is offset by ≈0.2″ from the CON, has a luminosity of ∼10 ⁹ L _⊙ , and may be a massive young stellar complex or intruding nucleus. Conclusions. The inner CMZ in IC 860 contains two luminous compact objects. The CON is identified for the first time as an obscured central source, while the structure of the CMZ+CON is complicated by the presence of IC860-a, compact object and possibly a massive young stellar system or second nucleus. The presence of IC 860-a in combination with the CON is a further signature of the unusual evolution of the gas-rich IC 860 CMZ.

A tight correlation between nuclear millimeter and X-ray emission has recently been found in nearby ( z < 0.01) and low-Eddington ratio ( λ _Edd < 0.1) radio-quiet active galactic nuclei (AGNs), suggesting a common origin in the hot X-ray corona. We test this relation in nine more distant AGNs ( z ∼ 0.06–0.16) with higher bolometric luminosities ( log ( L bol / erg s − 1 ) = 45.3 –46.3), Eddington ratios ( λ _Edd = 0.19–0.85), and X-ray bolometric corrections ( κ = 29–194), selected from the Burst Alert Telescope (BAT) survey. We obtained observations with Swift at 2–10 keV and quasi-simultaneous observations with the Atacama Large Millimeter/submillimeter Array at 100 GHz with high angular resolution ( . ″ 14 arcsec). We find that these high-luminosity AGNs lie above the millimeter/X-ray correlation defined by lower-luminosity sources. A joint fit to both samples yields a second-degree polynomial with an intrinsic scatter of 0.32 dex. Furthermore, the millimeter emission correlates linearly with both the UV disk luminosity and L _bol , with intrinsic scatters of 0.45 and 0.35 dex, respectively. We propose that the deviation from the linear millimeter/X-ray relation arises from a two-component coronal electron population: thermal electrons that produce X-rays, but become less efficient at higher luminosities, and nonthermal electrons that produce millimeter emission and remain tied to L _bol . Additional millimeter emission from outflow-driven shocks may also contribute, though spectral energy distribution modeling and spectral index studies favor a coronal origin.

Context. We investigate the binding energies of atoms to interstellar dust particles, which play a key role in their growth and evolution as well as the chemical reactions on their surfaces. Aims. We aim to compute the binding energies of abundant elements in the interstellar medium (C, N, O, Mg, Al, Si, S, Ca, Fe, and Ni) to silicate dust. Methods. We used the Geometries, Frequencies, and Non-covalent Interactions Tight Binding (GFN1-xTB) method to compute the binding energies. An FeMgSiO ₄ periodic surface model was amorphized using a molecular dynamics simulation. We then calculated the binding energies of each element to 81 local minima on the resulting surface. Results. A range of binding energies was found for each element. The mean of the binding energies follows the order Si (15.3 eV) > Ca (13.5 eV) > Al (12.8 eV) > C (9.2 eV) > O (8.1 eV) > N (6.4 eV) > Fe (5.9 eV) > S (5.2 eV) > Mg (2.6 eV). The probability distribution of binding energies for each element except Ca is statistically consistent with a log-normal distribution. Conclusions. In general, Si, Ca, and Al atoms have large binding energies, while the binding energies of the other atoms (C, N, O, Mg, S, Fe and Ni) are weaker. However, even the weakest computed binding energies for these elements are still far stronger than the energies associated with dust temperatures typical of the ambient interstellar medium, suggesting that silicate grains are generally stable against sublimation. We estimate sublimation temperatures for silicate grains to range from 1600 K to 3000 K depending on assumed grain size and lifetime. These binding energies on silicate dust grains, estimated from first principles for the first time, provide invaluable input to models of dust evolution and dust-catalyzed chemical reactions in the interstellar medium and grain dynamics in circumstellar environments such as asymptotic giant branch stars and protoplanetary disks.

Outflows are one of the most spectacular mechanisms through which active galactic nuclei (AGN) impact their host galaxy, though the role of AGN-driven outflows in global star formation regulation across the galaxy population is unclear. NGC 1266 is an excellent case study for investigating outflows and star formation quenching because it is a nearby ( D ∼ 30 Mpc) AGN host galaxy with an outflow driving shocks through the interstellar medium (ISM) and has recently quenched its star formation outside the nucleus. While previous works have studied the molecular outflow from its CO emission, to fully characterize the impact the outflow has on the ISM observations probing the dense, cold gas are necessary. Our Atacama Large Millimeter/submillimeter Array Cycle 0 observations do not detect a molecular outflow in ¹³ CO(2–1) and yield a lower limit of ¹² CO/ ¹³ CO ≥ 250, suggesting a highly optically thin CO outflow with low ¹³ CO abundance. In contrast, we detect substantial HCN(1–0) emission in the outflow, with an HCN(1–0)/ ¹² CO(1–0) ratio of 0.09, consistent with global measurements of many star-forming galaxies and luminous infrared galaxies. We conclude that the CO emission traces a diffuse component of the molecular gas with a low optical depth, whereas the HCN(1–0) traces dense clumps of gas entrained in the outflow. We measure an upper limit molecular outflow rate of M _⊙ yr ^–1 . Assuming the ongoing nuclear star formation and outflow continue at the same rates, NGC 1266 will deplete its gas reservoir in 450 Myr or longer, indicating that relatively low-level AGN feedback is capable of gradually expelling the molecular gas reservoir after a rapid quenching event.

Context. Mass loss in oxygen-rich asymptotic giant branch (AGB) stars remains a longstanding puzzle, as the dust species detected around these stars appear too transparent to drive winds through the absorption of radiation alone. The current paradigm consists of outflows driven by photon scattering and requires relatively large grains (∼0.3 μm). Whether the necessary number of grains with the required scattering properties exist around AGB stars remains to be determined empirically. Aims. We test whether the dust grains observed around the oxygen-rich AGB star R Doradus can drive its stellar wind by combining, for the first time, polarimetric constraints with elemental abundance limits and force balance calculations. We examine Fe-free silicates (MgSiO ₃ ), aluminium oxide (Al ₂ O ₃ ), and Fe-bearing silicates (MgFeSiO ₄ ) to determine whether any dust species can generate sufficient radiative pressure under physically realistic conditions. Methods. We analysed high-angular-resolution polarimetric observations obtained with SPHERE/ZIMPOL at the Very Large Telescope (VLT) and modelled the circumstellar dust using the radiative transfer code RADMC-3D. Dust optical properties were computed using Optool for both Mie and the distribution of hollow spheres (DHS) scattering theories. By systematically exploring a six-dimensional parameter space, we derived constraints on dust grain sizes, density profiles, and wavelength-dependent stellar radii. For models that successfully fit the observations, we analysed the results taking into consideration recent models for the gas density distribution around R Dor, and applied a multi-criteria zone analysis incorporating gas-depletion constraints and radiation pressure thresholds to assess dust-driven wind viability. Results. We find sub-micron MgSiO ₃ and Al ₂ O ₃ grains (up to 0.1 μm) regardless of scattering theory considered, and a two-layer dust envelope with steep density profiles (r ^−3.4 to r ^−4.1 ). Despite matching observed scattered-light patterns, these grains generate insufficient radiative force under physically realistic gas-to-dust mass ratios, even when assuming complete elemental depletion. Silicates containing Fe could theoretically provide adequate force, but would sublimate in critical acceleration regions and require implausibly high silicon-depletion levels. Conclusions. Our findings for R Doradus show insufficient radiation pressure from scattering on grains, suggesting that dust alone cannot drive the wind in this star and that additional mechanisms may be required.

We report the discovery of a highly polarized millimeter (mm) continuum source in the central region of NGC 4945 that was identified through ALMA Band 3 observations. This starburst Seyfert 2 galaxy contains numerous compact mm sources, but only one, located approximately 3 _. ^″ 4 (∼60 pc) from the galactic center and unresolved with ∼0 _. ^″ 1 resolution, exhibits an unusually high polarization degree of 50% ± 14% that likely originates from nonthermal synchrotron radiation. The source is faint, but clearly detected in two separate epochs of observation taken 14 days apart, with a flux of 0.104 ± 0.018 and 0.125 ± 0.016 mJy. It was also detected in earlier ALMA observations, where it showed no variability at any timescale. The spectral index remains stable within large uncertainties of −1.8 ± 2.5 and −1.3 ± 2.4. The source, which we further refer to as Punctum because it is so compact, revealed no clear counterparts in existing X-ray or radio observations. Assuming an association with the central region of NGC 4945, we estimated upper limits for its luminosity of ∼1 × 10 ³⁷ erg s ⁻¹ in the 3–6 keV X-ray band (from archival Chandra data) and ∼5 × 10 ³⁵ erg s ⁻¹ at 23 GHz (from archival ATCA data). A comparison of the radio, mm (including polarization), and X-ray properties with known astrophysical sources emitting synchrotron radiation, such as accreting neutron stars, supernova remnants, and nonthermal galactic filaments, revealed no clear match in any of these scenarios. The exact nature of this highly polarized source remains undetermined.

The compact millimeter emission ubiquitously found in radio-quiet active galactic nuclei (RQ AGNs) exhibits properties consistent with synchrotron radiation from a small region (≤1 light day) and undergoing self-absorption below ∼100 GHz. Several scenarios have been proposed for its origin, including an X-ray corona, a scaled-down jet, or outflow-driven shocks, which can be tested via millimeter polarimetry. In the optically thin regime, synchrotron emission is expected to show polarization up to ∼70%, but disordered magnetic fields and Faraday rotation reduce this to a few percent for jets and outflows, while an X-ray corona is likely to result in complete depolarization. To investigate this, we conducted the first ALMA Band 3 full-polarization observations of three RQ AGNs – NGC 3783, MCG 5–23–16, and NGC 4945. No polarized signal was detected in any of the AGNs, with an upper limit of 0.5–1.5%, supporting the X-ray corona scenario. However, we detected a compact source with 17% polarization in NGC 3783, 20 pc away from the AGN, co-spatial with the millimeter and narrow-line outflow, likely linked to a shock propagating through the outflowing material. Additionally, combining our data with archival ALMA observations, we found typical millimeter variability in RQ AGNs by a factor of 2.

Luminous and ultraluminous IR galaxies are critical for investigating feedback mechanisms due to a combination of intense star formation episodes and active galactic nuclei (AGN), particularly in the context of complex galaxy interactions. We conduct a detailed analysis of the II ZW 096 merging system using the Multi-Unit Spectroscopic Explorer on the Very Large Telescope, combining high-resolution narrow-field mode and large-area wide-field mode observations. We mapped the morphology, kinematics, and ionizing radiation of the system’s gas by fitting atomic emission lines and the optical continuum. We identify three or more distinct galaxies within II ZW 096, revealing rotational patterns and complex interactions consistent with a collapsing small galaxy group. The kinematics and ionization structures suggest high star formation rates and shock-driven processes, which align with this proposed scenario. Focusing on the D1 compact region, which contributes 40%–70% of the system’s IR emission, and combining information from archival multiwavelength observations, we find strong evidence of a heavily obscured AGN powering it. Our analysis of the internal structure, interactions, and merger state of II ZW 096 offers novel insights into the galaxy evolution processes in this dynamic and highly chaotic system.

Context. Active galactic nuclei (AGNs) host accreting supermassive black holes (SMBHs). The accretion process can lead to the formation of a hot, X-ray emitting corona close to the SMBH that can accelerate relativistic electrons. Observations in the millimeter band can probe its synchrotron emission. Aims. We intend to provide a framework to derive physical information of SMBH coronae by modelling their spectral energy distribution (SED) from radio to far-infrared frequencies. We also explore the possibilities of deriving additional information from millimeter observations, such as the SMBH mass, and studying high-redshift lensed sources. Methods. We introduce a corona emission model based on a one-zone spherical region with a hybrid thermal and non-thermal plasma. We investigated the dependence of the corona SED on different parameters such as size, opacity, and magnetic field strength. Other galactic emission components from dust, ionised gas, and diffuse relativistic electrons were also included in the SED fitting scheme. We applied our code consistently to a sample of radio-quiet AGNs with strong indications of a coronal component in the millimeter. Results. The detected millimeter emission from SMBH coronae is consistent with a non-thermal relativistic particle population with an energy density that is ≈0.5–10% of that in the thermal plasma. This requires magnetic energy densities close to equipartition with the thermal gas and corona sizes of 60–250 gravitational radii. The model can also reproduce the observed correlation between millimeter emission and SMBH mass when we accounted for the uncertainties in the corona size. Conclusions. The millimeter band offers a unique window into the physics of SMBH coronae, enabling the study of highly dust-obscured sources and high-redshift lensed quasars. Gaining a deeper understanding of the relativistic particle population in SMBH coronae can provide key insights into their potential multiwavelength and neutrino emission.