We present mid-infrared (MIR) observations of the Seyfert 1 galaxy NGC 4151 using the Large Binocular Telescope Interferometer (LBTI). We took open-loop Fizeau images with 66–104 mas (5.8–9.1 pc) resolution in the N -band (at 8.7 and 10.5 μ m), using the full resolution of the LBTI—equivalent to that of a 28.8 m telescope. These images were complemented by adaptive optics imaging in the LM -bands (3.7 and 4.8 μ m), with 50–62 mas (4.4–5.4 pc) resolution. These images bridge the scales between previous Very Large Telescope Interferometer (VLTI)/MIDI and VLT/VISIR data, delivering Extremely Large Telescope (ELT)–like imaging resolution in the N -band. We resolve a dusty torus (with a diameter of 32 pc and position angle of 125°), and detect dusty clouds within the narrow-line region. Matching the resolution across four bands, we measured spatially resolved SEDs of the central ∼100 pc. Modified blackbody fitting revealed dust temperature and extinction profiles, indicating both heating from the accretion disk and additional shock heating due to the radio jet. The spatial coincidence of ionized emission (e.g., [Fe II ] and [O III ]), extended MIR structures, and radio features further supports the interpretation of shock heating. Comparison with NGC 1068 tests the Unified Model of Active Galactic Nuclei: structures are similar, despite differences in orientation and Eddington ratio. NGC 4151’s torus is smaller than NGC 1068’s following a r ∝ L ^0.5 scaling. These ELT-like observations of NGC 4151 and NGC 1068 highlight the need to revise MIR radiative transfer models of AGN to account for jet-related heating.

Obtaining accurate fluxes for faint sources from the ground at near- and mid-infrared wavelengths is challenging because of rapidly varying atmospheric absorption, and is often limited by the lack of a nearby spectro-photometric calibrator. We present the STellar Absolute Reference Spectroscopic Flux Library (STARSFLUX), an all-sky catalogue of 64,484 calibrator spectra spanning 0.3–30 μm, based on the Mid-infrared stellar Diameters and Fluxes compilation Catalogue (MDFC). STARSFLUX combines Gaia DR3 stellar parameters, multi-band photometry from space- and ground-based surveys, and synthetic NewEra PHOENIX atmosphere models. For each star, we fit the observed SED with an interpolated model spectrum, angular diameter, and extinction to produce an absolutely flux-calibrated spectrum. The derived angular diameters can also be used to calibrate interferometric observations. We validate the catalogue using three independent tests. First, STARSFLUX angular radii agree closely with Gaia DR3 radii, with a median |ΔR|/R_ Gaia,DR3≃ 4.8 per cent. Second, integrated L-band (2.8–4.2 μm) fluxes for 12 stars in common with the Cohen infrared standards agree with Cohen values with a mean absolute percentage difference of 4.7 ± 2.6 per cent. Third, comparison with Gaia DR3 BP/RP spectra shows that the near-UV/visible/near-IR absolute spectro-photometric fluxes agree to approximately 3 per cent. The FITS spectra are available at https://home.strw.leidenuniv.nl/ gamez/ and will be submitted to VizieR.

Active galactic nuclei (AGNs) are a key component of galaxy evolution owing to feedback on the host from its supermassive black hole. The morphology of warm inflowing and outflowing dusty material can reveal the nature of the onset of feedback, AGN feeding and the unified model of AGN. Here we use the Large Binocular Telescope Interferometer (LBTI) to image the dense, obscuring disk and extended dusty outflow region of NGC 1068. In Fizeau imaging mode, the LBTI synthesizes the equivalent resolution of a 22.8 m telescope. The 8.7 μm Fizeau images of NGC 1068 have an effective resolution of 47 × 90 mas (3.3 × 6.2 pc) in a 5″ field of view after performing point spread function deconvolution techniques described here. This is the only extragalactic source to be Fizeau imaged using the LBTI, and the images bridge the scales measured with the Very Large Telescope Interferometer (0.5–5 pc) and those of single telescopes such as James Webb Space Telescope and Keck (>15 pc). The images detect and spatially resolve the low surface brightness mid-infrared features in the AGN disk/wind region that are overresolved by the Very Large Telescope Interferometer. The images show strong correlation between mid-infrared dust emission and near-infrared emission of highly excited atomic lines observed by SINFONI. Such LBTI imaging is a precursor to infrared imaging using the upcoming generation of extremely large telescopes, with angular resolutions up to six times better than James Webb Space Telescope, the largest space telescope in orbit. Detailed infrared images of the region around the supermassive black hole of NGC 1068 from the Large Binocular Telescope Interferometer reveal a dense disk, dusty outflow and emissions linked to radio jets, suggesting that active galactic nucleus torus models should include jet interactions.

Active galactic nuclei (AGNs) are a key component of galaxy evolution due to feedback on the host from its supermassive black hole. The morphology of warm, in- and outflowing dusty material can reveal the nature of the onset of feedback, AGN feeding, and the unified model of AGN. Here we use the Large Binocular Telescope Interferometer (LBTI) to image the dense, obscuring disk and extended dusty outflow region of NGC 1068. In Fizeau imaging mode the LBTI synthesizes the equivalent resolution of a 22.8 m telescope. The 8.7 μm Fizeau images of NGC 1068 have an effective resolution of 47×90 mas (3.3×6.2 pc) in a 5" field of view after performing PSF deconvolution techniques described here. This is the only extragalactic source to be Fizeau imaged using the LBTI, and the images bridge the scales measured with the Very Large Telescope Interferometer (VLTI; 0.5-5 pc) and those of single telescopes such as JWST and Keck (>15 pc). The images detect and spatially resolve the low surface brightness mid-infrared (MIR) features in the AGN disk/wind region that are over-resolved by the VLTI. The images show strong correlation between MIR dust emission and near-infrared (NIR) emission of highly excited atomic lines observed by SINFONI. Such LBTI imaging is a precursor to infrared imaging using the upcoming generation of extremely large telescopes, with angular resolutions up to 6x better than JWST, the largest space telescope in orbit.

Rocky planets form by the concentration of solid particles in the inner few au regions of planet-forming disks. Their chemical composition reflects the materials in the disk available in the solid phase at the time the planets were forming. Studying the dust before it gets incorporated in planets provides a valuable diagnostic for the material composition. We aim to constrain the structure and dust composition of the inner disk of the young Herbig Ae star HD 144432, using an extensive set of infrared interferometric data taken by the Very Large Telescope Interferometer (VLTI), combining PIONIER, GRAVITY, and MATISSE observations. We introduced a new physical disk model TGMdust to image the interferometric data, and to fit the disk structure and dust composition. We also performed equilibrium condensation calculations with GGchem to assess the hidden diversity of minerals occurring in a planet-forming disk such as HD 144432. Our best-fit model has three disk zones with ring-like structures at $0.15$, $1.3$, and $4.1$ au. Assuming that the dark regions in the disk at sim $0.9$ au and at sim $3$ au are gaps opened by planets, we estimate the masses of the putative gap-opening planets to be around a Jupiter mass. We find evidence for an optically thin emission ($ from the inner two disk zones ($r < 4$ au) at $ Our silicate compositional fits confirm radial mineralogy gradients, as for the mass fraction of crystalline silicates we get around $61$ in the innermost zone ($r<1.3$ au), mostly from enstatite, while only sim $20$ in the outer two zones. To identify the dust component responsible for the infrared continuum emission, we explore two cases for the dust composition, one with a silicate+iron mixture and the other with a silicate+carbon one. We find that the iron-rich model provides a better fit to the spectral energy distribution. Our GGchem calculations also support an iron-rich and carbon-poor dust composition in the warm disk regions ($r<5$ au, $T > 300$ K). We propose that in the warm inner regions ($r<5$ au) of typical planet-forming disks, most if not all carbon is in the gas phase, while iron and iron sulfide grains are major constituents of the solid mixture along with forsterite and enstatite. Our analysis demonstrates the need for detailed studies of the dust in inner disks with new mid-infrared instruments such as MATISSE and JWST/MIRI.

Context. The Very Large Telescope Interferometer (VLTI) has been providing breakthrough images of the dust in the central parsecs of active galactic nuclei (AGNs), which is thought to be a key component of the AGN unification scheme and AGN host galaxy interaction. In single infrared bands, these images can enjoin multiple interpretations, some of which could challenge the unification scheme. This is the case for the archetypal type 2 AGN of NGC 1068, whereby the degeneracy is reduced by multi-band temperature maps that are hindered by an ambiguity in the alignment between different single-band images. Aims. We aim to solve this problem by creating a chromatic model capable of simultaneously explaining the VLTI GRAVITY+MATISSE 2 μm–13 μm observations of AGNs hosted by NGC 1068. Methods. We employed a simple disk and wind geometry populated by spherical black-body emitters and dust obscuration to create a versatile multi-wavelength modelling method for chromatic IR interferometric data of dusty objects. Results. This simple geometry is capable of reproducing the spectro-interferometric data of NGC 1068 from the K through N bands. It explains the complex single band images with obscuration and inclination effects, and it solves the alignment problem between bands. We find that the resulting model disk and wind geometry is consistent with previous studies of comparable and larger scales. For example, compared to molecular gas emission, our model wind position angle (PA) of 2322° is close to the mas-scale outflowing CO(6–5) PA of ∼33° seen with ALMA. The equivalent 90° offset model disk PA is also consistent with the CO(6–5) disk axis of 112° as well as the mas-scale disk axis from CO(2–1), CO(3–2), and HCO+(4–3) of 115 ± 5°. Furthermore, the resulting model images visually resemble the multiple achromatic image reconstructions of the same data when evaluated at the same wavelengths. We conclude that the IR emitting structure surrounding the AGN within NGC 1068 can indeed be explained by the clumpy disk+wind iteration of the AGN unification scheme. Within the scheme, we find that it is best explained as a type 2 AGN and the obscuring dust chemistry can be explained by a mix of olivine silicates and 16 ± 1% amorphous carbon.

MATISSE is the 2nd generation mid-infrared (3.0 mu m to 12.0 mu m) spectro-interferometric instrument of the Very Large Telescope Interferometer (VLTI). It was designed to deliver its advertised performance when supported by an external fringe tracker. This proceeding gives an historical account of how the fringe tracker of the GRAVITY instrument, another 2nd generation K-band spectro-astrometric instrument of VLTI, became this external fringe tracker. For a more technical and performance-oriented description of the GRAVITY for MATISSE project, Woillez, Petrov, et al. (2024)(1) should be consulted.

Context. MATISSE, the mid-infrared spectro-imaging instrument of VLTI, was designed to deliver its designed performance when paired with an external second-generation fringe tracker. Science observations started in 2019, demonstrating imaging capabilities and faint science target observations. Now, the GRAVITY fringe tracker stabilises the MATISSE fringes, which allows the use of all spectroscopic modes and improves sensitivity and data accuracy. Aims. We present how the MATISSE and GRAVITY instruments were adapted to make the GRAVITY fringe tracker work with MATISSE, under the umbrella of the aptly named GRA4MAT project, led by ESO in collaboration with the two instrument consortia. Methods. We detail the software modifications needed to implement an acquisition and observing sequence specific to GRA4MAT, including simultaneous fringe tracking and chopping and a narrow off-axis capability inspired by the Galactic Centre and exoplanet capability of GRAVITY. We explain the modified data collection and reduction processes. We show how we leveraged the recent fringe tracker upgrade to implement features specific to its use with MATISSE, for example mitigation of fringe jumps with an improved group delay control, and simultaneous fringe tracking and chopping with a new state machine. Results. We successfully demonstrate significant improvements to the MATISSE instrument. Observations can now be performed at higher spectral resolutions of up to R similar to 3300 and across the full LM bands at once. Long detector integration times, made possible with stabilised fringes, have improved the LM-band sensitivity by a factor of 10. Low flux biases in coherently reduced N-band data have been eliminated. The L-band transfer function is now higher and more stable. We finally illustrate the scientific potential of GRA4MAT with a preview of the first exoplanet observation made by MATISSE on beta Pictoris b.

From Nov. 2019 to May 2020, the red supergiant star Betelgeuse experienced an unprecedented drop of brightness in the visible domain called the Great Dimming event (GDE). Large atmospheric dust clouds and large photospheric convective features are suspected to be responsible for it. To better understand the dimming event, we used mid-infrared long-baseline spectro-interferometric measurements of Betelgeuse taken with the Very Large Telescope Interferometer/Multi AperTure mid-Infrared SpectroScopic Experiment (VLTI/MATISSE) instrument before (Dec. 2018), during (Feb. 2020), and after (Dec. 2020) the GDE. We present data in the 3.98-4.15 mu m range to cover SiO spectral features molecules as well as adjacent continuum. We have employed geometrical models, image reconstruction, as well as radiative transfer models to monitor the spatial distribution of SiO over the stellar surface. We find a strongly inhomogeneous spatial distribution of SiO that appears to be looking very different between our observing epochs, indicative of a vigorous activity in the stellar atmosphere. The contrast of our images is small in the pseudo-continuum for all epochs, implying that our MATISSE observations support both cold spot and dust cloud model.

The Very Large Telescope Interferometer (VLTI) is a wonderful infrastructure for long-baseline interferometry. MATISSE, the Multi AperTure mid-Infrared SpectroScopic Experiment, installed at the VLTI focus, accesses high resolution imaging over a wide spectral domain of the mid-infrared. The instrument is a spectro-interferometric imager operating in the L, M, and N transmission windows and combining four optical beams from the VLTI's unit or auxiliary telescopes. We propose at the SPIE conference to advertise the use of the MATISSE instrument. We will illustrate the instrument capabilities through astrophysical results recently achieved (the focus on the astrophysical results is not reported in the article). We also show what are the expected future infrastructure optimizations and instrument adaptations (off-axis tracking, frame of GRAVITY+) that will permit to push the sensitivities and accuracies for the astrophysical programs in the context of the JWST.

In this paper we present the first-ever $L$- and $M$-band interferometric observations of Circinus, building upon a recent $N$-band analysis. We used these observations to reconstruct images and fit Gaussian models to the $L$ and $M$ bands. Our findings reveal a thin edge-on disk whose width is marginally resolved and is the spectral continuation of the disk imaged in the $N$ band to shorter wavelengths. Additionally, we find a point-like source in the $L$ and $M$ bands that, based on the $LMN$-band spectral energy distribution fit, corresponds to the $N$-band point source. We also demonstrate that there is no trace of direct sightlines to hot dust surfaces in the circumnuclear dust structure of Circinus. By assuming the dust is present, we find that obscuration of A$_{\rm V} \gtrsim 250$ mag is necessary to reproduce the measured fluxes. Hence, the imaged disk could play the role of the obscuring "torus" in the unified scheme of active galactic nuclei. Furthermore, we explored the parameter space of the disk + hyperbolic cone radiative transfer models and identify a simple modification at the base of the cone. Adding a cluster of clumps just above the disk and inside the base of the hyperbolic cone provides a much better match to the observed temperature distribution in the central aperture. This aligns well with the radiation-driven fountain models that have recently emerged. Only the unique combination of sensitivity and spatial resolution of the VLTI allows such models to be scrutinized and constrained in detail. We plan to test the applicability of this detailed dust structure to other MATISSE-observed active galactic nuclei in the future.

ABSTRACT Single dish observations of NGC 1068 have shown the presence of a 3.4 μm aliphatic hydrocarbon absorption feature similar to diffuse lines of sight towards the Galactic Centre. Both, the single dish spectra and the correlated fluxes obtained by MATISSE, present a broad absorption profile in the L band as well, that vary from baseline to baseline. A new CRIRES spectrum with a slit width comparable to the MATISSE aperture also reveals a wide feature around 3.4 μm. We aim to analyse these features to learn about their origin and the distribution of the carriers across the source. We argue that all these features are caused by absorption of aliphatic hydrocarbons. The 3.4 μm absorption features in the interferometric MATISSE correlated fluxes vary in optical depth over scales of 0.2-1.4 parsec in the plane of the sky, although probably placed at a larger distance from the AGN along the line of sight. The absorption in the extended areas seems to peak in the direction of the inner jet, close to the center of the ionization cone. These results support the theory of a clumpy torus. In comparing our results to the N-band surface density previously reported, we have discovered a mass ratio of carbonaceous to silicate olivine dust at ∼2 - 3.6 per cent, which is similar to those found in several lines of sight in the diffuse interstellar medium of our Galaxy.

In this paper we present the first-everL- andM-band interferometric observations of Circinus, building upon a recentN-band analysis. We used these observations to reconstruct images and fit Gaussian models to theLandMbands. Our findings reveal a thin edge-on disk whose width is marginally resolved and is the spectral continuation of the disk imaged in theNband to shorter wavelengths. Additionally, we find a point-like source in theLandMbands that, based on theLMN-band spectral energy distribution fit, corresponds to theN-band point source. We also demonstrate that there is no trace of direct sightlines to hot dust surfaces in the circumnuclear dust structure of Circinus. By assuming the dust is present, we find that obscuration ofAV ≳ 250 mag is necessary to reproduce the measured fluxes. Hence, the imaged disk could play the role of the obscuring “torus” in the unified scheme of active galactic nuclei. Furthermore, we explored the parameter space of the disk + hyperbolic cone radiative transfer models and identify a simple modification at the base of the cone. Adding a cluster of clumps just above the disk and inside the base of the hyperbolic cone provides a much better match to the observed temperature distribution in the central aperture. This aligns well with the radiation-driven fountain models that have recently emerged. Only the unique combination of sensitivity and spatial resolution of the VLTI allows such models to be scrutinized and constrained in detail. We plan to test the applicability of this detailed dust structure to other MATISSE-observed active galactic nuclei in the future.

The Very Large Telescope Interferometer (VLTI) has been providing breakthrough images of the dust in the central parsecs of Active Galactic Nuclei (AGN), a key component of the AGN unification scheme and AGN host galaxy interaction. In single IR bands, the images can have multiple interpretations some of which could challenge the unification scheme. This is the case for the archetypal type 2 AGN of NGC1068. The ambiguity is reduced by multi-band temperature maps which are hindered by uncertainty in intra-band alignment. We create a chromatic model capable of simultaneously explaining the VLTI GRAVITY+MATISSE 2μm-13μm observations of the AGN in NGC1068. We use a simple disk and wind geometry populated with spherical black body emitters and dust obscuration to create a versatile multi-wavelength model for IR interferometric data of dusty objects. This simple geometry is capable of reproducing the K-N-band VLTI data, explains the complex single band images, and solves the alignment between bands. We find that the resulting geometry is consistent with previous studies. Compared to molecular gas emission, our model wind position angle (PA) of 22^3_2 is close to the mas scale outflowing CO(6-5) PA of  33 seen with the ALMA. The equivalent 90 offset model disk PA is also consistent with the CO(6-5) disk axis of 112 as well as the mas scale disk axis from CO(2-1), CO(3-2), and HCO^+(4-3) of 115±5. Furthermore, the resulting model visually resembles the equivalent achromatic image reconstructions. We conclude that the IR emitting structure surrounding the AGN can indeed be explained by the clumpy disk+wind iteration of the AGN unification scheme. Within the scheme, we find it is best explained as a type 2 and the obscuring dust chemistry is consistent with a mix of olivine silicates and 16±1

Active galactic nuclei play a key role in the evolution of galaxies, but their inner workings and physical connection to the host are poorly understood due to a lack of angular resolution. Infrared interferometry makes it possible to resolve the circumnuclear dust in the nearby Seyfert 2 galaxy, Circinus. Previous observations have revealed complex structures and polar dust emission but interpretation was limited to simple models. MATISSE makes it possible to image these structures for the first time. We observed the Circinus Galaxy with VLTI/MATISSE, producing 150 correlated flux spectra and 100 closure phase spectra. We reconstructed images in the N-band at ~10 mas resolution. We fit blackbody functions with dust extinction to several aperture-extracted fluxes from the images to produce a temperature distribution of central dusty structures. We find significant substructure in the circumnuclear dust: central unresolved flux of ~0.5 Jy, a thin disk 1.9 pc in diameter oriented along ~45 deg,and a ~4x1.5 pc polar emission extending orthogonal to the disk. The polar emission exhibits patchiness, which we attribute to clumpy dust. Flux enhancements to the east and west of the disk are seen for the first time. We distinguish the temperature profiles of the disk and of the polar emission: the disk shows a steep temperature gradient indicative of denser material; the polar profile is flatter, indicating clumpiness and/or lower dust density. The unresolved flux is fitted with a high temperature, ~370 K. The polar dust remains warm (~200 K) out to 1.5 pc from the disk. The recovered morphology and temperature distribution resembles modeling of accretion disks with radiation-driven winds at large scales, but we placed new constraints on the subparsec dust. The subparsec features imaged here place new constraints on the physical modeling of circumnuclear dust in active galaxies.

In the widely accepted ‘unified model’ 1 solution of the classification puzzle of active galactic nuclei, the orientation of a dusty accretion torus around the central black hole dominates their appearance. In ‘type-1’ systems, the bright nucleus is visible at the centre of a face-on torus. In ‘type-2’ systems the thick, nearly edge-on torus hides the central engine. Later studies suggested evolutionary effects 2 and added dusty clumps and polar winds 3 but left the basic picture intact. However, recent high-resolution images 4 of the archetypal type-2 galaxy NGC 1068 5 , 6 , suggested a more radical revision. The images displayed a ring-like emission feature that was proposed to be hot dust surrounding the black hole at the radius where the radiation from the central engine evaporates the dust. That ring is too thin and too far tilted from edge-on to hide the central engine, and ad hoc foreground extinction is needed to explain the type-2 classification. These images quickly generated reinterpretations of the dichotomy between types 1 and 2 7 , 8 . Here we present new multi-band mid-infrared images of NGC 1068 that detail the dust temperature distribution and reaffirm the original model. Combined with radio data (J.F.G. and C.M.V.I., manuscript in preparation), our maps locate the central engine that is below the previously reported ring and obscured by a thick, nearly edge-on disk, as predicted by the unified model. We also identify emission from polar flows and absorbing dust that is mineralogically distinct from that towards the Milky Way centre.

Context: FS Canis Majoris (FS CMa, HD 45677) is an unclassified B[e] star surrounded by an inclined dust disk. The evolutionary stage of FS CMa is still debated. Perpendicular to the circumstellar disk, a bipolar outflow was detected. Infrared aperture-synthesis imaging provides us with a unique opportunity to study the disk structure. Aims: Our aim is to study the intensity distribution of the disk of FS CMa in the mid-infrared L and N bands. Methods: We performed aperture-synthesis imaging of FS CMa with the MATISSE instrument (Multi AperTure mid-Infrared SpectroScopic Experiment) in the low spectral resolution mode to obtain images in the L and N bands. We computed radiative transfer models that reproduce the L- and N-band intensity distributions of the resolved disks. Results: We present L- and N-band aperture-synthesis images of FS CMa reconstructed in the wavelength bands of 3.4-3.8 and 8.6-9.0 micrometer. In the L-band image, the inner rim region of an inclined circumstellar disk and the central object can be seen with a spatial resolution of 2.7 milliarcsec (mas). An inner disk cavity with an angular diameter of 6x12mas is resolved. The L-band disk consists of a bright northwestern (NW) disk region and a much fainter southeastern (SE) region. The images suggest that we are looking at the bright inner wall of the NW disk rim, which is on the far side of the disk. In the N band, only the bright NW disk region is seen. In addition to deriving the inclination and the inner disk radius, fitting the reconstructed brightness distributions via radiative transfer modeling allows one to constrain the innermost disk structure, in particular the shape of the inner disk rim.

AGB stars are one of the main sources of dust production in the Galaxy. However, it is not clear what this process looks like and where the dust is condensing in the circumstellar environment. By characterizing the location of the dust and the molecules in the close environment of an AGB star, we aim to achieve a better understanding the history of the dust formation process. We observed the carbon star R Scl with the VLTI-MATISSE instrument in L- and N-bands. The high angular resolution of the VLTI observations, combined with a large uv-plane coverage allowed us to use image reconstruction methods. To constrain the dust and molecules' location, we used two different methods: MIRA image reconstruction and the 1D code RHAPSODY. We found evidence of C2H2 and HCN molecules between 1 and 3.4 Rstar which is much closer to the star than the location of the dust (between 3.8 and 17.0 Rstar). We also estimated a mass-loss rate of 1.2+-0.4x10-6 Msun per yr. In the meantime, we confirmed the previously published characteristics of a thin dust shell, composed of amorphous carbon (amC) and silicon carbide (SiC). However, no clear SiC feature has been detected in the MATISSE visibilities. This might be caused by molecular absorption that can affect the shape of the SiC band at 11.3 micron. The appearance of the molecular shells is in good agreement with predictions from dynamical atmosphere models. For the first time, we co-located dust and molecules in the environment of an AGB star. We confirm that the molecules are located closer to the star than the dust. The MIRA images unveil the presence of a clumpy environment in the fuzzy emission region beyond 4.0 Rstar. Furthermore, with the available dynamic range and angular resolution, we did not detect the presence of a binary companion. Additional observations combining MATISSE and SAM-VISIR instrument should enable this detection in future studies.

Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification. Aims. We aim to spatially resolve the photospheric extent as well as the circumstellar environment. Methods. We used interferometric observations obtained with the MATISSE instrument in the L (3 to 4 {\mu}m), M (4.5 to 5 {\mu}m), and N (8 to 13 {\mu}m) bands. We reconstructed monochromatic images using the MIRA software. We used 3D radiation-hydrodynamics (RHD) simulations carried out with CO5BOLD and a uniform disc model to estimate the apparent diameter and interpret the stellar surface structures. Moreover, we employed the radiative transfer codes Optim3D and Radmc3D to compute the spectral energy distribution for the L, M, and N bands, respectively. Results. MATISSE observations unveil, for the first time, the morphology of VX Sgr across the L, M, and N bands. The reconstructed images show a complex morphology with brighter areas whose characteristics depend on the wavelength probed. We measured the angular diameter as a function of the wavelength and showed that the photospheric extent in the L and M bands depends on the opacity through the atmosphere. In addition to this, we also concluded that the observed photospheric inhomogeneities can be interpreted as convection-related surface structures. The comparison in the N band yielded a qualitative agreement between the N band spectrum and simple dust radiative transfer simulations. However, it is not possible to firmly conclude on the interpretation of the current data because of the difficulty in constraing the model parameters using the limited accuracy of our absolute flux calibration. Conclusions. MATISSE observations and the derived reconstructed images unveil the appearance of the stellar surface and circumstellar environment across a very large spectral domain for the first time.