arXiv subjects

Stefano Carniani

Publications and source records attributed to Stefano Carniani.

At least 19 recordsLinked to original sources

arXiv2026

Metal Mayhem at z~7-10: Diversity and Evolution of Gas-Phase Metallicity Gradients

We present a JWST/NIRSpec Integral Field Unit (IFU) study of metallicity gradients in seven low-metallicity systems at $z=7.2-9.5$. The main sample spans stellar masses of $\rm \log(M_*/M_{\odot}) \sim 7.8-9.5$, star formation rates (SFRs) of $\rm \log(\text{SFR} / M_{\odot} \text{yr}^{-1}) \sim 0.5-2.5$, and gas-phase metallicities of $4\%-13 \%~Z_\odot$. Within our sample, we also identify three low-metallicity satellite galaxies associated with two of our sources, providing a rare view of early-epoch interactions. The three satellites exhibit even more primordial properties, with metallicity $3\% -4\% ~Z_\odot$ and low star-formation activity ($\rm \log(\text{SFR} / M_{\odot} \text{yr}^{-1}) \sim -0.5$ to $-0.9$). We find that our galaxies, and especially the satellites, are significantly offset from the local Fundamental Metallicity Relation (FMR), with deviations reaching $Δ\text{FMR} \approx -1.0$ dex. This indicates that these galaxies are likely experiencing strong accretion of pristine gas. Overall, we observe a large scatter in radial metallicity gradients, ranging from positive to negative with an average metallicity gradient of $\rm -0.03 \pm 0.04 \ dex \ kpc^{-1}$. Flat gradients are found in systems with confirmed satellites, suggesting that tidal interactions and mergers drive the radial mixing necessary to homogenise the interstellar medium. The (tentative) presence of an AGN in two of our sources suggests that strong feedback may also be responsible for the observed flat gradients. Conversely, the detection of a positive gradient in one source points toward a direct funnelling of metal-poor gas inflow into the central region of the galaxy. These results show that galaxies in the first billion years grow through diverse, episodic processes, suggesting that early evolution is characterised by structural variety rather than a single, predictable path.

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arXiv2026

PRISMS. GHZ1. A standard tale of galaxy evolution with atypical ionizing conditions at $z=9.878$

[Abridged] The first operating years of the James Webb Space Telescope (JWST) have revealed a surprising number of $z \gtrsim 10$ UV-bright objects, many of them characterized by their compactness, anomalous chemical abundance patterns and complex ISM physics (e.g. N-enhanced systems). To assess whether these objects are a phase of early galaxy evolution or a peculiar population, it is essential to understand the physics of systems without anomalous abundance patterns in this epoch. We present a detailed analysis of GHZ1 ($z=9.878$), a moderately ($μ= 1.715$) lensed UV-bright ($M_{UV} = -20.07$) galaxy in the Abell2744 field, characterized by an extended size ($R_{e} \sim 410$ pc). These properties, which contrast with the properties of many of the N-enhanced galaxies, make GHZ1 an excellent laboratory to assess what are the major drivers of early galaxy evolution and the origin of the differences. By means of JWST spectroscopic observations from NIRSpec (PRISM-CLEAR configuration) and the MIR/LRS, we analyzed the full UV and optical emission from GHZ1. We used a spectro-photometric SED fitting to characterize the stellar evolution and the emission line spectra to constrain its chemical enrichment. We find that GHZ1 is among the most massive systems at $z\sim 10$ (log(M$_{\star}$ [M$_{\odot}$]) = 9.17$_{-0.26}^{+0.27}$), with a moderate amount of dust attenuation ($A_{V} = 0.21_{-0.12}^{+0.12}$~mag) and ongoing star formation (with a median SFR $= 8.75_{-1.72}^{+1.71}$ M$_{\odot}\cdot$yr$^{-1}$ over the last 10 Myr). Its chemical enrichment patterns (12+log(O/H) = $7.81_{-0.11}^{+0.12}$, log(C/O) = $-0.66_{-0.13}^{+0.13}$, log(N/O)$< -1.04$) are consistent with a standard evolution dominated by primary production from massive stars. We also derive an H$α$/H$β$ Balmer ratio below Case B predictions. We speculate that [...].

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arXiv2026

A Dusty Quenching-candidate AGN host at z=5.7: massive quiescent galaxies may quench already during the dust-obscured phase

We present a spectro-photometric analysis of RUBIES-EGS-9809, a broad-line AGN host at z=5.7 with a Balmer break and a dominant point source. The source is dust reddened ($A_V>$3 mag), shows hot- and cold-dust emission, and is X-ray Compton thick, with X-ray-to-bolometric ratio consistent with luminous AGN. The large $A_V$ agrees with strong NaI absorption (EW=17$\pm 2 Å$), possibly from a neutral-gas outflow (3 $σ$), while broad [O III] traces an ionized outflow, suggesting AGN feedback may be affecting the host. We estimate a stellar mass log(M*/M$_\odot$)=10.7$\pm$0.3, Lbol$\sim4\times10^{46}$ erg s$^{-1}$ , and black-hole mass log(M$_{BH}/M_\odot)\sim8.1$ (subject to large systematics). The resolved Balmer break (0.1 arcsec, 0.6 kpc) is consistent with a stellar origin, implying the galaxy is already old in stars while still heavily obscured. This shows that massive galaxies can host evolved stellar populations during their compact, dust-obscured phase, consistent with formation in an earlier dusty starburst. SED modelling suggests a declining SFR, but low-resolution spectroscopy alone cannot constrain the recent SFR, so we treat the source as a strong quenching candidate rather than a secure post-starburst system. Because quenching follows the decline in SFR, star-formation-driven outflows are disfavoured, leaving AGN feedback as a plausible driver. We speculate that such rapid feedback ($\sim$100 Myr timescale) may precipitate quenching while the post-quenching phase stays dust-enshrouded, concealing the UV-bright quenched phase that would otherwise be easily detected, if dust free. By the time dust clears, the UV-luminous stars may have already dwindled, leaving a classic dust-free quiescent galaxy. This 'dusty path' to quiescence may explain the lack of intermediate-age quenched progenitors (ages 50-100 Myr) linking dusty starbursts to massive quiescent systems at z=3-7.

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arXiv2026

Super-Eddington Accretion and Early-Stage Feedback in Ton S180

Narrow-line Seyfert 1 (NLSy1) galaxies are key laboratories for studying rapid supermassive black hole (SMBH) growth and active galactic nucleus (AGN) feedback at high accretion rates. We investigate the nearby NLSy1 Ton S180 with VLT-MUSE optical integral field spectroscopy to connect its nuclear accretion properties with the spatially resolved ionized gas and host-galaxy kinematics. We modeled the unresolved nuclear spectrum and applied a custom point spread function subtraction to recover the host-galaxy emission on kiloparsec scales. The nuclear spectrum requires a complex permitted-line decomposition and a blueshifted [O III] outflow component. Single-epoch estimators and the stellar velocity dispersion imply black hole masses in the range $\log(M_{\rm BH}/M_\odot) = 6.5 - 7.7$. Combined with the observed luminosity, this implies a dimensionless mass accretion rate of $\dot{M} / \dot{M}_{\rm Edd} = 4.1 - 980$, confirming the extreme accretion regime. The host galaxy shows a circumnuclear ring, an inner elongated structure consistent with a bar, and rotation-dominated gas and stellar kinematics. Simple inflow models do not significantly better reproduce the observed velocity field. We detect a resolved ionized outflow extending about 2 kpc west of the nucleus, with mildly blueshifted velocities (with a maximum of $v_{\rm max} \sim 340$ km s$^{-1}$). Its mass outflow rate is only $\sim 4.2 \times 10^{-4}\,M_\odot\,\mathrm{yr}^{-1}$, whereas the unresolved nuclear outflow reaches $v_{\rm max} \sim 1140$ km s$^{-1}$ and $\dot{M}_{\rm out} > 1.5\,M_\odot\,\mathrm{yr}^{-1}$. This contrast may reflect either weak ionized coupling from nuclear to galactic scales or different episodes of AGN activity over time. These results show that Ton S180 is undergoing rapid SMBH growth, while the observed ionized outflow remains confined to the inner few kiloparsecs and is weak on host-galaxy scales.

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arXiv2026

Little Red Dots: One Photometric Tag Concealing Diverse Spectroscopic Flavors of Massive Star Formation and Black Hole Activity

We compile JWST/NIRSpec prism and MIRI data for 249 Little Red Dots (LRDs) at 2.3<z<9.3, forming a representative spectroscopic subset of NIRCam-selected LRDs. We derive a median stacked spectrum covering rest-frame 0.09-1.2 $μ$m, with MIRI photometry extending the spectral energy distribution to 4 $μ$m. Four additional stacks for subsamples defined by optical-to-UV luminosity ratios show that LRDs form a heterogeneous population spanning diverse continuum slopes and line properties. Assuming LRDs host super-massive black holes (BHs) surrounded by dense gas clouds, and stars accompany this core, we infer masses of $M_{BH}\sim10^{6.0-6.5}$ M$_\odot$ and $M_\bigstar\sim10^{8.3}$ M$_\odot$, corresponding to BH-to-stellar mass ratios of 1-2%. The stacks show ubiquitous UV and optical FeII emission, indicating a direct view of the broad-line region and high (but sub-Eddington) accretion ($λ_{Edd}=0.6\pm0.2$). We find a significant stellar contribution in the far-UV, reaching $\sim80$% in the bluest systems. Possible Wolf-Rayet features (HeII$λ$4687, nitrogen lines) are identified, tracing a young (3-7 Myr) compact starburst event. We also detect strong Balmer breaks and atypical Balmer, Paschen, [OIII], and optical and near-infrared HeI line ratios, and an absorption at $\sim4550$ Angstrom (probably linked to FeII), all consistent with radiative-transfer effects in high-density gas with warm temperatures (4000-7000 K). We find a diversity of LRD flavors modulated by the luminosity ratio between between a short ($\lesssim20$ Myr) and intense phase of BH activity, the most extreme stage lasting $\sim3-7$ Myr, characterized by near-Eddington-limit radiation, and a nuclear and compact starburst dominated by massive stars (even super-massive, $\mathrm{M}_\mathrm{SMS}\sim10^{5}$ M$_\odot$), all embedded in dense gas with modest dust content producing a variety of optical depths.

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arXiv2026

JWST/NIRSpec Spectra for Three Ultracool Brown Dwarfs Detected in Extragalactic Surveys: Further Evidence for Phosphine Absorption

We present JWST NIRSpec prism spectra for three ultracool brown dwarfs discovered in extragalactic survey data, two from the JWST Advanced Deep Extragalactic Survey (JADES), and one from Public Release IMaging for Extragalactic Research (PRIMER) survey observed as part of the Mirage or Miracle (MoM) program. The spectra for these sources indicate that one is a T6 dwarf (JADES-GS-BD-11, T$_{\mathrm{eff}} = \sim 700$ K) and two are Y0-Y1 dwarfs (JADES-GS-BD-5, T$_{\mathrm{eff}} = \sim 400$ K, and MoM-239450, T$_{\mathrm{eff}} = \sim 500$ K). Model atmospheric fits with $\texttt{NIFTY}$ to the spectra are consistent with this classification, and indicate that JADES-GS-BD-5 is only $\sim 150$ pc from the Sun, MoM-239450 is $\sim 700 - 800$ pc from the Sun, and JADES-GS-BD-11 is $\sim 1$ kpc from the Sun, with these latter two more distant sources being best fit at sub-solar metallicities. JADES-GS-BD-5 has an observed spectrum with significantly weaker J and H band emission than Y dwarf atmospheric models, potentially indicating the presence of water ice clouds in the brown dwarf. The spectrum for JADES-GS-BD-11 has a feature at 4.3$μ$m consistent with absorption from the rarely seen phosphine molecule at $2.6σ$ confidence. Given the low metallicity for this source ([M/H] $ = -0.7$), our finding supports the theory that detecting phosphine in brown dwarf atmospheres is tied to atmospheric metallicity. JWST/NIRSpec spectroscopy continues to be a powerful tool for understanding the properties of these distant, and very cold brown dwarfs.

astro-ph.SR↗

arXiv2026

Doubling NIRSpec/IFS capability to calibrate the single-epoch black hole mass relation at high redshifts

The recent discovery of a large population of overmassive black holes (BHs) in the early Universe challenges the validity of the BH-host galaxy coevolution framework. However, the reliability of the estimated BH masses (M$_{BH}$) is being questioned, as these are typically derived using single-epoch (SE) relations calibrated locally. Calibrating SE relations at high redshift would therefore enable more accurate M$_{BH}$ estimates and help identify potential biases. In this work, we release a data-reduction technique for JWST/NIRSpec IFU observations that doubles the effective wavelength coverage, enabling detection of otherwise inaccessible emission. Whenever adjacent dispersers are required, observers should carefully evaluate the tradeoff between integrating longer in the bluer configuration alone versus distributing the exposure time across two dispersers. We apply this pipeline to a sample of 5 quasars at z~2 with M$_{BH}$ independently measured through reverberation mapping (RM). This enables a joint analysis of both H$β$ and H$α$; the latter lying beyond the nominal wavelength range. We assess the reliability of the most widely adopted SE calibrations, finding that H$β$ yields the closest agreement with RM-based M$_{BH}$ estimates, whereas H$α$-based estimators exhibit a larger scatter. For the least massive BH in our sample ($M_{BH,RM}$~$10^{7.5}M_\odot$), which is accreting at a rate close to the Eddington limit ($λ_{Edd}=0.8$), all SE calibrators overestimate M$_{\rm BH, RM}$ by one order of magnitude. This may indicate a systematic overestimation of M$_{BH}$ for highly accreting BHs at high redshift. Finally, we provide the first high-redshift SE calibration based on H$α$ and H$β$. Although a larger sample is needed to reduce the uncertainties, our calibration can already be applied to the newly discovered BH population in the early Universe.

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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$α$ emission, probably tracing the host galaxy. We measure a nearly grey continuum polarisation $p_{\rm cont}=1.53\pm0.04$(rand.)$\pm$0.20(syst.) per cent, while broad H$α$ is less polarised, $p_{\rm Hα}=0.58-0.84$ 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$α$ 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

Undermassive Hosts of $z = 4-6 $ AGN from JWST/NIRCam Image Decomposition with CONGRESS, FRESCO, and JADES

In the local Universe, supermassive black hole (SMBH) masses strongly correlate with their host-galaxies' stellar masses ($M_{*}$), but galaxies hosting faint AGN recently found by JWST may deviate from this relation. To constrain the M$_{\text{BH}}$-M$_{*}$ relation at high redshift, we performed AGN-host image decomposition for 17 low-luminosity AGN galaxies at $z$ $\sim$ 4-6 using NIRCam images in the JADES GOODS-N field. These sources are identified as AGNs from broad H$α$ emission lines detected by the CONGRESS and FRESCO surveys. We used galfit+MCMC to fit spatial profiles in 7 wide-band images and detected extended emission in 9 sources out of 17. The close spatial alignment between the extended components and the AGN centers indicates that this emission likely originates from the host galaxies. These sources are extended at 0.9-2.0~$μ$m, suggesting significant host-galaxy light in the rest-frame UV. For the sources with the host detection, the stellar mass inferred based on image decomposition result can be 1-2 dex lower than the results without image decomposition. The BH-to-stellar mass ratio spans $M_{\text{BH}}/M_\ast$ $\sim$ 0.01-1.48, placing them well above the local $M_{\text{BH}}$-$M_\ast$ relation. In contrast, the host-galaxy size-mass relation broadly agrees with previous measurements. Our results suggest that the host galaxies of these faint AGN are either genuinely under-massive compared to their black hole masses, or too compact to be spatially resolved.

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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

GA-NIFS and JADES: Confirmation of pristine gas near GN-z11

According to the leading cosmological model, a first generation of stars called Population III (PopIII), condensed almost entirely out of hydrogen and helium, must have initiated the creation of all heavier chemical elements. We report the detection of ionised hydrogen (H$γ_{4342}$) with a signal-to-noise ratio of $S/N$=5.9 in a region about 3 pkpc (projected) north-east from the z~10.6 galaxy GN-z11, where line emission compatible with doubly ionised helium (HeII$_{1640}$) has been found. Our new JWST/NIRSpec-IFU G395H data confirm the authenticity of the previous detection at a redshift of $z_{\rm Hγ}$=$10.5862$$\pm$$0.0003$. H$δ$ is marginally detected ($S/N$$\sim$$2$). No metal lines are detected in our observations spanning $λ_{\rm rest}$=$0.25$-$0.45μ$m. We derive a $3σ$ upper limit on the gas phase metallicity of 12+log(O/H)$<$7.0 ($Z_{\rm gas}$$<$$0.02$ $Z_\odot$). Through comparison with NIRCam imaging, we constrain a lower limit on the equivalent width of EW$_0$(H$γ$)$>$350Å. We compare our emission line constraints to model predictions and find them compatible with photoionisation by PopIII stars, possibly intermixed with next-generation (PopII) stars. We infer an upper limit on the dynamical mass of $M_{\rm dyn}$$\lesssim$$3$$\times$$10^8M_\odot$. Our data provide novel support for the presence of PopIII stars nearby GN-z11, 440 Myr after the Big Bang.

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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$σ$) 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

GA-NIFS: Powerful and frequent outflows in moderate-luminosity active galactic nuclei at $z\sim3-6$

The period between z ~ 3-6, a key transformational phase in galaxy evolution preceding 'cosmic noon' (z ~ 1-3), is very poorly explored in terms of feedback from AGN acting through gas outflows. In this work, we study the properties of outflows in AGN (mostly X-ray-selected) from the GOODS-S field, exploiting JWST NIRSpec IFU observations as part of the GA-NIFS GTO survey. Together with its twin subsample from COSMOS reported in a previous GA-NIFS work, this constitutes the largest spatially resolved sample of AGN outflows at these redshifts to date, comprising 16 targets with outflows (out of a total of 19 AGN), and probes the unexplored regime of AGN at z ~ 3-6 with bolometric luminosities ~$10^{45-46}$ erg/s. We mapped the rest-optical ionised gas emission lines at sub-kpc scales and spectrally isolated the broad wing tracing fast outflows from the gas at rest in the host galaxies. The incidence of ionised outflows in the GOODS-S + COSMOS GA-NIFS sample is high (>75%), among the highest at any redshift. We inferred outflow velocities between ~600-2000 km/s, maximum radii of <1-4 kpc, and ionised gas mass outflow rates of ~0.1-100 $M_\odot$/yr, which in some cases can exceed the host galaxy star formation rate (SFR). We find that the outflow properties inferred for the GOODS-S + COSMOS GA-NIFS AGN sample and their relations to $L_{\rm bol}$ and SFR generally align with those observed for other spatially resolved literature samples of AGN outflows across different redshifts and AGN luminosities. Nonetheless, after accounting for any luminosity bias, our analysis suggests a cosmic evolution of the outflow properties, with higher median mass outflow rates (and possibly also mass loading factors) at higher redshifts, especially at z>3, which indicates that AGN outflows were stronger in the early Universe than at later times, and thus potentially more capable of affecting their host galaxy.

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arXiv2026

GA-NIFS: Dissecting The Alchemised: JWST reveals turbulent metal-poor gas fuelling a co-spatial starburst in a complex system at $z=10.17$

Recent observations revealed that distant galaxies have bursty star formation histories, regulated by stellar or active galactic nuclei (AGN) feedback and gas inflows. According to theoretical models, feedback preferentially removes metal-rich gas, while subsequent starbursts are triggered by mergers and newly accreted gas that is generally less enriched than the galaxy's interstellar medium (ISM). Therefore, gas-phase metallicity provides key insights into the baryonic processes shaping early galaxies. We present the first NIRSpec/IFU study of spatially resolved ISM properties in the MACS0647-JD system ($z=10.17$). The system consists of two stellar components detected in NIRSpec/IFU and NIRCam photometry. The main component ($\log \left(M_{\ast}/M_{\odot}\right)=7.77 \pm0.09$; $12+\log\left(\rm O/H\right)=7.89 \pm 0.16$) is more massive and significantly more metal-rich compared to its companion ($\log \left(M_{\ast}/M_{\odot}\right)=7.42\pm0.07$; $12+\log\left(\rm O/H\right)=7.47 \pm 0.20$), suggesting an older stellar population and a prolonged chemical enrichment history. We find that the H$γ$ line emission centroid is spatially offset by $\sim 0.1^{\prime \prime}$ (150 pc in the source plane) from the stellar continuum centroid; the latter coincides with the location of the main stellar component. This offset provides possible evidence of a merger-driven starburst in this system. By comparing the spatial distributions of the metallicity, velocity dispersion, and the burstiness of star formation history, we infer the presence of turbulent, metal-poor gas outside the stellar components. %detected in both NIRSpec/IFU and NIRCam photometry. This metal-poor, dynamically unstable gas is likely responsible for the enhanced recent star formation in the north-east region of the system.

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arXiv2026

BlackTHUNDER Reveals a Massive Filament around a Compact AGN at $z\simeq5.23$

Despite the growing number of compact active galactic nuclei (AGN) at $z>4$ discovered by JWST, their formation and evolution remain poorly understood. This paper investigates the large-scale environment of GN-77652, a compact AGN at $z=5.229$ observed as part of the JWST NIRSpec IFU Large Program BlackTHUNDER and complemented by deep multi-band NIRCam imaging. GN-77652 lies in close proximity to a 12 kpc-long filament composed of multiple sources at $z\simeq5.23$, spanning a remarkable range in stellar masses ($M_{\star}=0.7-13 \times 10^8$ ${M_\odot}$), gas phase metallicities (12$+$log(O/H) $=$ 7.6-8.5) and star formation rates (SFR $=0.4-6$ ${M_\odot}$ yr$^{-1}$). The [OIII]$λ$5007 kinematics reveals a smooth large-scale velocity gradient centred on the central, massive ($M_{\star}\simeq1.1\times10^9$ ${M_\odot}$) and metal rich ($Z\sim0.6$ $Z_{\odot}$) system of the group. In this source, only 2.4 kpc (projected) from GN-77652, [OIII]$λ$4363 line diagnostics provide possible evidence for a second AGN. GN-77652 exhibits a shallow ($-30$ to $+20$ km s$^{-1}$) velocity gradient that is consistent with disk rotation according to dynamical modelling. The Lyman-Werner radiation field produced by the filament is too weak for the black hole (BH) in GN-77652 to have formed recently via direct collapse. However, the required conditions may have existed at earlier epochs, or alternative scenarios (e.g. a recoiling BH ejected from the filament) could also be plausible. The whole system is expected to coalesce in $150-440$ Myr, also motivating an exploration of its future evolution through toy-model extrapolations and numerical simulations. Our analysis suggests that the compact AGN appearance of GN-77652 represents a transient evolutionary phase, consistent with the apparent decline with redshift in number density of compact AGN identified with JWST.

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arXiv2026

Little Red and Blue Dots: AGN-excited narrow lines, Lyman-$α$ emission, and resemblance to standard quasars

We present an analysis of a sample of 36 Little Red and Blue Dots (LRDs and LBDs) at $2.26<z<7.89$, identified by JWST in the GOODS fields. While both categories are selected to have broad Balmer lines, both of them are extremely X-ray weak. Both classes share the same location on various diagnostic diagrams, consistent with AGN excitation (with some deviations which can be ascribed to low metallicity), although their weak HeII emission suggests a generally softer ionizing spectrum than ordinary AGN. LRDs display Ly$α$ emission stronger than normal star-forming galaxies, and with a broad component consistent with the broad component of H$α$. Overall, these findings indicate that LRDs and LBDs are both powered by growing black holes and their ionizing radiation escapes to ionize the surrounding interstellar medium (ISM). The broad Balmer lines ($Hα_b$ and $Hβ_b$) have different apparent properties: LBDs have EW(H$α_b$) and $Hα_b/Hβ_b$ broadly consistent with normal AGN, while LRDs have higher values of both quantities, although still in the tail of the quasars distribution. LRD models in which a gas envelope completely encases the black hole, are inconsistent with these results -- these scenarios need modification to include clumpiness, or a (classical) equatorial geometry, letting ionizing photons reach the ISM. The different broad Balmer properties imply that LBDs cannot simply be LRDs with more galaxy contribution. Scenarios in which LRDs are simply dust-obscured LBDs seem broadly consistent with the observations. Finally, these results indicate that LRDs' bolometric luminosities estimated assuming isotropic emission and complete covering by the absorber are inadequate. The few X-ray-detected LRDs suggest no deviation from the standard AGN bolometric corrections, once absorption is accounted for.

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arXiv2026

Beyond the Dot: an LRD-like nucleus at the Heart of an IR-Bright Galaxy and its implications for high-redshift LRDs

Little Red Dots (LRDs) are compact, red sources discovered by JWST at high redshift ($z \gtrsim 4$), marked by distinctive 'V-shaped' spectral energy distributions (SEDs) and often interpreted as rapidly accreting Active Galactic Nuclei (AGNs). Their true nature remains unclear, however, and their evolutionary connection to their lower-redshift counterparts is still poorly constrained. Thus, we present WISEA J123635.56+621424.2, here dubbed {\it the Saguaro}, a $z=2.0145$ galaxy in GOODS-North, as a possible analog of high-redshift LRDs and a potential missing link in their evolutionary path toward lower-redshift systems. It features a compact LRD-like nucleus surrounded by a face-on spiral host. Its connection to LRDs includes that: (1) its nuclear spectrum shows a clear `V-shaped'' SED; and (2) when redshifted to $z=7$, surface-brightness dimming makes the host undetectable, thus mimicking an LRD. This suggests that high-redshift LRDs may be embedded in extended hosts. To test this, we stack rest-frame UV images of 99 photometrically selected LRDs, revealing faint, diffuse emission. Stacking in redshift bins reveals mild radial growth, consistent with the expected galaxy size evolution. A simple analytic model confirms that surface-brightness dimming alone can explain their compact appearance. Lastly, we show that {\it the Saguaro} is not unique by describing similar objects from the literature at $z\lesssim3.5$. Taken together, our results support a scenario in which LRDs may not be a distinct population, but could instead be the visible nuclei of galaxies undergoing a short-lived, perhaps AGN-dominated, evolutionary phase, with their compact, red appearance driven largely by observational biases.

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arXiv2026

JADES: the mass-metallicity relation at $z=1-10$. New calibrations, extremely metal-poor galaxies, and chemical diversity

We present gas-phase metallicities of star-forming galaxies at $z=1$-10 with deep JWST/NIRSpec spectra from the JADES full data release, Dark Horse, and OASIS programmes. We stack $\sim$1500 medium-resolution spectra, yielding detections of the [OIII]$λ$4363 auroral line down to $12+\log(\mathrm{O/H})=7.0$ to derive stack-based strong-line calibrations over the metallicity range $12+\log(\mathrm{O/H})=7.0$-8.7. At a fixed metallicity, our stacks exhibit [OIII]$λ$5007/H$β$ and [OIII]$λ$5007/[OII]$λλ$3726,3729 values generally lower than calibrations based on high-$z$ individual auroral-line emitters, suggesting an observational bias towards higher excitation introduced when requiring auroral line detections in individual spectra. Based on our new calibrations, we obtain canonical mass-metallicity relations (MZRs) at z$=$1-10, identifying a decrease in metallicities from $z\sim0$ to z$\sim$4-10, without significant change in slope. Moreover, we identify 50 promising candidates of extremely metal-poor galaxies (EMPGs) with $12+\log(\mathrm{O/H})=6.7$-7.3 (1-4\% solar metallicity) at $z=1.2$-9.1. The MZRs of EMPGs are characterised by a large scatter, with those having lower metallicities generally exhibiting lower sSFRs, opposite of what expected from the local Fundamental Metallicity Relation. These results support a stochastic star-formation history involving gas consumption/ejection and metal-poor inflow, strongly affecting metallicities of low-mass galaxies. Furthermore, we identify two Little Red Dots in our EMPG candidates, both exhibiting broad H$α$ and prominent Ly$α$, offering insights into the early black-hole growth in extremely metal-poor environments.

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