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Darach Watson

Publications and source records attributed to Darach Watson.

At least 19 recordsLinked to original sources
arXiv2026

Chemical signatures from the first stars embedded in metal-poor gas in galaxies at cosmic dawn

The first generation of stars formed from pristine, neutral hydrogen gas. The most massive of these exploded as supernovae within a few million years of their birth, producing the first heavier elements and leaving distinct chemical signatures of their origin in the surrounding medium. However, chemical abundance studies have so far mainly relied on emission-line measurements, which are luminosity weighted and hence biased towards the most recently formed stars. Here we analyse near-infrared, medium-resolution spectroscopy from the JWST-SPURS program of three UV-bright galaxies at redshifts 7.8, 8.6, and 9.3, within the first 650 to 520 million years after the Big Bang. The chemical abundance patterns of the metal lines detected in absorption hint at extremely metal-poor gas, substantially lower than inferred from the emission lines tracing the central, star-forming regions. Further, they all exhibit super-solar [C/O] abundances, which is also imprinted in the averaged spectrum of a larger set of galaxies at similar redshifts. These results reveal the distinct chemical signatures of the first Population III supernovae explosions.

astro-ph.GA↗
arXiv2026

TBD LBD: The nature of `little blue dots'

Previous Sirocco radiative-transfer models of gas-cocooned AGN predicted lower-column counterparts to little red dots (LRDs): compact, X-ray-weak sources with bluer continuum slopes and Balmer jumps rather than Balmer breaks. The recently identified population of little blue dots (LBDs) closely resembles this predicted phase. Here we explore these lower-column-density cocoons in which nebular recombination emission remains visible while strong Balmer-continuum absorption is avoided. We find that a sequence of increasing column density connects more classical AGN spectra, Balmer-jump LBD-like spectra at $N_{\rm H}\!\sim\!{\rm few}\times10^{24} \mathrm{cm^{-2}}$, and Balmer-break LRD-like spectra at higher columns. In this sequence, electron scattering produces exponential line wings and suppresses X-ray emission before strong Balmer absorption features, characteristic of higher column densities, appear. We therefore propose that LBDs are lower-column analogues of LRDs within a common gas-cocooned AGN sequence. This interpretation predicts that Balmer-jump emission, X-ray weakness, permitted lines with exponential wings, He II $\lambda$4686 emission, smaller H$\alpha$ FWHM values and equivalent widths than in LRDs, and weak or absent absorption features are characteristic of LBDs. We compare to three example LBD spectra and identify Balmer-jump signatures in them.

astro-ph.GA↗
arXiv2026

Quenching of X-ray emission in little red dots by both Compton-thick gas and high accretion rates

Little red dots (LRDs) are candidate high-redshift supermassive black holes accreting in dense gas. They remain undetected in X-rays. In previous work, we provided the first quantitative models that reproduce the optical and near-infrared spectra of LRDs with the \sirocco radiative transfer code, thereby constraining the properties of the surrounding gas. Here, we use these constraints to predict the X-ray attenuation produced by dense gas cocoons, and explore its dependence on Balmer-break strength, metallicity, intrinsic X-ray spectral energy distribution, and observed bandpass as a function of redshift. The X-ray constraints are very tight, requiring extinction by a Compton-thick gas column ($N_{\rm H}\sim10^{25}\,{\rm cm}^{-2}$) with moderate metallicity $0.05$-$0.1\,Z_\odot$ and intrinsically weak X-ray emission (bolometric to X-ray luminosity ratio, $k_{\rm bol,X}\gtrsim 30$), as observed in narrow-line active galactic nuclei with high accretion rates, to make LRDs sufficiently faint to evade detection. Intrinsically bright X-ray emitters as seen in typical broad-line active galactic nuclei would be detected even behind the typical Compton-thick gas columns with modest metallicity that were inferred from the optical spectra. Very low metallicity objects might be detected in X-rays even with low intrinsic X-ray luminosities, suggesting that LRDs are not (currently) chemically pristine.

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arXiv2026

Paschen Jumps in Little Red Dots: Evidence for Nebular Continua

''Little Red Dots'' (LRDs) are broad-line sources at high redshift, initially identified by their compact morphologies, red colours and prominent Balmer breaks. The origin of their optical-to-near-infrared continua is debated, with proposed explanations ranging from direct recombination emission to thermalised blackbodies from stellar-like atmospheres. Here we report evidence for Paschen jumps in a subset of LRDs, consistent with free-bound recombination to hydrogen $n=3$. The Paschen and Brackett continuum shapes across the sample are consistent with minimally reddened emission from low-temperature gas with $T_e\lesssim10\,000$ K, while the presence of Paschen jump signatures limits scenarios in which the emission is thermalised. Further, the extreme H$\alpha$ equivalent widths and the tight observed correlation between H$\alpha$ and the continuum follow naturally if both originate in recombination emission. This provides an observational upper limit on the contribution of any direct AGN accretion component and any stellar-atmosphere-like component, as well as on the fraction of line emission that can be thermalised as it traverses the cocoon. Ultimately, nebular radiative-transfer models provide a self-consistent explanation of the continuum, line strengths and line profiles without requiring multiple separately fitted components.

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arXiv2026

Strontium and helium in the kilonova AT2017gfo: Origin of the 1{\mu}m feature constrained via NLTE calculations

Mergers of neutron stars are believed to be one of the primary sites for the synthesis of the universe's heavy elements via the rapid neutron capture process. AT2017gfo, the kilonova following GW170817 provided the first direct spectroscopic evidence of the $r$-process happening in the universe. A prominent line feature near $1\,\mu$m in its spectrum was attributed to strontium -- a claim that has been independently recovered by several teams. However, in recent years it has been debated whether the feature arises instead from helium. Here, we present non--local thermodynamic equilibrium (NLTE) radiative transfer modelling of the observed kilonova spectra, including detailed radiation-matter interaction physics for both strontium and helium. We make use of freshly calculated strontium atomic data for e$^-$ impact collisions, photoionization, and recombination processes. Our strontium model self-consistently reproduces the temporal evolution of the $1\,\mu$m feature at early times, with its absence at $0.92\,$days to its clear emergence at $1.17\,$days. This transition mimics LTE, because at early epochs ($t\lesssim 1.5\,$days) the radiation field dominates the ionization state of the ejecta over thermal and non-thermal electron collisions. We further test if helium can form the feature under the same plasma conditions. The helium mass required at $1.17\,$days is comparable to the total ejecta mass, while a few percent by mass of helium suffices at 4.4 days. On the other hand, the strength of the strontium lines decrease with time, and may require a radially stratified abundance to consistently produce the feature. We conclude that strontium is required to explain the onset of the feature at early times, but helium can contribute to, or even dominate the feature at later epochs.

astro-ph.HE↗
arXiv2026

A hot, nebular-dominated galaxy interacting with a pristine PopIII system uncovered by JWST

The discovery of galaxies with extremely strong nebular continuum emission at high redshifts provide novel, unique insights into the conditions under which the first super-massive stars formed. Here we identify a galaxy at redshift $z=5.124$ observed by the JWST CAPERS survey that exhibits a prominent turnover in the rest-frame UV continuum and a pronounced Balmer `jump'. We model the entire JWST/NIRSpec Prism spectrum from rest-frame UV to optical wavelength, finding that a dominant ($>95\%$) nebular continuum emission can accurately reproduce the spectral shape across all wavelengths. We tested an alternative model with strong damped Ly$\alpha$ absorption (DLA), but found that it is not able to match the shape of the turnover without invoking a large freedom in the redshift of the absorber. The nebular continuum emission model reveals a hot ($T = (5.3\pm 0.2)\times 10^{4}$ K) and dense ($n_e = (5.4\pm 0.8)\times 10^{3} {\rm cm^{-3}}$) nebular region powering the origin of the spectral shape. We also note the presence of a `blue' candidate companion source potentially at the same redshift, offset by 3 kpc to the main galaxy. Intriguingly, the spectrum of this source show several hints of hydrogen and helium lines, but no metal lines are detected. We theorize that this companion galaxy might be comprised mainly of Population III (PopIII) stellar remnants and potentially powers the nebular continuum emission seen in the main galaxy. These results have important implications for the presence of a potential dominant population of super-massive and PopIII stars and their consequent excess UV brightness for a significant fraction of galaxies at cosmic dawn.

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arXiv2025

Interstellar Ices as Carriers of Supernova Material to the Early Solar System

Planetary materials show systematic variations in their nucleosynthetic isotope compositions that resonate with orbital distance. The origin of this pattern remains debated, limiting how these isotopic signatures can be used to trace the precursors of terrestrial planets. Here we test the hypothesis that interstellar ices carried supernova-produced nuclides by searching for a supernova nucleosynthetic fingerprint in aqueous alteration minerals from carbonaceous and non-carbonaceous chondrite meteorites. We focus on zirconium, a refractory element that includes the neutron-rich isotope $^{96}$Zr formed in core-collapse supernovae. Leaching experiments reveal extreme $^{96}$Zr enrichments in alteration minerals, showing that they incorporated supernova material hosted in interstellar ices. We show that the Solar System's zirconium isotope variability reflects mixing between these ices and an ice-free rocky component. Finally, the presence of supernova nuclides in a volatile carrier supports models where the Solar System's nucleosynthetic variability was imparted by thermal processing of material in the protoplanetary disk and during planetary accretion.

astro-ph.EP↗
arXiv2025

Inefficient dust production in a massive, metal-rich galaxy at $z=7.13$ uncovered by JWST and ALMA

Recent observations have revealed a remarkably rapid buildup of cosmic dust in the interstellar medium (ISM) of high redshift galaxies, with complex dust compositions and large abundances already appearing at redshifts $z>6$. Here we present a comprehensive, joint analysis of observations taken with the {\em James Webb Space Telescope} (JWST) and the Atacama Large Millimetre/sub-millimetre Array (ALMA) of the highly magnified, dusty `normal' galaxy, A1689-zD1 at $z=7.13$. We perform detailed spectro-photometric modeling of the rest-frame UV to far-infrared spectral energy distribution (SED) based on archival photometry of the source and report new rest-frame optical strong-line measurements and metallicity estimates from recent JWST/NIRSpec IFU data. We find that despite its substantial dust mass, $M_{\rm dust}\sim 1.5\times 10^{7}\,M_\odot$, A1689-zD1 has remarkably low dust-to-gas and dust-to-metal mass ratios, ${\rm DTG} = (5.1^{+3.0}_{-1.9})\times 10^{-4}$ and ${\rm DTM} = (6.1^{+3.6}_{-2.3})\times 10^{-2}$, respectively, due to its high metallicity $12+\log({\rm O/H}) = 8.36\pm 0.10$ and substantial gas mass, $M_{\rm gas} = (2.8^{+0.2}_{-1.7})\times 10^{10}\,M_\odot$. The DTG and DTM mass ratios are an order of magnitude lower than expected for galaxies in the local universe with similar chemical enrichment. These low relative measurements are also corroborated by the deficit observed in the $A_V/N_{\rm HI}$ ratio of A1689-zD1 in the line-of-sight. We find that this deviation in the DTG and DTM mass ratios appears to be ubiquitous in other metal-rich galaxies at similar redshifts, $z\gtrsim 6$. This suggests that the processes that form and destroy dust at later times, or the dust emissivity itself, are drastically different for galaxies in the early Universe.

astro-ph.GA↗
arXiv2025

Early galaxy evolution: The complex interstellar medium distribution of the z~7 galaxy A1689-zD1

We observed the gravitationally lensed ($\mu = 9.6\pm0.19$) galaxy A1689-zD1 at $z = 7.1$ in bands 3, 6, and 8 of the Atacama Large Millimeter/submillimeter Array. These high-resolution observations ($\approx 200$ pc) enabled us to separate the source into five components in the [CII] 158$\mu$m and [OIII] 88$mu$m emission lines within a projected distance of 2 kpc. Even though these components appear to vary strongly from one another in both their line, continuum, and optical characteristics, the assembly of components do not show ordered rotation and appear consistent with simulations of a galaxy system undergoing the process of assembly. The total dynamical mass of the galaxy ($2\times10^{10}$ M$_\odot$) is an order of magnitude larger than the spectrally estimated stellar mass, suggesting a near-complete optical obscuration of the bulk of the stellar component. Comparing the line ratios as well as the line properties to other properties such as the star formation rate, we find that A1689-zD1 is consistent with the relations derived from local star-forming galaxies. Even though A1689-zD1 lies on local star formation scaling relations and has a high dust and stellar mass estimate, the kinematics suggest it is in an early assembly stage, which could lead to it becoming a disk galaxy at a later stage.

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arXiv2025

On the origins of oxygen: ALMA and JWST characterise the multi-phase, metal-enriched, star-bursting medium within a 'normal' $z > 11$ galaxy

The unexpectedly high abundance of galaxies at $z > 11$ revealed by JWST has sparked a debate on the nature of early galaxies and the physical mechanisms regulating their formation. The Atacama Large Millimeter/submillimeter Array (ALMA) has begun to provide vital insights on their gas and dust content, but so far only for extreme 'blue monsters'. Here we present new, deep ALMA observations of JADES-GS-z11-0, a more typical (sub-$L^*$) $z > 11$ galaxy that bridges the discovery space of JWST and the Hubble Space Telescope. These data confirm the presence of the [O III] 88 $\mu$m line at $4.5\sigma$ significance, precisely at the redshift of several faint emission lines previously seen with JWST/NIRSpec, while the underlying dust continuum remains undetected ($F_\nu < 9.0 \, \mathrm{\mu Jy}$), implying an obscured star formation rate (SFR) of $\text{SFR}_\text{IR} \lesssim 6 \, \mathrm{M_\odot \, yr^{-1}}$ and dust mass of $M_\text{dust} \lesssim 1.0 \times 10^{6} \, \mathrm{M_\odot}$ (all $3\sigma$). The accurate ALMA redshift of $z_\text{[O III]} = 11.1221 \pm 0.0006$ ($\gtrsim \! 5\times$ refined over NIRSpec) helps confirm that redshifts measured purely from the Lyman-$\alpha$ break, even spectroscopically, should properly take into account the effects of potential damped Lyman-$\alpha$ absorption (DLA) systems to avoid systematic overestimates of up to $\Delta z \approx 0.5$. The [O III] 88 $\mu$m luminosity of $L_\text{[O III]} = (1.1 \pm 0.3) \times 10^{8} \, \mathrm{L_\odot}$, meanwhile, agrees well with the scaling relation for local metal-poor dwarfs given the SFR measured by NIRCam, NIRSpec, and MIRI. The spatially resolved MIRI and ALMA emission also underscores that JADES-GS-z11-0 is likely to consist of two low-mass components that are undergoing strong bursts of star formation yet are already pre-enriched in oxygen (~20-30% solar), only 400 Myr after the Big Bang.

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arXiv2025

The day-long, repeating GRB 250702BDE / EP250702a: A unique extragalactic transient

Gamma-ray bursts (GRBs) are singular outbursts of high-energy radiation with durations typically lasting from milliseconds to minutes and, in extreme cases, a few hours. They are attributed to the catastrophic outcomes of stellar-scale events and, as such, are not expected to recur. Here, we present observations of an exceptional GRB\,250702BDE which triggered the {\em Fermi} gamma-ray burst monitor on three occasions over several hours, and which was detected in soft X-rays by the \textit{Einstein Probe} a day before the $\gamma$-ray triggers (EP250702a). We present the discovery of an extremely red infrared counterpart of the event with the VLT, as well as radio observations from MeerKAT. Hubble Space Telescope observations pinpoint the source to a non-nuclear location in a host galaxy with complex morphology, implying GRB 250702BDE is an extragalactic event. The multi-wavelength counterpart is well described with standard afterglow models at a relatively low redshift $z \sim 0.2$, but the prompt emission does not readily fit within the expectations for either collapsar or merger-driven GRBs. Indeed, a striking feature of the multiple prompt outbursts is that the third occurs at an integer multiple of the interval between the first two. Although not conclusive, this could be indicative of periodicity in the progenitor system. We discuss several possible scenarios to explain the exceptional properties of the burst, which suggest that either a very unusual collapsar or the tidal disruption of a white dwarf by an intermediate-mass black hole are plausible explanations for this unprecedented GRB.

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arXiv2025

Novel $z\sim~10$ auroral line measurements extend the gradual offset of the FMR deep into the first Gyr of cosmic time

The mass assembly and chemical enrichment of the first galaxies provide key insights into their star-formation histories and the earliest stellar populations at cosmic dawn. Here we compile and utilize new, high-quality spectroscopic JWST/NIRSpec Prism observations from the JWST archive. We extend the wavelength coverage beyond the standard pipeline cutoff up to 5.5$\mu$m, enabling a detailed examination of the rest-frame optical emission-line properties for galaxies at $z\approx 10$. The improved calibration allows us to detect H$\beta$ and the [OIII]$\lambda\lambda 4959,5007$ doublet and resolve the auroral [OIII]$\lambda 4363$ line for the 11 galaxies in our sample ($z=9.3-10.0$) to obtain direct $T_e$-based metallicity measurements. We find that all galaxies show high ionisation fields and electron temperatures, with derived metallicities in the range $12+\log {\rm (O/H)} = 7.1 - 8.3$, consistent with previous strong-line diagnostics at high-z. We derive an empirical relation for $M_{\rm UV}$ and 12+log(O/H) at $z\approx 10$, useful for future higher-z studies, and show that the sample galaxies are `typical' star-forming galaxies though with relatively high specific star-formation rates and with evidence for bursty star formation. Combining the rest-frame optical line analysis and detailed UV to optical SED modelling, we determine the mass-metallicity relation and the fundamental-metallicity relation of the sample, pushing the redshift frontier of these measurements to $z=10$. These results, together with literature measurements, point to a gradually decreasing MZR at higher redshifts, with a break in the FMR at $z\approx 3$, decreasing to metallicities $\approx 3\times$ lower at $z=10$ than observed during the majority of cosmic time at $z=0-3$, likely caused by massive pristine gas inflows diluting the observed metal abundances during early galaxy assembly at cosmic dawn.

astro-ph.GA↗
arXiv2025

The optical constants and grain sizes of interstellar dust measured directly using the dust scattered x-ray halo of GRB 221009A

X-ray scattering is a powerful probe of the optical constants and grain size distribution of interstellar dust. Bright, transient sources are excellent tools for this, since they fade rapidly, leaving only the expanding scattered x-ray halo. Here, we analyse the dust-scattered x-ray halo data of the unprecedentedly bright $\gamma$-ray burst, GRB 221009A, using anomalous diffraction theory to measure the grain size distribution of dust in the Galaxy as well as the complex refractive index, $m$, and use these results to infer the likely composition. We find a complex refractive index, $m=n+ik$ at several x-ray energies, finding $k_{1\,\mathrm{keV}}= (2.7 \pm 0.7)\times10^{-4}$ and $1-n_{1\,\mathrm{keV}}= (9 \pm 2)\times 10^{-4}$, strongly inconsistent with the commonly employed assumptions of the Rayleigh-Gans approximation. These results lie in the expected range for interstellar dust compositions dominated by carbon, magnesium silicates, and iron. The absorption results suggest a substantial mass fraction of iron at $35\pm7\%$. The MRN distribution fit returns a maximum grain radius, $a_{\rm max}=0.24\pm 0.01\,\mu$m; all fits strongly rule out models with $\sim0.4\,\mu$m grains for this sightline. The soft x-ray spectrum of the prompt GRB can also be inferred from the fitting, with the best-fit providing a spectral slope that is consistent with the slope of the low energy side of the best-fit Band model of the directly measured prompt emission. Forcing a different grain size or composition than the best fit results in an inferred prompt spectrum different to the observed prompt emission. The refractive index is consistent with standard average dust compositions, showing that x-ray scattering is an effective tool to measure interstellar dust optical properties. [abridged]

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arXiv2025

A Cosmic Miracle: A Remarkably Luminous Galaxy at $z_{\rm{spec}}=14.44$ Confirmed with JWST

JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, $z>10$, where few such sources were expected. Here we present the most distant example of this class yet -- MoM-z14, a luminous ($M_{\rm{UV}}=-20.2$) source in the COSMOS field at $z_{\rm{spec}}=14.44^{+0.02}_{-0.02}$ that expands the observational frontier to a mere 280 million years after the Big Bang. The redshift is confirmed with NIRSpec/PRISM spectroscopy through a sharp Lyman-$\alpha$ break and $\sim3\sigma$ detections of five rest-UV emission lines. The number density of bright $z_{\rm{spec}}\sim14-15$ sources implied by our "Mirage or Miracle" survey spanning $\sim350$ arcmin$^{2}$ is $>100\times$ larger ($182^{+329}_{-105}\times$) than pre-JWST consensus models. The high EWs of UV lines ($\sim15{-}35$ \AA) signal a rising star-formation history, with a $\sim10\times$ increase in the last 5 Myr ($\rm{SFR_{\rm{5Myr}}}/\rm{SFR_{\rm{50Myr}}}=9.9^{+3.0}_{-5.8}$). The source is extremely compact (circularized $r_{\rm{e}} = 74^{+15}_{-12}$ pc), and yet elongated ($b/a=0.25^{+0.11}_{-0.06}$), suggesting an AGN is not the dominant source of UV light. The steep UV slope ($\beta=-2.5^{+0.2}_{-0.2}$) implies negligible dust attenuation and a young stellar population. The absence of a strong damping wing provides tentative evidence that the immediate surroundings of MoM-z14 may be partially ionized at a redshift where virtually every reionization model predicts a $\sim100\%$ neutral fraction. The nitrogen emission and highly super-solar [N/C]$>1$ hint at an abundance pattern similar to local globular clusters that may have once hosted luminous supermassive stars. Since this abundance pattern is also common among the most ancient stars born in the Milky Way, we may be directly witnessing the formation of such stars in dense clusters, connecting galaxy evolution across the entire sweep of cosmic time.

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arXiv2025

A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that displays singular properties: among the largest Hydrogen Balmer breaks reported at any redshift, broad multi-peaked H$\beta$ emission, and Balmer line absorption in multiple transitions. We model this source as a "black hole star" (BH*) where the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free "atmosphere" around a supermassive black hole. This source may provide evidence of an early black hole embedded in dense gas -- a theoretical configuration proposed to rapidly grow black holes via super-Eddington accretion. Radiation from the BH* appears to dominate almost all observed light, leaving limited room for contribution from its host galaxy. We demonstrate that the recently discovered "Little Red Dots" (LRDs) with perplexing spectral energy distributions can be explained as BH*s embedded in relatively brighter host galaxies. This source provides evidence that black hole masses in the LRDs may be over-estimated by orders of magnitude -- the BH* is effectively dust-free contrary to the steep dust corrections applied while modeling LRDs, and the physics that gives rise to the complex line shapes and luminosities may deviate from assumptions underlying standard scaling relations.

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arXiv2025

Dissecting the massive pristine, neutral gas reservoir of a remarkably bright galaxy at z = 14.179

At cosmic dawn, the first stars and galaxies are believed to form from and be deeply embedded in clouds of dense, pristine gas. Here we present a study of the JWST/NIRSpec data of the most distant, spectroscopically confirmed galaxy observed to date, JADES-GS-z14-0 (GS-z14 for short), at $z=14.179$, combined with recent far-infrared measurements of the [OIII]-$88\mu$m and [CII]-$158\mu$m line transitions and underlying dust-continuum emission. Based on the observed prominent damped Lyman-$\alpha$ (DLA) absorption profile, we determine a substantial neutral atomic hydrogen (HI) column density, $\log (N_{\rm HI} / {\rm cm^{-2}}) = 22.27^{+0.08}_{-0.09}$, consistent with previous estimates though seemingly at odds with the dynamical and gas mass of the galaxy. Using various independent but complementary approaches, considering the implied neutral gas mass from the DLA measurement, the star-formation rate surface density, and the metal abundance, we demonstrate that the total gas mass of GS-z14 is of the order $\log (M_{\rm gas} / M_\odot) = 9.8\pm 0.3$. This implies a substantial gas mass fraction, $f_{\rm gas} \gtrsim 0.9$ and that the bulk of the interstellar medium (ISM) is in the form of HI. We show that the derived gas mass is fully consistent with the non-detection of [CII]-$158\mu$m, assuming an appropriate scaling to the neutral gas. The low dust-to-gas ratio, $A_V/N_{\rm HI} = (1.3\pm 0.6)\times 10^{-23}$\,mag\,cm$^2$, derived in the line-of-sight through the DLA further indicates that the absorbing gas is more pristine than the central, star-forming regions probed by the [OIII]-$88\mu$m emission. These results highlight the implications for far-infrared line-detection searchers attainable with ALMA and demonstrate that the bright, relatively massive galaxy GS-z14 at $z=14.179$ is deeply embedded in a substantial, pristine HI gas reservoir dominating its baryonic matter content.

astro-ph.GA↗
arXiv2024

COSMOS2020: Disentangling the Role of Mass and Environment in Star Formation Activity of Galaxies at $0.4<z<4$

The role of internal and environmental factors in the star formation activity of galaxies is still a matter of debate, particularly at higher redshifts. Leveraging the most recent release of the COSMOS catalog, COSMOS2020, and density measurements from our previous study we disentangle the impact of environment and stellar mass on the star formation rate (SFR), and specific SFR (sSFR) of a sample of $\sim 210,000$ galaxies within redshift range $0.4< z < 4$ and present our findings in three cosmic epochs: 1) out to $z\sim 1$, the average SFR and sSFR decline at extremely dense environments and high mass end of the distribution which is mostly due to the presence of the massive quiescent population; 2) at $1<z<2$, the environmental dependence diminishes, while mass is still the dominant factor in star formation activity; 3) beyond $z\sim 2$, our sample is dominated by star-forming galaxies and we observe a reversal of the trends seen in the local universe: the average SFR increases with increasing environmental density. Our analysis shows that both environmental and mass quenching efficiencies increase with stellar mass at all redshifts, with mass being the dominant quenching factor in massive galaxies compared to environmental effects. At $2<z<4$, negative values of environmental quenching efficiency suggest that the fraction of star-forming galaxies in dense environments exceeds that in less dense regions, likely due to the greater availability of cold gas, higher merger rates, and tidal effects that trigger star formation activity.

astro-ph.GA↗
arXiv2024

Helium as an Indicator of the Neutron-Star Merger Remnant Lifetime and its Potential for Equation of State Constraints

The time until black hole formation in a binary neutron-star (NS) merger contains invaluable information about the nuclear equation of state (EoS) but has thus far been difficult to measure. We propose a new way to constrain the merger remnant's NS lifetime, which is based on the tendency of the NS remnant neutrino-driven winds to enrich the ejected material with helium. Based on the He I $\lambda 1083.3$ nm line, we show that the feature around 800-1200 nm in AT2017gfo at 4.4 days seems inconsistent with a helium mass fraction of $X_{\mathrm{He}} \gtrsim 0.05$ in the polar ejecta. Our recent neutrino-hydrodynamic simulations of merger remnants are only compatible with this limit if the NS remnant collapses within 20-30 ms. Such a short lifetime implies that the total binary mass of GW170817, $M_\mathrm{\rm tot}$, lay close to the threshold binary mass for direct gravitational collapse, $M_\mathrm{thres}$, for which we estimate $M_{\mathrm{thres}}\lesssim 2.93 M_\odot$. This upper bound on $M_\mathrm{thres}$ yields upper limits on the radii and maximum mass of cold, non-rotating NSs, which rule out simultaneously large values for both quantities. In combination with causality arguments, this result implies a maximum NS mass of $M_\mathrm{max}\lesssim2.3 M_\odot$. The combination of all limits constrains the radii of 1.6 M$_\odot$ NSs to about 12$\pm$1 km for $M_\mathrm{max}$ = 2.0 M$_\odot$ and 11.5$\pm$1 km for $M_\mathrm{max}$ = 2.15 M$_\odot$. This $\sim2$ km allowable range then tightens significantly for $M_\mathrm{max}$ above $\approx2.15$ M$_\odot$. This rules out a significant number of current EoS models. The short NS lifetime also implies that a black-hole torus, not a highly magnetized NS, was the central engine powering the relativistic jet of GRB170817A. Our work motivates future developments... [abridged]

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