Fergus Cullen

“Little red dots” (LRDs) at 4 < z < 8 are one of the most challenging discoveries by JWST to date because their distinctive V-shaped spectra and compact morphologies defy conventional astrophysical interpretation. Previous attempts to explain LRDs as compact stellar systems, heavily cocooned black holes (BHs) with differential flows, supermassive stars, or more exotic objects like “black hole stars” cannot either show how they formed, explain the origin of the dense shells needed for the absorption features in their spectra, or account for their observed abundances or inferred lifetimes. Here we show that LRDs are simply direct-collapse BH galaxies in which the BH is still shrouded by the massive disk that created it. Our cosmological simulations yield spectra that are good matches to those of LRDs because high densities at the center of the disk trap X-rays from the BH and produce the observed Balmer absorption features while allowing UV, optical, and reprocessed IR flux to partly escape. The host galaxy forms a dense 10 ⁸ M _⊙ cluster of stars with a radius of 150 pc next to the BH, consistent with observations of LRDs. Our models reproduce a wide variety of LRD spectra, from typical objects like RUBIES-EGS-42046 at z = 5.28 to those with the strongest Balmer breaks such as MoM-BH ^* −1 at z = 7.76 and those at the highest redshifts like CAPERS-LRD-z9 at z = 9.29.

We present argon abundances from a sample of 46 star-forming galaxies at z = 2−3.5 from the Assembly of Ultradeep Rest-Optical Observations Revealing Astrophysics (AURORA) program. Although argon is an α –element produced by core collapse supernovae (CCSNe), the latest supernova yield models suggest additional argon production and enrichment by Type Ia supernovae (SNe Ia), unlike other α –elements such as oxygen. To study the relationship between argon and oxygen abundances, we construct two median-stacked composite spectra for separate z = 2.0−2.6 and z = 2.8−3.5 redshift bins, presenting more representative measurements than previous samples that require individual detection of faint lines. Abundance ratios were determined using an empirical calibration based on the strength of the [ArIII] λ 7137 emission line relative to the [O III ] λ 5008 emission line. With this calibration, we estimate argon abundances (Ar/O) of 0.4 2 − 0.10 + 0.12 (Ar/O) ⊙ for the 〈 z 〉 = 2.26 bin and 0.4 2 − 0.11 + 0.12 (Ar/O) ⊙ for the 〈 z 〉 = 3.15 bin, suggesting minimal SNe Ia and dominant CCSNe enrichment in this sample. Comparison of our abundance measurements of z ∼ 2−3 AURORA galaxies with chemical evolution modeling of Milky Way stars shows consistency with the Milky Way bulge component, suggesting a rapid star formation timescale. However, even larger samples of actively star-forming galaxies with available argon abundances, as well as comparisons between argon abundance and other critical galaxy properties (e.g., specific star formation rate, sSFR) and models (e.g., one tuned specifically to this redshift range) are needed to draw stronger conclusions on the role of argon in galactic chemical enrichment at cosmic noon.

We analyze eight spectroscopically confirmed little red dots (LRDs) at redshifts z = 5.1–8.7 with JWST/NIRCam, NIRSpec, and MIRI data. The LRDs have red NIRCam colors, F150W − F444W > 1, but flat NIRCam − MIRI colors, −0.5 < F444W − F770W < 0.5, suggesting weak warm/hot dust components. The LRDs have −1.0 < F1000W − F1500W < 1.1, suggestive of nonuniform rest near-IR properties within the sample. We model the spectral energy distributions (SEDs) of the LRDs using the CIGALE and P rospector codes to assess how the differing templates impact the interpretation for LRDs for cases of: (1) models with star-forming (SF) stellar populations only; (2) active galactic nuclei (AGN)-dominated models; and (3) composite AGN and SF models. Using the Bayesian information criterion, we find that six of the eight LRDs favor AGN models compared to SF models, though no model reproduces all of the observed properties. Two LRDs with pronounced Balmer breaks and broad H α have SEDs that are reproduced with hot, dense-gas ( log T / K = 5 – 5.7 , log n / cm − 3 = 9 – 11 ) models with low dust attenuation ( A ( V ) ≃ 0.5 mag). However, these models require an additional thermal component (800–1400 K) to account for the MIRI data, and fail to reproduce the rest-UV and narrow [O iii ] emission. The total bolometric emission from the dense-gas models, and possibly CIGALE AGN models, appears consistent with literature constraints in the far-IR and radio, and requires log L bol / L ⊙ < 12 . These results suggest that our LRDs cannot be modeled entirely with standard templates, but instead require a novel treatment of gas conditions, AGN, and star formation.

We present detections of auroral emission lines of [O iii ], [O ii ], [S iii ], and [S ii ] in deep JWST/NIRSpec spectroscopy for 41 star-forming galaxies at z = 1.4–7.2 from the AURORA survey. We combine these new observations with 98 star-forming galaxies at z = 1.3–10.6 with detected auroral lines drawn from the literature to form a sample of 139 high-redshift galaxies with robust electron temperature and direct-method oxygen abundance determinations. This sample notably covers a wider dynamic range in metallicity than previous work, spanning 0.02–0.9 Z _⊙ . We calibrate empirical relations between 19 emission-line ratios and oxygen abundance, providing a robust tool set to infer accurate gas-phase metallicities of high-redshift galaxies when auroral lines are not detected. While calibrations based on lines of α elements (O, Ne, S, Ar) appear reliable, we find significant scatter in calibrations involving lines of N driven by a high dispersion in N/O at fixed O/H, suggesting that N-based line ratios are less reliable tracers of the oxygen abundance at high redshift. These new high-redshift calibrations are notably offset from those based on typical z ∼ 0 galaxy and H ii region samples, and are better matched by samples of extreme local galaxies that are analogs of high-redshift sources. The new metallicity calibrations presented in this work pave the way for robust studies of galaxy chemical evolution in the early Universe, leading to a better understanding of baryon cycling and galaxy formation from Cosmic Noon through the Epoch of Reionization.

Overmassive black hole galaxies (OBGs) at redshifts z ∼ 10, or 450 Myr after the Big Bang, are one of the most puzzling discoveries by the James Webb Space Telescope to date because they formed by such early epochs and their black-hole-to-stellar-mass ratios are 100 times higher than those in galaxies today. Here we show that OBGs are simply the result of DCBH birth in primordial halos at early times. A 70,000 M _⊙ DCBH forming at z = 25.7 in our cosmological simulation grows at about half the Eddington rate to 6.0 × 10 ⁶ M _⊙ by z = 10.1. Its host galaxy reaches a stellar mass of 4 × 10 ⁸ M _⊙ , a metallicity Z = 0.1 Z _⊙ , a star formation rate of 2 M _⊙ yr ⁻¹ , and M _BH / M _* ∼ 0.01, on par with OBGs like GN-z11, UHZ1, and GHZ9 at z = 10.6, 10.1, and 10.2, respectively. Our simulation, the first to follow the coevolution of a DCBH and its host galaxy for several hundred Myr while resolving star formation in the earliest minihalos, shows that this ratio is a natural result of initial suppression of star formation by the DCBH and the later, violent blowout of metals by Population III supernovae. Our models yield an excellent match to the spectra of UHZ1 and GHZ9 at z = 10.1 and 10.4, respectively.

We present new constraints on the mass–metallicity (MZR) and fundamental metallicity relations (FMR) using a sample of 34 galaxies at 1.38 ≤ z ≤ 3.5 (median z = 2.28). These galaxies have direct T _e measurements from [O iii ]4363 Å and/or [O ii ]7320,7331 Å auroral emission lines detected with JWST/NIRSpec as part of the AURORA survey. The detection of both oxygen auroral lines allows for dual-zone direct T _e measurements and expands the dynamic range in 12 + log ( O / H ) (7.68–8.65 dex), stellar mass (10 ⁸ –10 ^10.4 M _⊙ ), and star formation rate (1–100 M _⊙ yr ⁻¹ ) compared to previous direct T _e studies of the high-redshift MZR and FMR. We characterize the z ∼ 2 MZR and find a slope of 0.27 ± 0.04 and normalization of 12 + log ( O / H ) = 8.44 ± 0.04 at10 ¹⁰ M _⊙ with an intrinsic scatter of 0.10 dex, consistent with past strong-line MZR measurements. Comparisons with z ∼ 2 predictions from six simulations reveal that none reproduce our observed MZR normalization evolution between z ∼ 0 and z ∼ 2. This discrepancy suggests current models do not fully capture the chemical enrichment and feedback processes occurring at cosmic noon. However, all 34 galaxies are on or above the star-forming main sequence such that our sample may be biased toward lower 12 + log ( O / H ) if the FMR persists at z ∼ 2. Correcting for this selection effect would increase O/H by ≈0.1 dex at 10 ^9.3 M _⊙ (the median mass of our sample), bringing our MZR into better agreement with that of Illustris TNG . Lastly, we find our z ∼ 2.3 sample is consistent with the z ∼ 0 FMR within 0.1 dex in O/H, indicating that the smooth secular mechanisms regulating chemical enrichment, star formation, stellar mass, and outflows were in place at cosmic noon.

We present a comprehensive analysis of the nitrogen-to-oxygen (N/O) abundance ratio in a sample of ∼660 star-forming galaxies at redshift z ∼ 1 − 6, with a median redshift of ⟨ z ⟩ = 3.0, using deep JWST/NIRSpec spectroscopy. Leveraging detections of faint auroral emission lines in 92 galaxies at z > 1 from both the MARTA survey and a large compilation of high-redshift literature objects, we derived direct electron-temperature-based abundances for nitrogen and oxygen using rest-frame optical lines. We establish the first high-redshift calibrations of strong-line N/O diagnostics based on ‘direct’ abundance measurements, finding a mild evolution for the N2O2 = log([N II ] λ 6585/[O II ] λλ 3727,3729) diagnostic and no clear evolution for the N2S2 = log([N II ] λ 6585/[S II ] λλ 6717,6731) diagnostic compared to local realisations. We then investigated the N/O–O/H relation across cosmic time using both ‘direct’ abundances and strong-line-based measurements (additional 535 galaxies). We find evidence of mild but systematic nitrogen enhancement at high redshift: galaxies at z > 1 exhibit N/O ratios elevated by ∼0.18 dex (median offset) at fixed O/H compared to local trends, with a more pronounced enhancement at low metallicity (i.e. 12 + log(O/H)≲8.1), where the offset reaches up to ∼0.4−0.5 dex. We considered several scenarios to explain the observed trends, including bursty star formation, differential metal loading, and inflows of pristine gas. Our results provide the most extensive confirmation of elevated N/O ratios at high-redshift to date based on rest-optical diagnostics and within a self-consistent frame.

We present JWST/NIRSpec PRISM observations of a robust galaxy candidate at z ≃ 14, selected from pure-parallel NIRCam imaging: PAN-z14-1. The NIRSpec spectrum allows confirmation of this source at z spec = 13.5 3 − 0.06 + 0.05 through modeling of the Ly α break. PAN-z14-1 is the fourth most distant galaxy known to date and is extremely luminous ( M _UV = −20.6 ± 0.2), with a blue UV-continuum slope ( β = −2.26 ± 0.08) and a large physical size ( r _c = 233 ± 10 pc). We fail to detect any rest-frame UV emission lines at ≥2 σ significance, with upper limits sufficiently constraining to exclude the possibility of strong line emission. In terms of its physical properties, PAN-z14-1 is remarkably similar to the previously confirmed z _spec = 14.18 galaxy GS-z14-0. The lack of strong emission lines and large physical size is consistent with an emerging picture of two potentially distinct galaxy populations at z > 10, distinguished by star formation rate surface density. In this scenario, PAN-z14-1 is a second example of a “normal” extended luminous star-forming galaxy at z ≃ 14, and differs markedly from the other class of extremely compact galaxies with strong emission lines recently uncovered at extreme redshifts with JWST. These results highlight the importance of further spectroscopic confirmation of z > 10 galaxy candidates in order to fully understand the diversity of properties displayed by the first galaxies.

We present an analysis of the gas-phase mass-metallicity relationship (MZR) and fundamental metallicity relationship (FMR) for 65 star-forming galaxies at 2 < z < 8 from the JWST/EXCELS survey. We calculate gas-phase metallicities (12 + log(O/H)) using strong-line calibrations explicitly tested against the EXCELS sample, and report direct-method metallicities for 19 galaxies. Our sample spans |$8.1<\log (\rm M_\star /M_\odot ) and |$0<\log (\rm SFR/M_\odot \, yr^{-1}), consistent with main-sequence star-forming galaxies at the same redshifts. We find a clear MZR at both 2 < z < 4 (〈z〉 = 3.2) and 4 < z < 8 (〈z〉 = 5.5), with consistent slopes and mild evolution in normalization of ≃ 0.1 dex, matching trends from simulations and recent observations. Our results demonstrate rapid gas-phase enrichment in the early Universe; galaxies at z ≃ 3 (within the first ≃ 15 per cent of cosmic time) are enriched to ≃ 40 per cent of the metallicity of equivalent mass galaxies at z = 0. We find tentative evidence for SFR-dependence in the MZR scatter, though results remain inconclusive and highlight the need for larger high-redshift samples. Comparison with locally derived FMRs reveals a clear offset consistent with other z > 3 studies. We discuss potential drivers of this offset, noting that high-redshift samples have significantly different physical properties compared to local samples used to define the z = 0 FMR. Our results confirm that low-mass, high specific star-formation rate galaxies common at high redshift are inconsistent with the equilibrium conditions underlying the local FMR, and highlight the rapid chemical enrichment at early cosmic epochs.