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Santiago Arribas

Publications and source records attributed to Santiago Arribas.

At least 19 recordsLinked to original sources
arXiv2026

GA-NIFS: sRMS, a new method for disentangling resolved and unresolved emission in integral field spectroscopy. Application to distant quasars

The interpretation of AGN emission-line profiles in type1 AGN is often complicated by the dominance of bright, spatially unresolved nuclear continuum and broad-line region (BLR) emission over narrower and blended components from the narrow-line region (NLR) and the host galaxy. This remains challenging even with integral-field spectroscopy (IFS), as the relevant spatial variations can occur on scales smaller than the angular resolution, particularly in the compact host of distant AGN. We introduce the spatial root-mean-square (sRMS) technique, a new approach to analyse single-epoch IFS that uses the spatial variance of spectra extracted from partially overlapping apertures to isolate off-nuclear emission of high-z BL-AGN, and investigate whether the resulting kinematic information can improve the decomposition of their integrated spectra. We construct sRMS spectra from ensembles of overlapping apertures centred on the unresolved nucleus. Emission that is spatially invariant on the scales probed by the apertures is consequently suppressed, whereas spatially varying emission is retained. We apply the procedure to JWST/NIRSpec data of two z~6.5 QSOs. We model the resulting sRMS spectra with multiple Gaussian components to determine the kinematics of the spatially varying emission, and use these kinematic constraints to model the integrated spectra. The sRMS technique identifies spatially varying narrow-line emission on scales up to ~5x smaller than the NIRSpec PSF and provides robust constraints on the kinematics of individual kinematic components. Imposing these kinematic constraints on the integrated-spectrum fit reduces significantly the degeneracy of multi-component decompositions. Simulations further demonstrate the potential of the method to detect spatially offset BLR emission from close dual BL-AGN, down to projected separations of ~200 pc at z~6.5 for a BLR flux ratio of ~10.

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arXiv2026

JADES: Dynamical Measurements of Dark Matter Halo Masses at $z \approx 7$ Using Galaxy Pairs

We present dynamical measurements of dark matter halo masses at $z\approx7$ using galaxy pairs. Galaxy pairs in the early Universe are usually in their first infall, before substantial orbital evolution or phase mixing. Their relative velocities closely trace the halo mass, especially when the pair separation is near the halo virial radius. In the TNG100 simulation, we find that dynamical mass estimators can recover halo masses with an intrinsic scatter of $\sim$0.2 dex. We apply this method to 10 pair systems at z=6-8 with [O III] spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES). We infer the stellar-to-halo mass relation while marginalizing over projection effects, measurement uncertainties, and intrinsic scatter. We find a mean halo mass of $\log\,(M_{200}/M_\odot)=11.06\pm0.21$ at $\log\,(M_{*}/M_\odot)=9$. These galaxies do not appear to inhabit unusually massive halos. Our results instead indicate a high stellar-to-halo mass ratio, corresponding to an integrated star formation efficiency of $5^{+3}_{-2}\%$, about twice the TNG100 prediction, although the current statistical significance is limited. Future observations with larger samples will tighten these constraints and directly examine whether enhanced star formation efficiency drives the overabundance of luminous galaxies at cosmic dawn.

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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

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

RIOJA. Environmental Effects on Stellar Populations and Ionized Gas in a Protocluster at $z=7.88$

Protoclusters in the epoch of reionization provide key laboratories for investigating how environment shapes early galaxy formation and evolution, and may also have contributed to cosmic reionization. We analyze 23 member galaxies of A2744-z7p9OD, a protocluster at $z=7.88$, using JWST/NIRCam and NIRSpec to investigate their stellar population properties, rest-frame UV sizes, and ionized-gas properties. We also quantify the internal structure of A2744-z7p9OD using the projected distance to the most massive galaxy ($D_{\rm YD4}$), and to the nearest neighbor ($D_{\rm nei}$), as global and local environmental indicators, respectively. Stellar mass, SFR on a 100 Myr timescale, dust attenuation, and galaxy size show significant correlations ($p<0.05$) with $D_{\rm YD4}$, but not with $D_{\rm nei}$, suggesting that these properties are primarily linked to the global protocluster structure. The member galaxies also show a large galaxy-to-galaxy variation in R23 ($=\log{(([\mathrm{O}\text{\textsc{iii}}]λ\lambda4960,5008\rmÅ+[\mathrm{O}\text{\textsc{ii}}]λ\lambda3727,3730\rmÅ)/\rm{H}β)}$), implying inhomogeneous chemical enrichment in the protocluster environment. O32 ($=\log{([\mathrm{O}\text{\textsc{iii}}]\lambda5008\rmÅ/[\mathrm{O}\text{\textsc{ii}}]λ\lambda3727,3730\rmÅ)}$) correlates with $D_{\rm YD4}$, indicating that the core region is characterized by low-ionization gas. Together with the non-detection of Ly$α$ emission, the possible neutral-gas reservoir traced by ALMA [C{\sc ii}]~$158μ$m emission, and evidence for high-column-density neutral hydrogen in the core, this suggests a neutral-gas-rich protocluster core where the current escape of ionizing photons may be suppressed, even in a overdense environment during the EoR.

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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

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: 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.

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arXiv2026

JWST Advanced Deep Extragalactic Survey (JADES) Data Release 5: Photometrically Selected Galaxy Candidates at z > 8

We present a sample of 2081 sources selected at photometric redshift $z_{\mathrm{phot}} > 8$ across the JADES DR5 data release in GOODS-S and GOODS-N over a total area of 469 square arcmin. These sources range from $M_{\mathrm{UV}} = -22$ to $M_{\mathrm{UV}} = -16$, with 19 objects at $z_{\mathrm{phot}} > 14$. We estimate the UV slopes for the full sample from fits to the photometry and find evidence for a steepening of the relationship between the UV continuum slope and $M_{\mathrm{UV}}$ to higher redshifts, a result that differs from prior analyses of brighter samples in the literature. We provide evidence that over one quarter of our sources have evidence for being morphologically extended, with many galaxies showing multiple bright knots or clumps even out to $z \sim 13 - 14$, an indication of how galaxies at Cosmic Dawn are growing and evolving. We discuss JADES-GN+189.15982+62.28899, a GOODS-N F200W dropout galaxy at $z_{\mathrm{phot}} \sim 15 - 18$ which has been observed spectroscopically with JWST/NIRSpec in prism mode, resulting in a very low signal-to-noise spectrum that is consistent with the photometry and rules out a number of low-redshift solutions for the source. Finally, we use a subsample of 123 objects in our sample with spectroscopic redshifts to explore the usage of alternate fitting templates and a prescription for Ly-$α$ damping wing absorption, finding that both produce significant improvements to the estimated photometric redshifts.

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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.

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arXiv2026

Near-infrared diagnostic diagrams of the gas ionization sources in nearby galaxies: a JWST NIRSpec view

Buried active galactic nuclei and obscured young stellar clusters pose significant challenges to traditional optical diagnostic diagrams. Therefore, developing new tools to trace the excitation sources across the spectrum is a necessary effort for the advancement of the field. Our goal is to explore the full spectral range of the JWST-NIRSpec data, searching for alternative diagnostic diagrams in the less-explored NIR, and to investigate the nature of the ionizing and heating source. We analyze the high-resolution spectra of the circum-nuclear regions of nine local (z < 0.1) U/LIRGS, investigating potential emission-line ratios to trace the excitation mechanisms acting on the line-emitting gas. We investigate these objects using the well-established [Fe ii]/PaB versus H2/Bry diagram, and attempt to correlate its classifications with other emission features across the spectrum. We then compare the empirical classifications with photo-ionization models, in order to evaluate how accurately the data can be reproduced. Finally, we compare the line width at 80% of the total flux (W80 ) of selected emission lines with the corresponding gas excitation mechanisms. We propose two line-excitation diagnostic diagrams based on [C i]/Pay and H2 1-0 O(5)/PAH 3.3 um ratio, which we found to directly correlate with the hardness of the radiation field, and therefore with the gas excitation mechanisms. In addition, the line-kinematics analysis shows that the W80 values of regions excited by star formation are on average slightly lower than those of AGN-excited regions, while shock-excited regions display distinctively higher W80 values compared to the other two groups. Our JWST-NIRSpec results reinforce previous studies showing that the H2 emission in the central regions of AGN hosts is complex and likely produced by multiple excitation mechanisms.

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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.

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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.

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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$λ$1640 emitter located at 3 pkpc from the galaxy GN-z11, at z=10.6. The detection, based on JWST NIRSpec-IFU high-resolution spectroscopy, confirms a previous claim based on medium-resolution spectroscopy. The HeII$λ$1640 identification is further supported by the independent detection of H$γ$ obtained by Übler 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

JADES Dark Horse: demonstrating high-multiplex observations with JWST/NIRSpec dense-shutter spectroscopy in the JADES Origins Field

We present JWST/NIRSpec dense-shutter spectroscopy (DSS). This novel observing strategy with the NIRSpec/MSA deliberately permits a high number of controlled spectral overlaps to reach extreme multiplex while retaining the low background of slit spectroscopy. In a single configuration over the JADES Origins Field, we opened shutters on all faint (mF444W$<$30 mag) z$_\mathrm{phot}>$3 candidates, prioritising emission-line science and rejecting only bright continuum sources. Using 33.6 and 35.8 ks on-source in G235M and G395M, we observed a single mask with $\sim$850 sources, obtaining spectroscopic redshifts for $\sim$540 galaxies over 2.5$<$z$<$8.9. The per-configuration target density in DSS mode is 4-5x higher than standard no- and low-overlap MSA strategies ($<$200 sources), with no loss in redshift precision or accuracy. Line-flux sensitivities are 30 percent lower at fixed exposure time, matching the expected increase in background noise, but the gain in survey speed is 5x in our setup, more than justifying the penalty. The measured line sensitivity exceeds NIRCam/WFSS by at least $\sim$5x ($\sim$25x in exposure time) at $λ\sim4\,μ$m, demonstrating that DSS is a compelling method to gain deep, wide-band spectra for large samples. Crucially, NIRSpec/MSA could deliver even higher target allocation densities than those used here. We derive H$α$-based SFRs, gas-phase metallicities (including a large sample suitable for strong-line calibrations), and identify rare mini-quenched galaxies and broadline AGN. DSS is immediately applicable wherever deep imaging enables robust pre-selection and astrometry, providing an efficient method to obtain large samples of faint emission-line galaxies, a compelling middle ground between the completeness of slitless surveys and the sensitivity and bandwidth of NIRSpec/MSA.

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arXiv2026

GA-NIFS: interstellar medium properties and tidal interactions in the evolved massive merging system B14-65666 at z=7.152

We present JWST/NIRSpec IFU observations of the z=7.152 galaxy system B14-65666, as part of the GA-NIFS survey. Line and continuum emission in this massive system (log10(M*/Msol)=9.8+/-0.2) is resolved into two strong cores surrounded by diffuse emission, as seen in recent JWST/NIRCam imaging. Our dataset contains detections of [OII]3726,3729, [NeIII]3869,3968, Balmer lines, [OIII]4959,5007, HeI5875, and weak [OIII]4363. Each spectrum is fit with a model that consistently incorporates interstellar medium conditions (i.e., electron temperature, Te, electron density, ne, and colour excess, E(B-V)). The resulting line fluxes are used to constrain the gas-phase metallicity (Zg~0.2-0.3 solar) and HBeta-based SFR for each region. Common line ratio diagrams (O32-R23, R3-R2, Ne3O2-R23) reveal that each line-emitting region lies at the intersection of low- and high-redshift galaxies, suggesting low ionisation and higher metallicity compared to the predominantly lower-mass galaxies studied with the JWST/NIRSpec IFU so far at z>5.5. Spaxel-by-spaxel fits reveal evidence for both narrow (FWHM<400 km s-1) and broad (FWHM>500 km s-1) line emission, the latter of which likely represents tidal interaction or outflows. Comparison to ALMA [CII]158um and [OIII]88um data shows a similar velocity structure, and we explore optical-far-infrared diagnostics. The two core galaxies both lie on the mass-metallicity relation at z>4, but show contrasting properties (e.g., M*, Zg), suggesting distinct evolutionary pathways. Combining the NIRSpec IFU and ALMA datasets, our analysis opens new windows into the merging system B14-65666.

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arXiv2026

The density-bounded twilight of starbursts in the early Universe

The peculiar nebular emission displayed by galaxies in the early Universe presents a unique opportunity to gain insight into the regulation of star formation in extreme environments. We investigate 500 (109) galaxies with deep NIRSpec/PRISM observations from the JADES survey at $z>2$ ($z>5.3$), finding 52 (26) galaxies with Balmer line ratios more than $1σ$ inconsistent with Case B recombination. These anomalous Balmer emitters (ABEs) cannot be explained by dust attenuation, indicating a departure from Case B recombination. To address this discrepancy, we model density-bounded nebulae with the photoionisation code CLOUDY. Density-bounded nebulae show anomalous Balmer line ratios due to Lyman line pumping and a transition from the nebulae being optically thin to optically thick for Lyman lines with increasing cloud depth. The H$α$/H$β$ versus H$γ$/H$β$ trend of density-bounded models is robust to changes in stellar age of the ionising source, gas density, and ionisation parameter; however, increasing the stellar metallicity drives a turnover in the trend. This is due to stronger stellar absorption features around Ly$γ$ reducing H$β$ fluorescence, allowing density-bounded models to account for all observed Balmer line ratios. ABEs show higher [OIII]/[OII], have steeper ultra-violet slopes, are fainter, and are more preferentially Ly$α$ emitters than galaxies which are consistent with Case B and little dust. These findings suggest that ABEs are galaxies that have become density bounded during extreme quenching events, representing a transient phase of $\sim$20 Myr during a fast breathing mode of star formation.

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