Brian C. Lemaux

We present the discovery and first characterization of Nāpōwawā‘enakaulua , the most compact known quasar pair candidate in the Epoch of Reionization, with a projected separation of only 0 . ″ 98 (≈5.56 pkpc at z ∼ 6.08) and a velocity offset of Δ v ≃ 15km s ⁻¹ . Using spectroscopy from JWST/NIRSpec integral field unit, as part of the Aether survey , Gemini/GNIRS and GMOS, we characterize the continuum and emission line properties of the two quasars, enabling constraints on their black hole masses ( M _BH ≃ 1.5 × 10 ⁹ and 4.0 × 10 ⁸ M _⊙ ), accretion states ( λ _Edd ≃ 1.23 and 1.07), and systemic redshifts ( z _sys ≃ 6.0781 and 6.0785). We place this pair in the cosmological context by comparing the chance of occurrence to extrapolations of high- z quasar clustering measurements at larger scales, resulting in a ∼2–3 order of magnitude excess with respect to expectations. This excess hints at an underlying population of merger-triggered quasar pairs at cosmic dawn or to a denser-than-average galactic environment. Nāpōwawā‘enakaulua thus represents a crucial signpost to investigate the mass assembly of the first massive galaxies and black holes.

We present three outliers on the quasar main sequence at z > 6, as revealed by JWST/NIRSpec integral field unit (IFU) observations from the Aether survey, a program targeting ∼200 quasars at cosmic dawn. The quasars, J0216−5226, J0320−3521, and J1429+5447, exhibit exceptionally strong Fe ii emission and weak H β and [O iii ] lines, indicating rapid chemical enrichment within the first Gyr of cosmic history. Their Fe ii strengths ( R _{Fe II, λ 4570} ≡ EW _{Fe II} /EW _{H β} ) range between ∼2.5 and 3.6, placing them among the top ∼3% of Sloan Digital Sky Survey quasars at (0 ≲ z ≲ 1). The H α emission lines of J0216−5226 and J1429+5447 imply black hole masses of ∼1–7 × 10 ⁹ M _⊙ with λ _Edd ∼ 0.1–0.6, comparable to typical high- z quasars, although virial black hole masses may be systematically overestimated in strong Fe ii emitters. In contrast, J0320−3521 hosts a lower-mass black hole of ∼0.2–2 × 10 ⁹ M _⊙ , suggesting possible super-Eddington rate accretion with λ _Edd ∼ 0.5–6.0. The IFU data of J0320−3521 reveal a distinct emission-line region at a projected distance of ∼1.5 kpc, consistent with either an interacting companion or ionized gas associated to the quasar. We further identify a complex environment around J1429+5447, including two candidate companion galaxies or extended ionized gas structures. These quasars with bright emission-line regions, J0320−3521 and J1429+5447, are radio-loud sources, while J0216−5226 remains undetected in radio ( R _{5GHz, 3 σ} ≲ 4). Together, these results confirm previous findings that highly enriched, rapidly growing black holes were already in place at cosmic dawn. Despite the small sample size, our results further highlight the diversity in environment and radio-jet properties among quasars that otherwise share the same extreme region of the quasar fundamental plane.

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts z > 10 and as high as z = 14. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at z > 15 may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright z ∼ 14 galaxies should be visible at redshifts as high as z ∼ 20–30, showing in the data as F277W and F356W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with Y dwarf templates with temperatures 272–351 K and and 445–525 K (using ATMO2020 and Sonora Elf Owl models) and distances of ∼150–650 pc. The first is one of the lowest-temperature brown dwarfs known, and the lowest-temperature brown dwarf detected spectroscopically outside the solar neighborhood. With additional NIRCam imaging taken ∼1 yr later, we also detect their proper motions of (49 ± 8) mas yr ⁻¹ and (24 ± 3) mas yr ⁻¹ , further indicating that at least some F277W and F356W dropouts are substellar cold Milky Way objects such as brown dwarfs.

Context . Galaxy protoclusters (PCs) at z ≳ 2 are dense regions extending up to a few megaparsec in which the availability of cold gas and the elevated rates of galaxy interactions trigger intense often dust-obscured star formation in the member galaxies. These same mechanisms are also expected to promote supermassive black hole (SMBH) growth, but this possible effect remains unclear, largely due to the heterogeneous galaxy selections and active galactic nucleus (AGN) identification methods employed in previous studies. Aims . We quantitatively assessed the effect of the PC environment on SMBH growth by measuring the incidence of X-ray AGN among dusty star-forming galaxies (DSFGs) in PCs and in a homogeneously selected control sample of field galaxies. We also investigated the physical mechanisms that drive any difference between the two environments. Methods . We considered the DSFG population of ALMA-detected galaxies in the submillimeter/millimeter continuum of seven PCs at 2 < z < 4.5 and constructed a selection-matched control field sample from the COSMOS survey. We statistically compared the incidence of X-ray selected AGN in the PC and control samples and the physical properties of the host galaxies as obtained through uniform spectral energy distribution fitting. Results . We found a significantly higher X-ray AGN fraction in PCs by ≈2.7×, which is different from the field expectation with a Poisson significance p = 3×10 ⁻⁴ . Similar values were obtained by splitting the sample into two redshift bins, ≈2.7× at z = 2–3 and ≈2.6× at z = 3–4.5, with significances of p = 0.003 and 0.03, respectively. The field and PC DSFG samples are well matched in stellar mass, star-formation rate, and dust mass, indicating that the observed enhancement is not driven by selection effects or systematically higher host masses. In particular, AGN in PCs are hosted by galaxies with similar stellar masses and star formation rates as field AGN. Conclusions . Our results provide quantitative and unbiased evidence that the dense PC environment enhances the AGN incidence and in turn SMBH growth among DSFGs beyond what is expected from host galaxy properties alone, likely through an increased gas availability and interaction-driven fueling. At higher redshift, current statistics limit firm conclusions, but the observed trends suggest that environmental triggering of AGN can operate already at z ≳ 4. This work represents a first step toward a homogeneous assessment of the overall environmental effects on SMBH growth across cosmic time.

While compact, star-forming galaxies are believed to play a key role in cosmic reionization, the physical mechanisms enabling the escape of ionizing photons through the galactic interstellar medium remain unclear. Supernova (SN) feedback is one possible mechanism for clearing neutral gas channels to allow the escape of Lyman continuum photons. Here we use SN discoveries in low-redshift analogs of high-redshift star-forming galaxies—Green Pea galaxies and their even lower-redshift counterparts, Blueberry (BB) galaxies—to understand how SNe shape the properties of their host galaxies at high redshifts. We cross-match 1242 BB galaxies with transient discovery reports and identify 11 SNe, 10 of which are likely core-collapse SNe, and compare their hosts to the larger BB population. We find that SN-hosting BBs exhibit elevated star formation rates, burstier star formation histories within the past ∼50 Myr, and higher stellar masses. We estimate the occurrence rates of SNe in BB galaxies, finding that the SN rate may be slightly suppressed in BBs compared to a controlled galaxy sample of similar mass, but we are unable to fully control for observational uncertainties. Finally, SN hosts show bluer UV slopes than nonhost BB galaxies at 2.1 σ significance and lower ionizing photon production efficiency at 7.9 σ significance; the former result may support the hypothesis that SN-driven feedback plays a role in facilitating the escape of ionizing photons, while the latter may imply that SN-driven quenching decreases the rate of ionizing photon production in compact star-forming galaxies during the epoch of reionization.

Context. Not all galaxies at cosmic noon (2 ≲ z ≲ 3) evolve in the same way. Particularly, it remains unclear how and to what extent the local environment – especially the extreme overdensities of protoclusters – affects the stellar mass assembly of its constituent galaxies at high redshifts. The imprint of these early processes is encoded in the galaxy stellar-mass function (SMF); comparing SMFs across environments therefore reveals differences in evolutionary history. Aims. We present the SMF of the Hyperion proto-supercluster at z ∼ 2.5, one of the largest and most massive protostructures in the early universe. This dataset yields the most statistically robust SMF of a single protostructure at z ≳ 2. By comparing the SMF of the overdense peaks within Hyperion to the coeval field, we begin to answer the question of how early, and how strongly, a dense environment tilts the balance in favor of massive galaxies Methods. Given that Hyperion resides in the field of the Cosmic Evolution Survey (COSMOS), we combined the extensive COSMOS2020 photometric catalog with ground-based spectroscopy and new grism spectroscopy from the Hubble Space Telescope ( HST ). The structure of Hyperion is defined based on a three-dimensional overdensity map, allowing us to place galaxies into (i) the highly overdense peaks of Hyperion, (ii) the less-overdense outskirts of Hyperion, or (iii) a coeval field. We performed 100 Monte Carlo realizations of the data to propagate redshift and stellar mass uncertainties, refitting galaxy properties in each realization. After constructing SMFs for the outskirts and peaks of Hyperion, we normalized them to that of the field to highlight differences in the underlying shape of the SMFs. Results. The overdense peaks of Hyperion host a striking excess of massive galaxies relative to the field: the number densities of log ₁₀ ( M _* / M _⊙ )∼11 galaxies are ∼10× higher than the coeval field, whereas log ₁₀ ( M _* / M _⊙ )∼9.5 galaxies are enhanced by only ∼3.5×. On the other hand, both the SMF of the outskirts of Hyperion and the SMF of Hyperion as a whole mirror the overall shape of the coeval field. Conclusions. Environmental effects that govern stellar mass growth are already well established by z ∼ 2.5. The densest regions of Hyperion host galaxies that have already experienced accelerated stellar mass growth. Furthermore, this impact is largely masked in the total SMF of Hyperion, highlighting the necessity of deep spectroscopic surveys when uncovering environmental trends at high redshifts. These findings imply that high-redshift protostructures begin sculpting the high-mass end of the SMF well before the epoch when local clusters experience widespread quenching, and may provide the appropriate laboratories for producing the elevated star formation observed at cosmic noon.

Context. As galaxies evolve in dense cluster and protocluster environments, they interact and quench their star formation. This gradually transforms the dominant galaxy population from star-forming galaxies to quiescent red galaxies. This transformation is identifiable by observing the galaxy colors and can also be seen in the morphological transformation of late-type galaxies into early-type galaxies, which creates the morphology-density relation observed when populations in clusters are compared to field galaxies at a given epoch. However, high- z ( z > 2) galaxy morphology studies are hindered by the high angular resolution necessary to characterize the morphology. Aims. We studied HST WFC3 F160W observations of protoclusters from the MAMMOTH survey (BOSS1244 and BOSS1542) at z ∼ 2.23 with populations of previously identified H α emitters. Methods. By measuring the Sérsic index of 151 H α emitters, we searched for the early morphological transformation of high- z star-forming galaxies in these well-studied large nonvirialized protoclusters, which we think are the precursors of present-day galaxy clusters. Results. The morphology of the populations of star-forming galaxies in protoclusters does not differ from that of their field counterparts. However, we also identified a population of clumpy potentially merging galaxies, which might increase the population of early-type galaxies within these structures. In BOSS1244, which has two previously identified massive quiescent galaxies including a brightest cluster galaxy (BCG), we additionally found an abundance of early-type galaxies near the BCG and two coeval high- z quasars. Conclusions. Although the morphology of field and protocluster galaxies are very similar, the population of early-type star-forming galaxies surrounding the spectroscopically confirmed quiescent BCG in BOSS1244, which is not observed in BOSS1542, might indicate differences in the evolutionary state of these two coeval protoclusters. It might be a sign of an early forming cluster core in BOSS1244.