Darach Watson

Previous S IROCCO radiative-transfer models of gas-cocooned active galactic nuclei (AGNs) 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 _H ∼ few × 10 ²⁴ cm ⁻² , and Balmer-break LRD-like spectra at higher columns. Along this sequence, electron scattering produces exponential line wings and suppresses X-ray emission before strong Balmer absorption features emerge. The models predict that LBDs should share X-ray weakness and exponential line wings with LRDs, while being distinguished by Balmer jumps in emission, weak or absent absorption features, potential He II λ 4686 emission, lower H α luminosities, and narrower H α lines. We compare with spectra of eight LBDs and find that significant Balmer-jump signatures are common. We therefore suggest that LBDs are lower-column analogues of LRDs within a common gas-cocooned AGN sequence.

The physical processes that led to the formation of billion-solar-mass black holes within the first 700 million years of cosmic time, a period known as cosmic dawn, remain a puzzle ¹ . Several theoretical scenarios have been proposed to seed and rapidly grow black holes ^2–4 , but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that exhibits singular properties: among the largest hydrogen Balmer breaks reported at any redshift, broad multi-peaked Hβ emission, and Balmer line absorption in several transitions. We model this source as an enshrouded black hole in which the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free envelope around a supermassive black hole ^5,6 . This source may provide evidence of an early black hole embedded in dense gas—a theoretical configuration proposed to rapidly grow black holes by super-Eddington accretion ^7,8 . Radiation from the black hole seems to dominate almost all observed light, leaving limited room for contribution from its host galaxy. If the source merged with its brighter neighbour, it would resemble the recently discovered ‘little red dots’ with perplexing spectral energy distributions ^9–11 . The redness of the black hole is due to gas, not dust ^12,13 , and scattering, not kinematics, gives rise to the complex line shapes and luminosities—black hole masses of these sources may therefore be overestimated by orders of magnitude.

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 ),cm,(^ )) with moderate metallicity (0.05--0.1,(Z_⊙)) and intrinsically weak X-ray emission (the ratio of bolometric to X-ray luminosity is (k_ ≳ 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. 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_ ̊m H 25 -2 ̊m bol,X

Context. Little Red Dots (LRDs) are a new class of compact extragalactic objects with a v-shaped optical spectral energy distribution breaking close to the Balmer break wavelength and broad, typically exponentially shaped lines. They are believed to be supermassive black holes surrounded by very dense ionised gas, leading us to search for any departures from case B recombination in these systems. Aims. For this paper we examined the ratios of multiple hydrogen Balmer lines (H α , H β , H γ , and H δ ) to determine whether there is indication of deviation from simple case B recombination processes. Methods. We analysed a dozen high-S/N LRDs with JWST/NIRSpec and measured Balmer ratios in the seven objects with coverage of at least three lines. We decomposed the line ratios into their respective broad and narrow components. Results. Broad-line ratios are consistent with case B plus severe dust extinction in all objects but one, RUBIES-EGS-4233_49140 at z = 6.68; in this case the ratios deviate by more than 5 σ and are inconsistent with all known dust curves. The narrow components are consistent with minimal dust attenuation, while two objects exhibit narrow H α /H β ≈ 1.8. Conclusions. Such low decrements are observed in highly ionised density bounded nebulae, associated with starburst environments. Nevertheless, the two flat decrement cases can be reconciled assuming an unresolved absorption feature. RUBIES EGS-49140 shows a high broad H α /H β , but the H γ /H α and H δ /H α ratios are lower than expected for extinction-modified case B, hinting at an unphysically steep dust law. These line ratios may be due to increased optical depth in the Balmer lines as a direct effect of high-density gas (log n _e > 9) surrounding the black hole. If case B recombination does hold in most LRDs, they must be moderately to heavily dust obscured ( A _V ≃ 1−8), while the host-galaxy should be dust-free, suggesting that the extinction in the broad lines is local to the LRD and is not due to the general interstellar medium of the host galaxy. Nevertheless, this dust-based scenario poses several caveats. The apparent general consistency of LRDs with case B may be an artefact of H γ and H δ non-detections.

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 utilise new, high-quality spectroscopic JWST/NIRSpec Prism observations from the JWST archive. In particular, we extend the wavelength coverage beyond the standard pipeline cut-off (5.3 μm) up to 5.5 μm, which enables for the first time a detailed examination of the rest-frame optical emission-line properties for galaxies at z ≈ 10. Crucially, the improved calibration allows us to detect H β and the [O III ] λλ 4959, 5007 doublet and resolve the auroral [O III ] λ 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 the interstellar medium (ISM) of all galaxies shows high ionisation fields and electron temperatures, with derived metallicities in the range 12 + log(O/H) = 7.1 − 8.3 (3–50% solar), consistent with previous strong-line diagnostics based on JWST data at high redshifts. We derive an empirical relation for M _UV and 12 + log(O/H) at z ≈ 10, useful for future higher-redshift studies, and show that the sample galaxies are ‘typical’ star-forming galaxies though with relatively high specific star formation rates (median sSFR = SFR _{H β} / M _★ = 38 Gyr ⁻¹ ) and with evidence of bursty star formation on 10 Myr versus 100 Myr timescales (log ₁₀ (SFR ₁₀ /SFR ₁₀₀ )≈0.7). Combining the rest-frame optical line analysis and detailed UV to optical spectro-photometric modelling, we determine the mass-metallicity relation (MZR) and the fundamental metallicity relation (FMR) of the sample, pushing the previous 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 ≈ 3, decreasing to metallicities ≈3× lower at z = 10 than observed in galaxies 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.

The James Webb Space Telescope (JWST) has uncovered many compact galaxies at high redshift with broad hydrogen and helium lines, including the enigmatic population of little red dots (LRDs) ^1,2 . The nature of these galaxies is debated and is attributed to supermassive black holes (SMBHs) ^3,4 or intense star formation ⁵ . They exhibit unusual properties for SMBHs, such as black holes that are overmassive for their host galaxies ⁴ and extremely weak X-ray ^6–10 and radio ^6,11–13 emission. Here we show that in most objects studied with the highest-quality JWST spectra, the lines are broadened by electron scattering with a narrow intrinsic core. The data require very high electron column densities and compact sizes (light days), which, when coupled with their high luminosities, can be explained only by SMBH accretion. The narrow intrinsic line cores imply black hole masses of 10 ⁵⁻⁷ M _⊙ , two orders of magnitude lower than previous estimates. These are the lowest mass black holes known at high redshift, to our knowledge, and suggest a population of young SMBHs. They are enshrouded in a dense cocoon of ionized gas producing broad lines from which they are accreting close to the Eddington limit, with very mild neutral outflows. Reprocessed nebular emission from this cocoon dominates the optical spectrum, explaining most LRD spectral characteristics, including the weak radio and X-ray emission ^14,15 .

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 μ m line at 4.5 σ significance, precisely at the redshift of several faint emission lines previously seen with JWST/NIRSpec, while the underlying dust continuum remains undetected ( F ν < 9.0 μ J y ), implying an obscured star formation rate (SFR) of SFR IR ≲ 6 M ⊙ y r − 1 and dust mass of M dust ≲ 1.0 × 10 6 M ⊙ (all 3 σ ). The accurate ALMA redshift of z [O III] = 11.1221 ± 0.0006 ( ≳ 5 × refined over NIRSpec) helps confirm that redshifts measured purely from the Lyman- α break, even spectroscopically, should properly take into account the effects of potential damped Lyman- α absorption (DLA) systems to avoid systematic overestimates of up to Δ z ≈ 0.5 . The [O III] 88 μ m luminosity of L [O III] = ( 1.1 ± 0.3 ) × 10 8 L ⊙ , 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 ( ∼ 30 % solar), only 400 Myr after the Big Bang.

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.

Context. The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons, thus effective probes of the distant Universe. The most luminous bursts are typically detected at moderate and high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as those at TeV energies. Aims. Here we present a spectroscopic redshift measurement for the exceptional GRB 221009A, the brightest GRB observed to date, with emission extending well into the TeV regime. Methods. We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-infrared (NIR) spectroscopy of the burst afterglow 0.5 days after the explosion. Results. The spectra exhibit both absorption and emission lines from material in a host galaxy at z_GRB = 0.15095 ± 0.00005. Thus, GRB 221009A was a relatively nearby burst with a luminosity distance of d_L = 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of long GRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, E_iso > 5 × 10⁵⁴ erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also most energetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every few decades and once per millennium.