arXiv subjects

Sandro Tacchella

Publications and source records attributed to Sandro Tacchella.

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.

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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.

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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.

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arXiv2026

What shapes the bright end of the UV luminosity function? Stellar mass, star-formation variability, and dust in JADES

JWST surveys find more UV-bright galaxies at $z>10$ than galaxy-formation models predict, but the roles of recent star formation, stellar mass and dust remain uncertain. We analyse 1365 spectroscopically confirmed JADES galaxies at $3\leq z\leq9$, fitting NIRCam photometry and NIRSpec emission lines with \ceri. Our model includes flexible star-formation histories (SFHs) under three priors, two-component dust attenuation and nebular emission. We account for spectroscopic selection through population reweighting and assess the reliability of inferred trends. At $z=3$--5, rising SFHs are more common in low-mass galaxies, whereas most galaxies above $10^{9.5}\,{\rm M}_{\odot}$ have declining SFHs. The apparent increase in rising fractions with redshift cannot be distinguished from the adopted SFH prior alone. At fixed stellar mass, recent star formation is the strongest predictor of UV brightness. UV-brighter galaxies have higher recent SFRs, younger stellar populations and less attenuation at fixed SFR. Below $10^9\,{\rm M}_{\odot}$, they are also more likely to have rising SFHs, with odds increasing by up to a factor of 2.3 per magnitude brighter, although the strongest trend requires caution. In contrast, galaxies with $M_{\mathrm{UV}}<-20$ at $z=3$--5 are predominantly massive, and approximately half have declining SFHs. Dust partly offsets burst-driven UV brightening: attenuation increases with recent SFR and declines gradually after the inferred SFR peak. In model sensitivity tests, dust has the largest effect on bright-end counts, whereas removing galaxy-to-galaxy SFH scatter changes them by at most 0.17~dex. Thus, bursts can make low-mass galaxies unusually UV-bright for their mass, while the absolute bright end of the UVLF is primarily shaped by stellar mass and dust attenuation.

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arXiv2026

How Steep is your Dust? Direct Measurements of Nebular Attenuation Curves at Cosmic Noon with JWST

Tracing the shape of the nebular attenuation curve, which is crucial for accurately measuring intrinsic emission-line fluxes, has proven challenging both in the local universe and at high redshift. Here we present nebular dust attenuation curves measured directly for 24 star-forming galaxies ($9.3<\log(\rm M{\star}/M_\odot)<11.1$) at cosmic noon ($1.8<z<3.3$) from the Blue Jay survey. Using deep JWST/NIRSpec spectra, we measure up to eight hydrogen recombination lines from H$δ$ to Pa$β$ per galaxy and find little diversity in the curve shape, with most galaxies agreeing with the \citet{calzetti_dust_2000} attenuation curve. For over $90\%$ of the sample, the nebular reddening from the Balmer lines is consistent with that from the Paschen lines. Using \texttt{Prospector} spectral energy distribution (SED) fits, we obtain the stellar reddening and find that the difference between stellar and nebular reddening increases with stellar mass, and that each reddening measurement is independent of inclination, which disagrees with the two-component dust model. We then measure the impact of using the derived nebular attenuation curve or a literature curve when determining metallicity using the strong-line method, and find that for some diagnostics (e.g., S32, S3, S23, and S3O3), the difference can be as large as $5σ$. Caution is needed when using these diagnostics if the nebular attenuation curve cannot be directly measured, since the resulting uncertainty can exceed the measurement uncertainty on the emission-line fluxes even with high signal-to-noise Balmer decrement measurements. These results show that the shape of the nebular attenuation curve is poorly understood and that constraining it is important for reducing systematic uncertainty in galaxy measurements.

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arXiv2026

Rubin J122659.4+090236: An Extremely Low Surface Brightness Galaxy Candidate Discovered in the Rubin LSST Early Data Preview 2

We report the serendipitous discovery of an exceptionally low surface brightness galaxy (LSBG) candidate, Rubin J122659.4+090236, in Rubin Observatory imaging of the interacting NGC 4410 system, identified in the Cosmic Treasure Chest public release. 2D Sérsic modelling of the Rubin g, r, and i images reveals a nearly round system with a shallow profile (n ~ 0.4), an effective radius of R_e ~ 6'', and central surface brightnesses of $μ_{0,g}=27.52\pm0.04$, $μ_{0,r}=27.62\pm0.07$, and $μ_{0,i}=27.04\pm0.08$ mag arcsec$^{-2}$. EAZY photo-z fitting favours an intermediate-z solution at z~0.3, while a low-redshift solution at z~0.028, consistent with the NGC 4410 system, is also permitted by a restricted EAZY fit over 0<z<0.1 without imposing a redshift prior. These alternatives imply substantially different physical interpretations, ranging from a diffuse dwarf-like system to an exceptionally extended background LSBG. This discovery demonstrates Rubin's sensitivity to extremely diffuse galaxies and highlights the potential of the LSST survey to uncover large samples of such elusive systems across wide areas, enabling systematic studies of the LSBG population and its role in galaxy evolution.

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arXiv2026

The THESAN-ZOOM project: the delayed rise of Lyman continuum escape fractions in bursty high-redshift galaxies

The ionizing radiation that galaxies release into their surroundings depends on whether efficient Lyman continuum (LyC) escape coincides with strong photon production. Stellar feedback can open escape paths as the source population fades, linking the escaping radiation to a galaxy's recent history. We investigate this timing in thirteen high-redshift (z > 3) target haloes from the THESAN-ZOOM radiation-hydrodynamic simulations, post-processed with COLT radiative transfer and sampled at ~10 Myr intervals. The clearest delayed relationship links $\text{SFR}_{10}/\text{SFR}_{100}$ to the LyC escape fraction. Selecting the nearest positive cross-correlation peak gives typical lags of ~16 Myr and peak correlations of 0.4-0.6 in massive haloes, with longer and less coherent delays towards lower masses. The fitted delays scale approximately as $M_\star^{-0.19}$ and $M_\text{halo}^{-0.27}$, although the shortest lags approach the native snapshot spacing. Recurrent variations span multiple amplitude-dependent timescales, consistent with repeated episodes of star formation and feedback. Recent star formation closely tracks both intrinsic and escaping photon rates, while the escape fraction reflects the evolving gas transparency. The H$α$-to-UV luminosity ratio retains a delayed association with escape, offering an observable connection between burst phase and leakage. High transparency during a fading phase can nevertheless yield little escaping radiation. The temporal overlap of photon production and transmission therefore shapes both the amount and timing of escaping radiation, informing the interpretation of observed leakers and their contribution to cosmic reionization.

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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.

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arXiv2026

The dawn of the first galaxies

Understanding how the first galaxies assembled their stellar mass at cosmic dawn is a central frontier of modern astrophysics. JWST has now revealed an unexpected population of ultraviolet-bright galaxies at z>10, whose intense emission points towards a rapid stellar mass growth that challenges current models of early galaxy assembly. Yet, the cold gas fueling this growth remains hidden: current ALMA sensitivity restricts routine detection to only the brightest ionized-gas tracers such as [OIII] 88$μ$m, while [CII] 158$μ$m, a key coolant and gas-mass proxy of the neutral interstellar medium (ISM), is undetected beyond $z\sim8$. This leaves fundamental questions unanswered: how quickly primeval galaxies gathered cold gas and turned it into stars; what physical conditions (density, temperature, radiation field) characterized their ISM; how fast metals and dust built up; and how feedback regulated early gas assembly. Only a transformational sub-millimeter facility like ALMA2040 can address these questions, enabling systematic surveys of [CII] in the first galaxies and providing a representative census of gas fractions and depletion times. A simulated galaxy at $z=11.3$ shows that ALMA2040 not only will detect, but also spatially resolve its merging cold-gas structure in [CII] on $\sim$200 pc scales in 5 hours (below 100 pc in 10 hours). ALMA2040 will also spectrally resolve multiple lines simultaneously, akin to integral-field spectroscopy, providing the key diagnostics needed to break density-metallicity-radiation-field degeneracies and map the multi-phase ISM of z>10 galaxies. The combination of dramatically increased line sensitivity and wide instantaneous bandwidth will push the search for the first galaxies out to the earliest cosmic times, with [OIII] detectable up to $z\sim20$. ALMA2040 will thus be transformative for understanding how the first galaxies assembled at cosmic dawn.

astro-ph.IM↗

arXiv2026

How bright can a galaxy be? Extreme value predictions for high-redshift survey observables

Assessing the significance of extreme galaxy candidates has typically relied on redshifts and stellar / halo masses inferred through inverse modelling, which carries substantial systematic uncertainty. We combine the halo mass function with a conditional model for galaxy emission and apply the Extreme Value Statistics (EVS) formalism to predict the distribution of the observed flux of the brightest galaxy expected in a given survey area, redshift interval and photometric band. We find the choice of parametric star formation history (SFH) dominates the modelling uncertainty, introducing a spread approaching $\sim 1 \, \mathrm{dex}$ by $z \sim 18$, whilst the stellar population synthesis model, initial mass function and metallicity each contribute $\lesssim 0.3 \, \mathrm{dex}$. The luminous $z \gtrsim 9$ candidates in COSMOS-Web cluster about the median prediction for the area they were drawn from, and lie at least an order of magnitude below the $3σ$ limit, a margin comparable to the SFH systematic. Of the observational samples reaching beyond $z \sim 13$, the two spectroscopically confirmed galaxies at $z \simeq 14$ lie close to the median prediction for their area, whereas photometric candidates at $z \gtrsim 16$ reach $+4σ$ (a probability to exceed of $\sim 10^{-5}$), the brightest lying at the limit obtained by converting all halo baryons into stars. Since the prediction bounds the flux far more tightly than the photometry bounds the redshift, such offsets are most naturally resolved by errors in the inferred redshifts. This approach allows extreme candidates to be assessed directly in the observational plane, making explicit the modelling assumptions common to forward and inverse approaches.

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arXiv2026

Probing cold gas and dust in quiescent galaxies across cosmic time

The discovery of massive quiescent galaxies at high redshift by JWST has revealed that star formation can cease rapidly in the early Universe, yet the physical mechanisms responsible for quenching remain unknown. A key missing element is the systematic measurement in statistically representative quiescent samples of the cold gas and dust content, which are crucial for distinguishing quenching mechanisms and evolutionary pathways. Such observations are currently not feasible because state-of-the-art sub-millimeter facilities (e.g., ALMA) cannot routinely detect the faint molecular gas and dust reservoirs of quiescent galaxies, nor pinpoint outflowing molecular gas. A next-generation facility (ALMA2040) capable of providing transformative access to the cold interstellar medium of quiescent galaxies across redshift is therefore essential. In particular, it will allow the community to probe the molecular gas content of quiescent galaxies at cosmic noon and beyond by targeting the CO, [CI], and [CII] emission lines, as well as unveiling the physical conditions of the cold interstellar medium by constraining the CO spectral line energy distribution. It will also reveal the dust content of the passive population, a crucial ingredient to constrain the different quenching mechanisms. Finally, it will enable probing the molecular gas kinematics and identify molecular outflows driven by supermassive black holes. When combined with the rich legacy of JWST, ELT, and SKAO surveys, ALMA2040 will deliver a comprehensive, multi-phase view of feedback and quenching, allowing us to establish when, how, and why galaxies shut down their star formation across cosmic time.

astro-ph.IM↗

arXiv2026

Mergers lighting the early Universe: modest star formation enhancement without evidence for AGN or Ly$α$ triggering

Galaxy mergers and interactions are often invoked to explain enhanced star formation, black hole growth, and mass build-up of galaxies at later cosmic times, but their effect is poorly understood at high redshift ($z>2$). We use JADES data to analyse a mass-complete sample of 1968 galaxies at $z=3-9$ with ${\rm log}(M_\star/{\rm M_\odot}) = [8, 10]$, identifying major merger pairs (projected separation of $5-100$ pkpc, mass ratio $\geq 1/4$) using a probabilistic method. To look for signatures of enhancement in multiple physical properties, we carefully build a control sample of non-pairs that are simultaneously matched in redshift, stellar mass, isolation, and environment to the pair sample. We find a moderate enhancement in specific star-formation rate (sSFR) of a factor of $1.10\pm 0.05$ at separations $\lesssim 20$ kpc in pairs compared to their matched controls. We find that the interaction-related sSFR enhancement is most apparent at longer SFR averaging time-scales (50-100 Myr), whereas the enhancement is not detected on 5-10 Myr time-scales, where bursty star formation might be driven by internal processes. By averaging star formation histories, we find two distinct populations: one with monotonically rising SFRs and another with an earlier peak in SFR, potentially corresponding to different interaction stages. Finally, we find no significant excess of AGN in pairs, consistent with galaxy interactions not strongly triggering black hole activity at the separations probed ($>5$ kpc). Similarly, we also do not detect an excess in the fraction of Lyman-$α$ emitters in pairs, suggesting that at the probed separations, galaxy interactions are not efficient at enhancing Lyman-$α$ photon production and escape, which may only become important at the smallest physical scales.

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arXiv2026

Discovery of the Lyman-$α$ Forest in a Star-Forming Galaxy at $z \approx 7.57$: Implications for the Timing and Topology of Cosmic Reionization

The escape of ionizing photons from galaxies during the Epoch of Reionization is often inferred indirectly because the intervening neutral hydrogen in the intergalactic medium (IGM) is expected to absorb all flux blueward of Lyman-$α$ at these redshifts. However, in this work, we report the significant detection ($\approx7σ$) of flux blueward of Ly$α$ in a massive ($M_{\ast}\approx10^{9.1}\ M_{\odot}$) star-forming galaxy at $z\approx7.57$, JADES-GS-z7-LAF, using JWST/NIRSpec PRISM spectroscopy. We independently confirm this detection of the Ly$α$ forest (LAF) using JWST/NIRCam F090W imaging ($\approx10σ$). The blueward flux in F090W is spatially coincident with the galaxy and emerges $\approx0.4$ pkpc from its UV centroid at $λ_{\mathrm{rest}}\approx1500\ \mathrm{\mathring{A}}$. We find no evidence for a foreground interloper producing the observed flux, although we cannot formally reject this interpretation with the existing data. The transmitted flux implies an extraordinarily transparent sightline ($x_{\mathrm{HI}}\ll0.1\%$) through the IGM at $z \approx 5.5-7.5$, a sightline that is spatially and kinematically coincident with numerous high-redshift galaxy overdensities, which possibly explains the emergence of this flux. We observe a smooth and gradual turnover around the Ly$α$ break that is either consistent with an extreme damping-wing ($N_{\mathrm{HI}}\approx10^{23.3}\ \mathrm{cm}^{-2}$) or a strong nebular continuum (contributing $\approx60-70\%$ of the flux at $λ_{\mathrm{rest}}=1500\ \mathrm{\mathring{A}}$). JADES-GS-z7-LAF is far more luminous ($M_{\mathrm{UV}}\approx-21.5$, similar to GN-z11) than the faint sources usually invoked as the drivers of cosmic reionization. If such systems are common in the early Universe, massive galaxies may contribute more to the ionizing photon budget than suggested by other indirect constraints.

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arXiv2026

Probing the molecular gas content of galaxies in an over-dense group at z~0.7: A test case for environmental quenching

We investigated the impact of group environment on molecular gas reservoirs at an intermediate redshift by observing the CO(2-1) emission from the galaxy group COSMOS-Gr30 at $z \sim 0.7$ with the Institut de Radioastronomie Millimetrique (IRAM) NOrthern Extended Millimeter Array (NOEMA) and 30m telescope. This dense environment, located at the intersection of large-scale cosmic web filaments hosts a large ($\sim 10^{4}$ kpc$^{2}$) ionized gas structure revealed by MUSE. We detect CO emission in four galaxies in the group with a signal-to-noise ratio (S/N) greater than five and derive upper limits for the remaining group members with secure spectroscopic redshifts. Stacked measurements indicate that group galaxies exhibit, on average, molecular gas contents reduced by $\sim 0.5$ dex relative to field scaling relations, corresponding to gas fractions that are $20- 40\%$ of those found in typical main-sequence galaxies. Although the uncertainties are significant, this suggests that environmental processes efficiently deplete molecular gas reservoirs in the galaxies of this group. The 30m observations place an upper limit on the molecular gas associated with the extended ionized structure, $M_{\rm gas} < 2 \times 10^{10} \rm M_\odot$, implying that less than a third of the gas in the intragroup medium is in a cold, star-forming phase. Together, these results show how environmental mechanisms in dense group environments act to remove or suppress molecular gas within galaxies, capturing quenching processes in action.

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arXiv2026

Source--Line Associations in JWST/NIRSpec Dense-Shutter Spectroscopy

We demonstrate the feasibility of source--line association in JWST/NIRSpec dense-shutter spectroscopy, with $\sim$2000 sources observed simultaneously per pointing regardless of spectral overlap. While JWST wide-field slitless spectroscopy (WFSS) can disentangle overlapping spectra using two grisms with perpendicular dispersion directions, NIRSpec can achieve this using gratings with different dispersions. Compared with WFSS, NIRSpec dense-shutter spectroscopy offers substantially higher sensitivity and broader wavelength coverage. We develop a Bayesian framework that combines cross-dispersion offsets, wavelength consistency, and changes in relative line positions between gratings to jointly determine source associations and redshifts. We test the method using mock observations based on the observed source distribution and emission-line fluxes from the JWST Advanced Deep Extragalactic Survey (JADES), accounting for the NIRSpec optical model, MSA operability, measurement noise, false-positive detections, and bad pixels. Although the spectral traces overlap extensively, fewer than 0.5% of the simulated emission lines are blended with another line on the detectors in NIRSpec grating spectroscopy. Applying our method to these simulations, we obtain redshifts, at an accuracy of 98%, for the $\sim$1800 sources that have detectable emission lines, of which $\sim$1500 are classified as confident and are 99.9% accurate. We further investigate how the association accuracy depends on the number of grating configurations. By combining high multiplexing, high sensitivity, and broad wavelength coverage, NIRSpec dense-shutter spectroscopy enables efficient, highly complete spectroscopic surveys for the high-redshift Universe.

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arXiv2026

CERIDWEN: Fast and Flexible GPU-Accelerated Stellar Population Inference

JWST has increased both the number of high-redshift galaxies with high-quality spectral energy distributions (SEDs) and their information content. In parallel, wide-area surveys from Euclid, Rubin's LSST, and Roman will increase galaxy samples by orders of magnitude. Analysing these datasets requires stellar-population models that are both flexible and computationally efficient. We present CERIDWEN, a GPU-native SED fitting framework written in JAX, with an end-to-end differentiable forward model spanning stellar populations, nebular emission, dust attenuation and emission, and projection into the observer frame. Its vectorised, compiled architecture lets nested sampling replace a batch of live points in parallel on the GPU, which makes flexible stellar-population models tractable under full Bayesian inference. Automatic differentiation also provides exact gradients for the gradient-based samplers in the package. We jointly infer time-dependent chemical-enrichment histories instead of a single stellar metallicity, and demonstrate non-parametric star-formation histories (SFHs) with $\sim$120 age bins. Using $α$-enhanced stellar libraries from FSPS, CERIDWEN can sample stellar [$α$/Fe] jointly with [Fe/H], mass, and SFH, so that the joint posterior represents the [Fe/H]-[$α$/Fe] degeneracy explicitly. In controlled mocks, CERIDWEN recovers parameters with well-calibrated posterior uncertainties, while fits to real JWST observations reproduce posteriors from the established Prospector framework: on a single GPU, CERIDWEN completes a fit in a median sampling time of $\sim$4 min, $\sim$134$\times$ faster per fit than equivalent CPU-based Prospector runs. CERIDWEN therefore makes full Bayesian inference practical for larger galaxy samples and more flexible stellar-population models, reducing computational constraints on the physical complexity explored in SED fitting.

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arXiv2026

Disc-mediated angular momentum transport resolves the mass-gain problem in rotating binaries

A large fraction of stars interact with a close companion during their lifetime, in which the transfer of mass and angular momentum shapes their evolution and final fate. Standard rotationally limited accretion models predict that accretors reach critical rotation after gaining only a small fraction of their mass, severely suppressing further accretion. This is in tension with observations of post-interaction systems that require substantial mass gain. We introduce a disc-mediated angular momentum transport prescription for mass-transfer onto stellar companions. This is based on a novel perturbative, analytic star-disc boundary model that allows for continued mass inflow as the accretor approaches critical rotation. Furthermore, the model reproduces previous numerical results in which perturbations to super-critical rotation result in negative torques exerted by the disc, extracting excess angular momentum while allowing continued mass inflow. We implement this mechanism in detailed binary evolution calculations with differential rotation using MESA, and compute grids spanning primary mass, orbital period, and mass ratio. Whereas rotationally limited models predict $β_{\rm eff}\lesssim 0.1$, the disc model yields sustained mass inflow near critical rotation, with effective mass-transfer efficiencies of $β_{\rm eff}\sim 0.4$-$1$ across much of the parameter space. The resulting accretor properties are broadly consistent with observed post-interaction sdOB+Be binaries. Disc-mediated angular momentum transport may therefore represent a key missing ingredient in standard binary evolution models, with important implications for rapidly rotating stars and compact-object progenitors.

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