arXiv subjects

Mirko Curti

Publications and source records attributed to Mirko Curti.

At least 19 recordsLinked to original sources

arXiv2026

Tiny Galaxies, Mighty Nitrogen: Linking Chemical Anomalies to Compact Galaxies at High Redshift using JWST/JADES

The origin of anomalous chemical enrichment in high-redshift galaxies discovered by JWST remains a key question in galaxy evolution. Enhanced nitrogen abundances have been identified in several galaxies at early cosmic times, but the physical conditions driving this enrichment, and their connection to galaxy morphology, remain unclear. We test whether morphologically compact galaxies at 4 < z < 7 show stronger chemical anomalies than extended systems. Using JWST/NIRSpec spectroscopic data, we construct samples of compact and extended galaxies and analyse their rest-frame spectra. We stack the spectra of each population and measure key UV and optical emission lines. We detect nitrogen emission in the compact galaxy stack, including NIV], and [NII], while also detecting key carbon and oxygen lines. The detection of [OIII]λ4363 enables a direct-Te determination of the oxygen abundance. From these measurements, we infer chemical abundance ratios, including log(N/O) and log(C/O). We find that compact galaxies exhibit elevated log(N/O) (by ~ 0.7 dex) and log(C/O) (by ~ 0.5 dex) abundance ratios relative to extended systems. The abundance patterns of compact galaxy stacks are comparable to those of previously identified individual nitrogen-enhanced galaxies at high redshift and second-generation globular-cluster stars in the Milky Way. These results suggest a possible link between galaxy compactness and chemical enrichment pathways in the early Universe. Deeper JWST NIRSpec IFU observations of compact high-redshift galaxies will be crucial for determining the origin, spatial distribution, and evolutionary significance of these abundance patterns.

astro-ph.GA↗

arXiv2026

OASIS I: Spectroscopy of Mini-Quenched and Lulling Galaxies Constrains the Quiet Side of Bursty Star Formation at z > 4

JWST has revealed that star formation in early galaxies is bursty, including episodes of violent suppression, so-called mini-quenching. But these were largely serendipitous discoveries in samples of UV-bright, emission-line-dominated galaxies, so that the quiet side of the burst cycle has remained poorly explored. With the JWST Cycle 3 program OASIS (GO 5997), we searched for such galaxies deliberately, using deep 28-hour NIRSpec/PRISM spectroscopy across the full burst cycle at z>4. We identify three mini-quenched galaxies with log(M*/Msun) = 8.3-8.9 and no detectable Balmer emission, at least 1.2-1.7 dex below the star-forming main sequence (SFMS), five lulling galaxies with weak Balmer emission above the quenching threshold, and 17 nearly-main-sequence galaxies just below the SFMS. Finding them required spectroscopy: seven of our ten photometric candidates for suppressed (mini-quenched and lulling) galaxies turned out to be nearly-MS or above the SFMS, and photometric fits overestimate their current star-formation rates and cannot distinguish lulling from mini-quenched ones. The suppressed galaxies are common: five of the eight come from tiers not selected on star-formation state, where they make up 29% of the galaxies at z = 4-5.5 and log(M*/Msun) = 8-9.5, a number density of ~1.6x10^-3 Mpc^-3 dex^-1. Simulations predict them to be an order of magnitude rarer at these masses and to occur mainly at lower masses. The suppressed galaxies are also old: Balmer-break strength and stellar age both increase as galaxies fall further below the SFMS, with light-weighted ages of 5 Myr for galaxies above it, ~30 Myr for the lulling galaxies and above 100 Myr for the mini-quenched ones. They show little stellar reddening (E(B-V) < 0.2), consistent with depleted gas reservoirs. Suppressed phases at z>4 are therefore neither rare nor brief, but a common and lasting part of early galaxy growth.

astro-ph.GA↗

arXiv2026

OASIS II: The Ionized Gas Traces Feedback-Driven Declines into the Quiet Side of Bursty Star Formation at z > 4

Models of bursty star formation in low-mass (10^8-10^9.5 Msun) galaxies at z>4 differ most in the suppressed phases: what drives the decline, how fast star formation falls, and how long it stays low. OASIS Paper I showed that such quiet galaxies are common and old; here we measure how they got there and how their ionized gas responds. We compare Balmer-line star-formation rates (SFRs) with 100-Myr averages from the stellar continuum, in NIRSpec/PRISM spectra of 54 OASIS galaxies at z>4, including three mini-quenched galaxies below the quenching threshold and five lulling galaxies at least 0.6 dex below the star-forming main sequence (SFMS) but still star-forming. Galaxies above the SFMS rise and fade equally often, whereas those below it are mostly fading: the lulling galaxies formed little more than a tenth of their mass in the last 100 Myr, and their decline began tens to ~100 Myr before observation. Suppressed phases thus last of order 100 Myr, as long as or longer than in most simulations, whereas recovery is much faster: few galaxies below the SFMS are rising. The ionized gas tracks these changes in star formation. [OIII]/[OII] falls from galaxies above the SFMS to the lulling ones, even at fixed mass and after dust correction, and follows the recent change in SFR (SFR_10/SFR_100) more closely than the position relative to the SFMS: two rising galaxies below the SFMS are highly ionized. This is expected if fewer and softer ionizing photons than in strongly star-forming galaxies meet gas depleted by earlier star formation. Lulling galaxies show ISM conditions distinct from those of strongly star-forming galaxies. The declines are faster than passive gas exhaustion predicts, favoring feedback that expels the gas or keeps it from forming stars. Quiet phases are thus long, feedback-driven episodes, and the ionized gas traces where a galaxy is in its burst cycle.

astro-ph.GA↗

arXiv2026

OASIS III: Bursty star formation drives observed galaxy sizes at $z\geq4$

We employ JWST/NIRSpec MSA PRISM spectroscopy together with NIRCam imaging to investigate the impact of star-formation variability on galaxy sizes for a representative sample of 51 galaxies at $z\geq4$. The OASIS survey enables us to probe low-star-formation-rate (SFR) lulling and mini-quenched galaxies down to $\log_{10}(M_\star/M_\odot)=8.3$ and $\log_{10}(\mathrm{SFR}/M_\odot\,\mathrm{yr}^{-1})<-1.6$, i.e. more than one order of magnitude below the star-forming main sequence (MS). This enables us to probe objects that are often missed by shallower spectroscopic surveys, in addition to the MS and starburst systems, yielding an SFR- and mass-representative sample at high redshift. We derive galaxy half-light sizes using ForcePho, enabling us to resolve the highly compact nature of these sources, and we analyse their star-formation histories and interstellar medium (ISM) properties with the full-spectral-fitting code ppxf. We find a clear anti-correlation between burstiness, as traced by the ratio of the SFR from H$α$ compared to that from non-parametric spectral fitting averaged over 100 Myr, and galaxy UV/optical half-light radii as measured with ForcePho from a combination of well-sampled NIRCam photometry. Using partial correlation coefficients we show that the anti-correlation between burstiness (as traced by the SFR ratio) and size cannot simply be explained by secondary dependencies on stellar mass or redshift. We interpret this relation as the effect of central starbursts in SFR-rising (bursting) systems, where intense central star formation reduces measured galaxy half-light sizes. This burstiness then regulates galaxy sizes at high redshift. Should the same mechanism be present at $z>10$, this could explain the size bimodality seen in the UV luminous galaxies as simply stemming from a more fundamental relationship between burstiness and size.

astro-ph.GA↗

arXiv2026

MARTA: A "little" black hole hidden in a clumpy, metal-poor galaxy at z~3.8

JWST has extended the search for accreting black holes into the high-redshift, low-mass regime. We report a low-mass black hole candidate in MARTA 3887, a clumpy, metal-poor galaxy at $z=3.7694$, combining deep JWST/NIRSpec spectroscopy, HST and JWST imaging, and archival VLT/MUSE data. We detect a broad H$α$ component with $L_{\rm Hα,b}=2.35\times10^{41}$ erg s$^{-1}$ and FWHM $=1270^{+260}_{-250}$ km s$^{-1}$, with no [O III] counterpart of comparable width. Under a broad-line-region interpretation, virial calibrations give $\log(M_{\rm BH}/M_\odot)=6.52^{+0.18}_{-0.23}$ and $λ_{\rm Edd}=0.07$, or up to 0.38 for alternative calibrations. The host comprises at least three compact clumps, with $\log(M_\star/M_\odot)=8.16^{+0.09}_{-0.10}$. If stellar photoionization dominates the narrow lines, the gas is highly ionized and metal poor, with $12+\log({\rm O/H})=7.68\pm0.04$ ($\simeq0.10\,Z_\odot$). The inferred $M_{\rm BH}/M_\star=2.3\%$ lies about 2 dex above the extrapolated local relation, suggesting that black-hole growth has outpaced the assembly of the observed host. If growth began only when the first 10% of the observed stellar mass had formed ($t_{10}=92$ Myr ago), a $10^2\,M_\odot$ seed would require a lifetime-mean Eddington ratio $\simeq5.7$; earlier growth or a more massive, e.g. direct-collapse, seed would allow sub-Eddington mean accretion. MUSE reveals extended Ly$α$ emission ($\simeq6.5$-6.9 proper kpc), offset by $+64^{+10}_{-8}$ km s$^{-1}$ from systemic, with a provisional escape fraction of $\simeq0.44$. Together with a Mg II doublet ratio close to the optically thin limit and a high [O III]/[O II] ratio, this makes MARTA 3887 a laboratory to study black-hole growth, stellar assembly, and ionizing-photon escape jointly.

astro-ph.GA↗

arXiv2026

The clumpy fraction of galaxies at $z=1-9$ with JADES: evidence for a mass-dependent transition in clump origin

We measure the fraction of clumpy galaxies ($f_{\rm clumpy}$) over the redshift range $z=1-9$ and stellar masses $\log_{10}(M_\star/{\rm M_\odot}) \geq 8$ using JWST/NIRCam imaging from JADES. We define clumps as compact off-centre surface-brightness peaks with characteristic sizes of a few hundred parsecs, and develop a dendrogram-based clump-finding method with astrodendro to identify them directly in rest-frame UV, blue, and optical images. Clump-detection completeness is quantified through injection-recovery simulations, which we use to define ($M_\star$, $z$)-dependent 50 per cent completeness-limited $f_{\rm clumpy}$, alongside a probability-weighted completeness-corrected statistic for comparison. For the full sample, the blue and optical $f_{\rm clumpy}$ reach $\approx55$ per cent at $z\approx1-2$ and decline to $\lesssim5$ per cent by $z\approx8-9$, while the UV fraction remains higher at $\approx12$ per cent. At $z\approx1-2$, $f_{\rm clumpy}$ increases strongly with stellar mass, from $\approx35$ per cent in the lowest-mass bin to $\approx85$ per cent at $\log_{10}(M_\star/{\rm M_\odot})\geq10$. Most clumpy galaxies host only one or two robustly detected off-centre clumps, while higher-multiplicity systems occur preferentially in massive, strongly star-forming galaxies at low to intermediate redshift. Comparison with major merger rates suggests that mergers could account for a substantial fraction of low-mass ($\log_{10}(M_\star/{\rm M_\odot})<8.5$), high-redshift clumpy galaxies, while the observed fractions in more massive systems ($\log_{10}(M_\star/{\rm M_\odot})>9$) require an additional in situ clump-formation channel. Our results are consistent with a transition from merger-dominated clumpy morphologies at early times and low stellar masses to an increasing contribution from in situ clump-formation channels, such as disc instabilities, in more massive systems.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2026

Trading oxygen for iron II. Oxygen- versus iron-dependent cosmic star formation history

Due to their different nucleosynthetic origin, a stellar population produces oxygen (O) and iron (Fe) on different timescales and their relative abundance can deviate strongly from solar. Galaxy formation models should treat these elements separately, as they play a distinct role in shaping physical phenomena. For example, oxygen mainly sets the gas cooling rate, while the iron abundance sets stellar atmosphere opacities impacting stellar evolution, spectra and feedback. Observations of star-forming galaxies usually only constrain gas-phase oxygen abundance, vastly limiting our capabilities of separating the cosmic evolution of oxygen and iron. Here, we present an observationally-motivated framework to scale the cosmic evolution of O and Fe abundances. We apply the relation between the alpha-enhancement and galaxies' specific star formation rate ([O/Fe]-sSFR; Chruslinska et al. 2024) to derive the Fe and O-dependent cosmic star formation history (cSFH). We find that star formation with near-solar O/Fe is rare: at least 70% of the integrated cosmic stellar mass forms at non-solar O/Fe. The cosmic average metallicity is generally lower in [Fe/H] than in [O/H] (by up to a factor 3), with the offset increasing from redshifts z=0 to z~3 and then approaching the core-collapse O/Fe ratio. We validate our results against samples that probe the Fe-dependent cSFH in different regimes such as absorption-derived <[Fe/H]> from long gamma-ray bursts. Our results impact the interpretations of stellar and galaxy spectra and the predicted rates of transients, especially those linked to metal-poor progenitors (e.g., black hole mergers).

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2026

BlackTHUNDER: Evidence of three massive black holes in a z~5 galaxy

We present observational evidence of three massive, accreting black holes (BHs) in the z=5.0167 galaxy J0148-4214 from JWST/NIRSpec-IFU spectroscopy. The BHs are revealed through broad H$α$ emission (FWHM=430-2920 km/s) without a forbidden-line counterpart in the bright [OIII] doublet. Channel maps of the asymmetric central H$α$ profile isolate two spatially distinct broad-line regions (BLRs), separated by $190\pm40$ pc, while a third BLR is found in the galaxy outskirts with a projected separation of 1.7 kpc. We discuss whether this emission could be due to supernovae, shocks, winds, or massive stars, but find the BLR origin most likely. Using single-epoch virial relations, we estimated BH masses of $\log(M_\bullet/M_\odot)=7.9\pm0.4$ (primary), $5.8\pm0.5$ (secondary), and $6.3\pm0.5$ (third off-nuclear). We argue that the two central BHs will likely rapidly merge, with a simple dynamical friction time estimate of the order of $\lesssim700$ Myr. Assuming that the third off-nuclear BH is also in the process of sinking towards the centre, it will likely lead to a second merger, and we investigated the detection probability of such mergers with LISA. Alternatively, the third BH may be the result of a previous central three-body interaction or a gravitational recoil, where our observations would provide evidence that such BHs may retain their accretion discs and BLRs even in the aftermath of such extreme dynamical interactions. The possible discovery of a BH triplet at high z, together with other recent results on distant BH pairs, indicates that multiple massive BH systems were likely common in the early Universe. Our results highlight the importance of IFU observations for the detection of massive BH multiplets in distant galaxies, the progenitors of massive BH mergers that may be detected with next-generation gravitational wave observatories.

astro-ph.GA↗

arXiv2026

JADES Data Release 4 Paper I: Sample Selection, Observing Strategy and Redshifts of the complete spectroscopic sample

This paper accompanies Data Release 4 of the JWST Deep Extragalactic Survey (JADES), which presents the full NIRSpec spectroscopy of the survey. We provide spectra of 5190 targets across GOODS-North and GOODS-South (including the Hubble Ultra Deep Field), observed with the low-dispersion (R $\sim$ 30-300) prism and three medium-resolution (R $\sim$ 1000) gratings spanning 0.8 $< λ<$ 5.5 microns; 2654 were also observed with the higher-resolution (R $\sim$ 2700) G395H grating. The tiered survey design obtained more than 20 hr exposures for $\sim$ 700 galaxies in the Deep and Ultra Deep tiers, and shallower observations ($\sim$ 1-3 hr per setting) of $>$ 4400 galaxies in the Medium tiers. Targets were selected from photometric redshifts or colours, with priority given to rest-UV-selected galaxies at $z > 5.7$ and F444W-selected galaxies at $1.5 < z < 5.7$. We describe the full target selection and present spectroscopic redshifts and success rates. In total we obtain robust redshifts for 3297 galaxies, including 396 at $z > 5.7$ and 2545 at $1.5 < z < 5.7$. To facilitate uniform analyses, we define 'gold' sub-samples based on UV- and F444W-selection. Using the parent samples and redshift success rates, we construct rest-UV luminosity functions at $6 \lesssim z \lesssim 9$ from the Medium- and Deep-JWST tiers. Our number densities agree well with previous determinations from both photometric and spectroscopic samples, with modest interloper fractions confirming the reliability of photometric UV-bright galaxy selections at these redshifts.

astro-ph.GA↗

arXiv2026

Intense and extended CIII] emission suggests a strong outflow in JADES-GS-z14-0

JWST has revealed an overabundance of very bright, blue galaxies at z>10, raising fundamental questions about how star formation and feedback operate at Cosmic Dawn. We present new JWST/NIRSpec MSA PRISM/CLEAR spectroscopy of JADES-GS-z14-0 (z=14.18) obtained with the JADES and OASIS programmes. While the rest-frame UV continuum flux level and shape are consistent between the two datasets, the OASIS spectrum shows a 10$σ$ detection of the CIII]$λ\lambda1907,1909$ emission line, with a luminosity three times higher than that measured in the JADES data. This difference is naturally explained by the offset in shutter placement between OASIS and JADES, implying that the CIII] emission is spatially displaced by $\sim400$ pc from the stellar continuum. The non-detection of CIII] in NIRCam medium-band imaging indicates that the emitting region is extended on scales $\gtrsim165$ pc, with a surface brightness below the detection threshold. Interpreting this diffuse, carbon-enriched gas as the result of ongoing or past outflows, we infer a mass outflow rate of $\dot{M}_{\rm out}\sim160~{\rm M_\odot\,yr^{-1}}$. We compare it with the star-formation rate (SFR) and derive a mass-loading factor of $η= \dot{M}_{\rm out}/{\rm SFR} = 4-15$, suggesting highly efficient feedback at very early times. Finally, we show that, if outflows are one of the mechanisms regulating star formation in JADES-GS-z14-0, the instantaneous star-formation efficiency in massive haloes is constrained to $ε_\star\lesssim0.08$. These results support a scenario in which outflows play a crucial role during the earliest phases of galaxy formation. Comparing our results with the current theoretical galaxy formation model, we conclude that a combination of moderate star-formation efficiency and reduced dust attenuation can account for the emergence of luminous galaxies at the highest redshifts.

astro-ph.GA↗

arXiv2026

A black hole in a near-pristine galaxy 700 million years after the Big Bang

The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow H$β$ emission indicates extremely low metallicity, about $4\times 10^{-3}$ solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.

astro-ph.GA↗