arXiv subjects

Alessandro Marconi

Publications and source records attributed to Alessandro Marconi.

At least 19 recordsLinked to original sources

arXiv2026

The Physical Conditions of Low-Mass Galaxies at z=3.5-7.0 from JWST Spectroscopy: The Behaviour of Spectral Line Ratios with Burstiness

We present a study of the physical conditions of galaxies at redshifts $z=3.5-7.0$, based on emission-line diagnostics of JWST NIRSpec spectra for 1889 star-forming sources spanning more than 4~dex in stellar mass. We find that the median H$\alpha$/H$\beta$ and [OIII]/H$\beta$ are constant throughout these redshifts, indicating a slow evolution of the interstellar medium conditions from the Epoch of Reionization to Cosmic Noon. The stellar-mass evolution of these line ratios follow the expected trend for chemical enrichment and dust build-up, with \ohb~being maximum for galaxies with $\rm M_{\star} \approx 10^9 \, \rm M_\odot$, and the medians of H$\alpha$/H$\beta$ and [OII]/H$\beta$ monotonically increasing from $\rm{M}_{\rm \star} \approx 10^7$ through $ \approx 10^{10}\, \rm M_\odot$. Focusing on the 1552 low stellar-mass ($<10^9 \, \rm M_\odot$) galaxies, we particularly investigate the behaviour of the main spectral-line ratios with burstiness. We find a significantly higher degree of scatter in [OIII]/H$\beta$ and [OII]/H$\beta$ for high-burstiness than for low-burstiness galaxies. Values of log([OIII]/H$\beta$)~$\gtrsim 0.8$ are almost exclusively seen amongst those with high burstiness. Meanwhile, log([OIII]/H$\beta$)~$\lesssim 0.3$ are virtually not found amongst high-burstiness galaxies, except at the lowest stellar masses $\mathrm{log(M_\star/M_\odot)}<7.5$. Our results are consistent with a scenario in which star-formation bursts produce temporary chemical enrichment that is quickly lost via stellar winds. They also suggest that metallicity measures in bursting galaxies are unstable, and that the search for the most metal-poor objects should optimally be conducted amongst low-burstiness or very low-stellar mass sources.

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arXiv2026

Misaligned or chaotic? A strong break of axial symmetry in the local LRD J1025 revealed with VLT/FORS2 spectropolarimetry

Little Red Dots (LRDs) are compact active galactic nuclei (AGN) with unusual spectral energy distributions and broad Balmer emission, candidate signposts of rapid black-hole growth. We present VLT/FORS2 optical linear spectropolarimetry of the closest known LRD, SDSS J102530.29+140207.3, at z=0.1. In total light, we detect spatially extended narrow-H$\alpha$ emission, probably tracing the host galaxy. We measure a nearly grey continuum polarisation $p_{\rm cont}=1.53\pm0.04$(rand.)$\pm$0.20(syst.) per cent, while broad H$\alpha$ is less polarised, $p_{\rm H\alpha}=0.58-0.84$ per cent. We rule out polarisation by Milky Way dust and dilution by an unpolarised line. The polarised continuum with a depolarised line resembles local Seyfert-1 nuclei and favours a single dominant source over multi-source explanations. After Stokes-continuum subtraction, broad H$\alpha$ shows no blue-to-red swing, but there is a significant (48$\pm$4)$^\circ$ continuum-to-line offset in polarisation angle. This implies a break of axial symmetry inside this object, posing interesting geometrical challenges to all existing LRD models and frameworks. We discuss different possible origins of this symmetry break and possible paths to discriminate between them with future observations.

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arXiv2026

Unsupervised selection and characterisation of Little Red Dots in JWST surveys with manifold learning

Little Red Dots (LRDs) are compact, red sources discovered at high redshift by JWST whose physical nature and selection function remain debated. We investigate whether an unsupervised machine-learning approach applied to multi-band photometry can identify LRD-like objects, and other populations, without relying on predefined colour cuts. Using UMAP, a manifold-learning (dimensionality-reduction) method, we place ~242,000 isolated, well-measured sources from the ASTRODEEP-JWST catalogue on a two-dimensional map, where objects with similar broadband colours, morphology, and photometric redshift lie close together. We then use spectroscopically confirmed LRDs to identify where LRD-like objects lie within this map, compare the resulting areas with published colour cuts, and validate our data-driven selection against archival NIRSpec spectra from the DJA. We find that the spectroscopically selected LRDs concentrate in two well-defined regions with no colour cut imposed, tracing populations that differ mainly in redshift, a difference imprinted in their broadband colours. The main region reaches a purity of ~0.78 at ~0.82 completeness on the spectroscopically classified subset, competitive with, or cleaner than, literature colour cuts, and yields ~100 additional candidates. We also test the method as a general tool for population discovery: the manifold recovers the locations of brown dwarfs and broad-line AGN with no explicit criterion, and isolates rare pathological outliers. Overall, unsupervised manifolds, anchored by sparse high-confidence spectroscopic labels, provide an efficient, assumption-light framework for characterising populations, comparing selection methods on a common basis, and discovering rare objects in large photometric datasets.

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arXiv2026

An (in)complete NIRSpec census of Balmer absorption in Type 1 AGN -- radiation-driven outflows in little red dots, quasars and variable stars

A notable achievement of the first generation of JWST surveys was the discovery of an abundance of faint high-redshift ($z > 4$) broad-line active galactic nuclei (AGN) in the $L_{\rm bol} < 10^{45}$~erg~s$^{-1}$ regime, previously only accessible at $z < 1$. The high prevalence of absorption features in their broad hydrogen lines is one of the peculiarities of a significant fraction of this new population. In this paper, we conduct a broad census of Balmer absorption in a sample of 47 Type-1 AGN spanning $2 < z < 7$. Accounting for incompleteness of JWST spectroscopic data, we estimate that $44_{-6}^{+21}$~\% of Little Red Dots (LRDs) have absorption in \Has while the incidence rate is $< 25$~\% (at 2$\sigma$) in Little Blue Dots (LBDs). Additionally, R1000 JWST/NIRSpec data is strongly resolution limited, implying an inability to disentangle Doppler broadening in the absorption from optical depth effects. This is alleviated with R2700 observations, although some degeneracies remain. We find a significant correlation between the velocity of the Balmer absorption and the [OIII]5007 narrow line luminosity, suggesting a common driving mechanism. We do not find any other significant correlations (and do not confirm previously claimed correlations) between Balmer absorption and other spectral properties of LRDs (except for the expected correlation between \Has and \Hbs absorption velocities). Comparing LRDs, quasars and stellar \Has absorbers, we establish that absorption in both LRDs and broad absorption line quasars is consistent with radiatively driven outflows, echoing the physics of variable stars yet occurring at vastly different scales.

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arXiv2026

Milky-Way-like stars in a galaxy core 8 billion years ago revealed by gravitational lensing

The assembly of stellar-dominated cores in elliptical galaxies is key to understanding how cosmic structures evolved. Gravitational lensing offers unique insights into the nature of their stars. We report the discovery of the smallest known quadruply lensed quasar (radius ~0.2"), whose lensing galaxy at redshift 1.055 (5.5 billion years after the Big Bang) features a lensing mass of only ~2x10^10 M_sun. A Bayesian analysis, based on the system's exceptional properties and standard scaling relations, allowed us to sample the central galactic initial mass function with unmatched accuracy and in a previously uncharted regime in terms of mass and redshift. We found it consistent with the Milky Way one, while excluding bottom-heavy functions. This suggests that the core either grew slowly or underwent early disruptive events altering its stellar build-up, in contrast with the classical view that bulges form rapidly and remain unchanged by later interactions.

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arXiv2026

The search for Population III: Confirmation of a HeII emitter with no metal lines at z=10.6

We report the confirmation of a HeII$\lambda$1640 emitter located at 3 pkpc from the galaxy GN-z11, at z=10.6. The detection, based on JWST NIRSpec-IFU high-resolution spectroscopy, confirms a previous claim based on medium-resolution spectroscopy. The HeII$\lambda$1640 identification is further supported by the independent detection of H$\gamma$ obtained by \"Ubler et al. (2026) at the same location. The HeII emission is spectrally resolved in two components separated by 120 km/s. The Equivalent Width of the HeII emission is extremely high ($>$20 A). No metal lines are detected. We argue that Population III stars are the most plausible explanation for the observed He II emission, with no satisfactory alternative from other classes of sources or mechanisms.

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arXiv2026

The Pristine HeII Emitter near GN-z11: Constraining the Mass Distribution of the First Stars

The properties of the first metal-free stars remain largely unknown, and so far, the only data-driven constraints on their mass distribution (IMF) come from near-field cosmology. Here, we interpret new observations of the C1 and C2 components of Hebe, the HeII emitter near the galaxy GN-z11. Using a locally calibrated model, we robustly confirm the pristine (PopIII) nature of both components, showing that the measured upper limits on metal lines can only be reproduced by galaxies with $>50\%$ of their stellar mass in PopIII stars. We find that C1 is consistent with a purely PopIII system and adopt a simple parametric approach to infer the implications for the PopIII IMF and stellar mass. The observed $\rm HeII/H_\gamma$ ratio excludes steep IMFs, favoring top-heavy distributions, especially for young stellar ages ($\leq 1$ Myr). Combined with the HeII luminosity, this implies a total PopIII stellar mass of $2 \cdot 10^4 < M_\star/M_\odot < 6 \cdot 10^5$. While degeneracies between IMF, stellar mass, and age remain, adopting the lower stellar masses predicted by simulations ($M_\star < 10^5\,M_\odot$) strengthens the preference for top-heavy IMFs. Combining these results with near-field constraints, which instead exclude the flattest IMFs, we define a data-driven range of viable PopIII IMFs, linking characteristic mass and slope. This work demonstrates that direct observations of high-$z$ PopIII systems can place independent constraints on the IMF of the first stars, opening a new window on their formation and properties.

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arXiv2026

One cloud is not enough: extreme conditions bias chemical abundances in high-redshift galaxies

Since its launch, JWST has opened an unprecedented opportunity to characterise the ionised ISM of high-redshift galaxies using well-established rest-frame UV/optical diagnostics from the local Universe. At the same time, these observations challenge the validity of such classical methods when applied to the extreme environments typical at high redshift. We present an in-depth analysis of the ISM in three representative case studies at $z=2 - 6$ (MARTA 4327, the Sunburst Arc and RXCJ2248-ID) conducted within a multi-cloud photoionisation modelling framework (HOMERUN). We show that even a small fraction of unresolved high-density clumps can contribute more than half of the observed flux of auroral lines, while only negligibly to standard optical density tracers. As a result, $T_{\mathrm{e}}$-method metallicities can be underestimated by $\sim 0.15 - 0.3$ dex, as for MARTA 4327. By modelling rest-frame UV and optical data, we demonstrate that discrepancies between abundances obtained from diagnostics tracing different zones do not necessarily imply chemical inhomogeneities. In RXCJ2248-ID, the disagreement between UV and optical N/O may naturally arise from ionisation and density structure alone. In contrast, we find evidence for genuine chemical stratification in the Sunburst Arc, where a component enriched in nitrogen coexists with a chemically normal one. Finally, we argue that very-high-ionisation lines may be explained within a pure star-formation scenario invoking matter-bounded regions. However, in the case of RXCJ2248-ID, we cannot rule out a minor contribution from an AGN based solely on the observed fluxes. These results indicate that classical diagnostics can be significantly biased in high-redshift galaxies and that self-consistent, physically motivated tools are therefore essential to properly interpret the complex ISM conditions and chemical enrichment in the early Universe.

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arXiv2025

Signatures of star formation inside galactic outflows

Observations have suggested that galactic outflows contain substantial amounts of dense and clumpy molecular gas, creating favourable conditions for igniting star formation. Indeed, theoretical models and hydrodynamical simulations have suggested that stars could form within galactic outflows, representing a new mode of star-formation that differs significantly from the typical star formation in star forming discs. In this paper, we examine 12 local galaxies with powerful Active Galactic Nuclei and high star-formation rate using spectroscopic data from the X-shooter spectrograph at the Very Large Telescope. We investigate the excitation mechanism and physical properties of these outflows via spatially resolved diagnostic diagrams (along with tests to rule out contribution by shocks and external photoionisation). Out of the seven galaxies with clearly detected outflows, we find robust evidence for star formation within the outflow of one galaxy (IRAS 20551-4250), with two additional galaxies showing tentative signs (IRAS 13120-5453 and F13229-2934). Therefore, our findings support previous results that star formation inside outflows can be a relatively common phenomenon among these active galaxies and may have played an important role in the formation and evolution of the spheroidal component of galaxies.

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arXiv2025

BlackTHUNDER: Shedding light on a dormant and extreme little red dot at z=8.50

Recent photometric surveys with JWST have revealed a significant population of mysterious objects with red colours, compact morphologies, frequent signs of active galactic nucleus (AGN) activity, and negligible X-ray emission. These 'Little Red Dots' (LRDs) have been explored through spectral and photometric studies, but their nature is still under debate. As part of the BlackTHUNDER survey, we have observed UNCOVER_20466, one of the most distant LRDs known (z=8.5), with the JWST/NIRSpec IFU. Previous JWST/NIRCam and JWST/NIRSpec MSA observations of this source revealed its LRD nature, as well as the presence of an AGN. Using our NIRSpec IFU data, we confirm that UNCOVER_20466 is an LRD (based on spectral slopes and compactness) that contains an overmassive black hole. However, our observed Balmer decrements do not suggest strong dust attenuation, resulting in a lower Hbeta-based bolometric luminosity and Eddington luminosity (~10%) than previously found. This source lies on local relations between M_BH-sigma_* and M_BH-M_Dyn, suggesting that this could be a progenitor of the core of a lower-redshift galaxy. We explore the possible evolution of this source, finding evidence for substantial black hole accretion in the past and a likely origin as a heavy seed at high redshift (~10^3Msol). Lyman-alpha emission is strongly detected, implying f_esc,Lya~30%. The extremely high [OIII]4363/Hgamma ratio is indicative of not only AGN photoionization and heating, but also extremely high densities (ne~10^7cm-3), suggesting that this black hole at such high redshift may be forming in an ultra-dense protogalaxy.

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arXiv2025

MARTA: The connection between chemical enrichment, feedback, and dust in a Wolf-Rayet galaxy at z${\sim}$2

We present the analysis of the stellar and interstellar medium (ISM) properties of MARTA-4327, a star-forming galaxy at z=2.224 observed by means of deep JWST/NIRSpec spectroscopy in both medium- and high-resolution gratings as part of the "Measuring Abundances at high Redshift with the Te Approach" (MARTA) programme. We report one of the highest-redshift detections of the Wolf-Rayet (WR) blue and red bumps in a non-lensed system. The broad He ii${\lambda}$4686 feature is consistent with a young (${\sim 5-6}$ Myr) burst dominated by WN stars, although both SSP models and empirical templates struggle to reproduce the nitrogen stellar features at ${\approx}$ 4640 A. Based on the relative strength of the available optical stellar features, we disfavor the presence of very massive stars (VMS) in this system. Elemental abundance ratios such as Ne/O, N/O, and Ar/O align with observations of local star-forming galaxies (including WR galaxies), suggesting that any impact of the WR population on the chemical enrichment of the ISM is strongly localized. However, the gas-phase Fe/O ratio appears enhanced compared to local galaxies of similar metallicity, which we interpret as evidence for reduced Fe depletion onto dust grains, possibly linked to localized destruction in WR-driven wind environments. In addition, we detect a broad and blueshifted (~70 km/s) H${\alpha}$ component, revealing the presence of an ionized outflow with a mass loading factor ${\eta \sim 0.2}$. Finally, we report the robust detection of O I${\lambda}$8446 emission (among the firsts at high redshift), which we interpret as originating from Ly${\beta}$ fluorescence and/or collisional excitation in dense clumps. Overall, MARTA-4327 represents a unique system for studying the role of massive stars in shaping the ISM in galaxies at Cosmic Noon.

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arXiv2025

A direct black hole mass measurement in a Little Red Dot at the Epoch of Reionization

Recent discoveries of faint active galactic nuclei (AGN) at the redshift frontier have revealed a plethora of broad \Halpha emitters with optically red continua, named Little Red Dots (LRDs), which comprise 15-30\% of the high redshift broad line AGN population. Due to their peculiar spectral properties and X-ray weakness, modeling LRDs with standard AGN templates has proven challenging. In particular, the validity of single-epoch virial mass estimates in determining the black hole (BH) masses of LRDs has been called into question, with some models claiming that masses might be overestimated by up to 2 orders of magnitude, and other models claiming that LRDs may be entirely stellar in nature. We report the direct, dynamical BH mass measurement in a strongly lensed LRD at $z = 7.04$. The combination of lensing with deep spectroscopic data reveals a rotation curve that is inconsistent with a nuclear star cluster, yet can be well explained by Keplerian rotation around a point mass of 50 million Solar masses, consistent with virial BH mass estimates from the Balmer lines. The Keplerian rotation leaves little room for any stellar component in a host galaxy, as we conservatively infer $M_{\rm BH}/M_{*}>2$. Such a ''naked'' black hole, together with its near-pristine environment, indicates that this LRD is a massive black hole seed caught in its earliest accretion phase.

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arXiv2025

A deep dive down the broad-line region: permitted OI, CaII and FeII emission in an AGN Little Red Dot at z=5.3

We present a spectroscopic analysis of a broad-line active galactic nucleus (AGN) selected as little red dot at $z = 5.3$ behind the Bullet cluster (Bz5.3), based on JWST/NIRCam and NIRSpec data. The detection of strong FeII, OI, and CaII triplet emission lines, along with the evidence of broad Balmer lines, provides strong evidence of a broad-line region (BLR) and an accreting supermassive black hole. Notably, we report the first detection of the $\lambda1304$ bump (i.e., blend of OI$\lambda$1304 and SiII) at high redshift, a feature commonly seen in local AGNs but not yet reported in the early Universe. The OI$\lambda$1304/$\lambda8446$ photon ratio provides an independent measurement of dust attenuation in galaxies. In Bz5.3, this ratio is highly suppressed (0.1--0.3), implying significant internal dust extinction, with estimated dust attenuation $A_V \sim 0.4$--$1.0$. We identify Ly$\beta$ fluorescence as the dominant excitation mechanism of the low-ionization lines, with additional contributions from collisional excitation. High OI$\lambda$8446 equivalent width and weak OI$\lambda$7774 support this interpretation. The detection of iron emission, whether from broad permitted or narrow forbidden lines, supports the presence of a stratified BLR, as also recently proposed in local LRDs. Photoionization modeling of OI$\lambda$8446 and CaII further suggests the coexistence of multiple gas phases with distinct densities and ionization states, highlighting the complexity of the BLR. Bz5.3 thus offers a rare window into early AGN activity and BLR physics at early times.

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arXiv2025

Metal-polluted Population III galaxies and How to Find Them

Observing Population III (hereafter PopIII) galaxies, the hosts of first-generation stars, remains challenging even with the JWST. The current few candidates have been identified through the combination of a prominent HeII emission and the absence of metal lines, a well-known but extremely brief signature of metal-free systems. Here, we accurately model the evolution of the emission from PopIII galaxies to increase the number of candidates in JWST observations. To achieve this, we employ a locally calibrated galaxy-formation model that self-consistently follows the star formation and chemical evolution initiated by the first stars. We find that PopIII galaxies can emit metal lines in their ``self-polluted'' phase, while galaxies host only metal-free stars but the gas has been chemically-enriched by the first supernovae. In this phase, PopIII galaxies have $\rm [OIII]/H\beta \approx 1$, which opens the pool of candidates to more easily detectable sources. We predict that the high HeII emission of PopIII galaxies can last up to $\rm \approx 20 \, Myrs$ and that it is partly maintained in the ``hybrid'' phase, when PopIII and PopII stars co-exist in the host galaxy. We propose novel diagnostics involving UV metal lines to select PopIII candidates in high-z JWST surveys. In JADES, we identify 9 candidate galaxies with $>25\%$ of their stellar mass in metal-free stars, showcasing the effectiveness of our method. Ultimately, the key to discovering PopIII galaxies could be to catch them during their first episodes of chemical enrichment.

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arXiv2025

Unveiling the Fast Acceleration of AGN-Driven Winds at Kiloparsec Scales

Supermassive black holes at the centre of galaxies gain mass through accretion disks. Models predict that quasi-spherical winds, expelled by the black hole during active accretion phases, have a key role in shaping galaxy evolution by regulating star formation, the distribution of metals over kiloparsec scales, and by sweeping ambient gas to the outskirts and beyond of galaxies. Nonetheless, the mechanism driving these outflows and the amount of energy exchanged between the wind and the galaxy's interstellar medium remain unclear. Here, we present a detailed analysis of the kinematical properties of winds in a sample of nearby active galaxies using the novel kinematic tool MOKA3D, which takes into account the clumpy nature of the ISM. We find remarkable similarities among the properties of the outflows in all the galaxies examined. In particular, we provide the first evidence that outflows exhibit a regular trend in radial velocity, initially constant or slightly decreasing, followed by rapid acceleration starting at approximately 1 kpc from the nucleus, despite the seemingly complex kinematics observed. The observed behavior aligns with our current theoretical understanding of Active Galactic Nuclei outflows, where a momentum-driven phase transitions to an energy-conserving phase just beyond approximately 1 kpc. The constant velocity of the momentum-driven wind is then rapidly accelerated following the inefficient Compton cooling of post-shock material and the transition to energy conservation. The measured radial terminal velocities of the outflows are always larger than the escape velocities from the host galaxies, confirming the key role of outflows in shaping the galaxy properties and evolution, as a manifestation of AGN feedback. Our results, only made possible by our novel kinematic analysis tool, are crucial to understand the origin and the powering mechanism of these winds.

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arXiv2025

BlackTHUNDER strikes twice: rest-frame Balmer-line absorption and high Eddington accretion rate in a Little Red Dot at $z=7.04$

JWST has revealed a population of 'Little Red Dots' (LRDs): compact, red objects at redshifts z=2-9 with 'v'-shaped spectral energy distributions, broad permitted lines, and, often, hydrogen Balmer absorption. We use NIRSpec/IFS data from the BlackTHUNDER survey to study the H$\alpha$ line in the LRD Abell2744-QSO1 at z=7.04, which is a confirmed AGN due to time-variable equivalent width (EW) in its broad emission lines. The H$\alpha$ spectral profile is non-Gaussian, requiring at least two Gaussian components. We also detect a narrow-line Gaussian component, and strong H$\alpha$ absorption (EW relative to the continuum $\sim 22_{-7}^{+12} \mathring{\mathrm{A}}$), confirming a connection between the strong Balmer break and line absorption. The absorber is at rest with respect to broad H$\alpha$, suggesting that the gas cannot be interpreted as an inflow or outflow, forming instead a long-lived structure. Its velocity dispersion is $\sigma_{\rm abs}=110_{-10}^{+20}$ km s$^{-1}$, consistent with the value inferred from the analysis of the Balmer break. Based on H$\alpha$, we infer a black hole mass of log($M_{\rm BH}/{\rm M_\odot}$)=7.2, smaller but close to the previous estimates based on H$\beta$. The Eddington ratio is 0.09. Combining the high signal-to-noise ratio of the narrow H$\alpha$ line with the spectral resolution R=3,700 of the G395H grating, we infer a narrow-line intrinsic dispersion $\sigma_{\rm n}=22_{-6}^{+5}$ km s$^{-1}$, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be $M_{\rm BH}/M_{\rm dyn}$=0.15-1.2, confirming the overmassive nature of the black hole and potentially leaving little room for a host galaxy.

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arXiv2025

Classifying spectra of emission-line regions with neural networks -- An application to integral field spectroscopic data of M33

Emission-line regions are key to understanding the properties of galaxies, as they trace the exchange of matter and energy between stars and the interstellar medium (ISM). In nearby galaxies, individual nebulae can be identified as HII regions, planetary nebulae (PNe), supernova remnants (SNR), and diffuse ionised gas (DIG) with criteria on single or multiple emission-line ratios. However, these methods are limited by rigid classification boundaries, the narrow scope of information they are based upon, and the inability to account for line-of-sight nebular superpositions. In this work, we use artificial neural networks to classify these regions using their optical spectra. Our training set consists of simulated spectra, obtained from photoionisation and shock models, and processed to match observations obtained with MUSE. We evaluate the performance of the network on simulated spectra for a range of signal-to-noise (S/N) levels and dust extinction, and the superposition of different nebulae along the line of sight. At infinite S/N the network achieves perfect predictive performance, while as the S/N decreases, the classification accuracy declines, reaching an average of ~80% at S/N(H$\alpha$)=20. We apply our model to real spectra from MUSE observations of the galaxy M33, where it provides a robust classification of individual spaxels, even at low S/N, identifying HII regions and PNe and distinguishing them from SNRs and diffuse ionized gas, while identifying overlapping nebulae. We then compare the network's classification with traditional diagnostics and find satisfactory agreement. Using activation maximisation maps, we find that at high S/N the model mainly relies on weak lines (e.g. auroral lines of metal ions and He recombination lines), while at the S/N level typical of our dataset the model effectively emulates traditional diagnostic methods by leveraging strong nebular lines.

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arXiv2024

The JWST/NIRSpec view of the nuclear region in the prototypical merging galaxy NGC 6240

Merger events are thought to be an important phase in the assembly of massive galaxies. At the same time, Active Galactic Nuclei (AGN) play a fundamental role in the evolution of their star formation histories. Both phenomena can be observed at work in NGC 6240, a local prototypical merger, classified as an UltraLuminous InfraRed Galaxy (ULIRG) thanks to its elevated infrared luminosity. Interestingly, NGC 6240 hosts two AGN separated by 1.5''(~ 735 pc), detected in both X-ray and radio band. Taking advantage of the unprecedented sensitivity and wavelength coverage provided by the Integral Field Unit (IFU) of the NIRSpec instrument onboard JWST, we observed the nuclear region of NGC 6240 in a FoV of 3.7'' x 3.7''(1.9 x 1.9 kpc^2), to investigate gas kinematics and InterStellar Medium (ISM) properties with a high spatial resolution of ~ 0.1'' (or ~ 50 pc). We separated the different gas kinematic components through multi-Gaussian fitting and studied the excitation properties of the ISM from the NIR diagnostic diagram based on the H_2 1-0 S(1)/BrGamma and [Fe II]1.257micron/PaBeta lines ratios. We isolated the ionization cones of the two nuclei, and detected coronal lines emission from both of them. Using H_2 line ratios, we found that the molecular hydrogen gas is excited mostly by thermal processes. We computed a hot molecular gas mass of 1.3 x 10^5 M_sun and an ionized gas mass in the range of 10^5 - 10^7 M_sun. We studied with unprecedented spatial resolution and sensitivity the kinematics of the molecular and ionized gas phases. We revealed the complex structure of the molecular gas and found a blueshifted outflow near the Southern nucleus, together with filaments connecting a highly redshifted H_2 cloud with the two nuclei. We speculate on the possible nature of this H_2 cloud and propose two possible scenarios: either outflowing gas, or a tidal cloud falling onto the nuclei.

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