Simona Gallerani

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.

The large population of broad-line active galactic nuclei (AGNs) observed with the James Webb Space Telescope (JWST) at z ≳ 4 provides a new view of the black hole–galaxy connection in the first gigayear of cosmic history. We used the JADES survey-level dataset and developed a forward-modelling Bayesian framework that explicitly accounts for broad H α detectability and incorporates selection effects into the likelihood function. With this approach, we constrain the black hole–stellar mass ( M _BH – M _★ ) relation to be log M _BH = −4.06 ^+0.50 _−0.51 + 1.17 ^0.06 _−0.06 log M _★ , with an intrinsic orthogonal scatter of σ _int = 0.63 ^+0.14 _−0.11 dex. The slope and normalization are consistent with local determinations, indicating that the average scaling was already established by z ∼ 4–6. This suggests that the primary evolution of the relation occurs in its dispersion rather than in its mean normalization. In contrast, the substantially larger intrinsic scatter relative to the nearby Universe reveals a wider diversity of black hole–galaxy growth histories, likely driven by bursty accretion, delayed feedback, and differences in merger or seeding histories. Future JWST samples will be crucial to test whether this increased scatter is a persistent feature of the high-redshift Universe.

We report JWST/NIRSpec integral field unit (IFU) observations of the quasar J1512+4422 at z ∼ 6.2 from the Aether survey. At ∼900 Myr after the Big Bang, this object already lies on the M _BH – σ _* relation found in the local Universe, with an M _BH ≃ 8.9 × 10 ⁸ M _⊙ and a stellar velocity dispersion σ _* ≃ 288 km s ⁻¹ . We detect an outflow with a velocity of ∼478 km s ⁻¹ in the nuclear region, which likely extends to ∼3.2 kpc in projection and has a median velocity of ∼352 km s ⁻¹ . The outflow dynamical timescale (∼9 Myr) is consistent with the timescale of the current quenching process based on the star formation history as reported previously. The total mass outflow rate (92.6 − 74.1 + 92.6 M _⊙ yr ⁻¹ ) is larger than the current star formation rate (0.9 − 0.8 + 3.8 or 4.3 − 3.7 + 5.8 M _⊙ yr ⁻¹ ), and the total kinetic energy outflow rate (0.6% − 0.5 % + 0.6 % of quasar luminosity) meets the threshold for negative quasar feedback as suggested by simulations. These results suggest that the outflow is capable of suppressing or quenching the star formation activity within the host galaxy. Furthermore, J1512+4422 exhibits σ _* , stellar mass, and size similar to those of z ≳ 3 quiescent and poststarburst galaxies, implying a link between the two. Overall, for objects like J1512+4422, the evolution of their supermassive black holes and host galaxies appears to be tightly coupled within the first billion years. The quasar feedback likely plays a critical role in both placing them on the M _BH – σ _* relation and quenching.

Context. Lyman α (Ly α ) emission is highly sensitive to dust and neutral hydrogen and is believed to be suppressed in dusty or hydrogen-rich galaxies – especially during the epoch of reionization (EoR). Yet, moderately dusty Ly α emitters (LAEs) are observed at this epoch, suggesting that complex interstellar medium (ISM) geometries and feedback-driven outflows facilitate Ly α escape. Aims. We investigated the dust, gas, and stellar properties of the gravitationally lensed LAE HCM 6A at z = 6.5676 to characterize its multiphase ISM structure and the physical conditions that regulate Ly α escape. Methods. We combined JWST/NIRISS slitless spectroscopy, HST+JWST/NIRCam imaging, and JWST/NIRSpec slit spectra from the CANUCS program. Using a customized BAGPIPES spectral-energy-distribution-fitting framework with a flexible attenuation law, we derived spatially resolved stellar, nebular, and dust properties on integrated (≈1 kpc), slit-level (≈0.1 kpc), and pixel-level (≈25 pc) measurements, thanks to strong lensing with a magnification of μ ≈ 8.3 − 9.1. A high-quality Ly α map from SLEUTH , a tool for extracting spatially resolved emission-line maps from slitless spectroscopy, traces the spatial distribution of Ly α emission. Results. We measure an unlensed stellar mass of log M _* = 8.3 − 8.4 and an intrinsic UV magnitude of M _UV = −19.8 ± 0.1. Slit-level measurements show that the oldest, most massive region (S1) is moderately dusty with consistent stellar ( A _V , β ) and nebular ( A _V ^B , line-emission) indicators, implying a uniform ISM geometry, while the dust tracers of the youngest region (S3) are highly mismatched, revealing a complex, feedback-shaped multiphase ISM. Ly α emission arises primarily from S3. Pixel-level maps reveal a dust-cleared central clump (C3) where Ly α emerges, encircled by dustier outskirts, consistent with a very recent (≲10 Myr) starburst that created Ly α escape channels. Next, slit-level maps show Calzetti-like attenuation curves with a UV bump that increases with stellar age and decreases with V -band attenuation ( A _V ), with a tentative detection of a UV bump in S1 at ∼25% of the Milky Way amplitude. Pixel-level maps reveal that both the curve slope ( S ) and the UV bump ( B ) peak in the region between two clumps (C1 and C2), indicating dust-grain processing in a merger-driven starburst. Conclusions. Our observations provide a uniquely detailed, spatially resolved view of a moderately dusty LAE at the EoR, demonstrating how the interplay between multiphase ISM geometry and feedback governs Ly α escape. Constraining some key quantities requires fully resolved spectroscopy, which future JWST/NIRSpec integral field unit observations will provide.

Context. A large variety of dust attenuation and/or extinction curves has been observed in high-redshift galaxies. Some studies investigated their correlations with fundamental galaxy properties, which yielded mixed results. These variations are likely driven by underlying factors such as the intrinsic dust properties, the total dust content, and the spatial distribution of dust relative to stars. Aims. We investigate the correlations between the shape of dust attenuation curves, defined by the UV-optical slope (S) and the UV bump parameter (B), and fundamental galaxy properties. Our goal is to identify the key physical mechanisms that shape the dust attenuation curves through cosmic time in the broader context of galaxy formation and evolution. Methods. We extended the analysis of 173 dusty high-redshift (z ∼ 2 − 11.5) galaxies, whose dust attenuation curves were inferred by fitting James Webb Space Telescope (JWST) data with a modified version of the spectral energy distribution (SED) fitting code BAGPIPES. We investigate the trends between the dust attenuation parameters and different galaxy properties as inferred from the SED fitting: V-band attenuation (A_V), star formation rate (SFR), stellar mass (M_*), specific SFR (sSFR = SFR/M_∗), mass-weighted stellar age (⟨a⟩_*^m), ionization parameter (log U), and metallicity (Z). For a subset of sources, we additionally explored the trends with oxygen abundance (12 + log(O/H)), which we derived using the direct T_e-based method. Results. We report moderate correlations between S and A_V, and B and A_V. Galaxies characterized by lower (higher) A_V exhibit steeper (flatter) slopes and stronger (weaker) UV bumps. These results agree with radiative transfer (RT) predictions that account for the total dust content and the relative spatial distribution of dust with respect to stars. Additionally, we find that S flattens with decreasing ⟨a⟩_*^m and increasing sSFR. These two trends can be explained if the strong radiation fields associated with young stars (low ⟨a⟩_*^m) and/or bursty galaxies (high sSFR) preferentially destroy small dust grains, which would shift the size distribution toward larger grains. Finally, the positive correlation between B and 12 + log(O/H) that emerged from our analysis might be driven by variations in the intrinsic dust properties with the gas metallicity. Conclusions. The shape of the dust attenuation curves primarily correlates with four key galaxy properties: (1) With the redshift, which traces variations in the intrinsic and/or reprocessed dust properties, (2) with A_V, which reflects RT effects, (3) with the mass-weighted stellar age or sSFR, which might be driven by the radiation field strength, and (4) with the oxygen abundance, which might be linked to intrinsic dust properties. The overlap between some of these mechanisms makes it difficult to isolate their contributions, however. Further progress requires a combination of observations of a larger galaxy sample, point-like sources, spatially resolved galaxy studies, and theoretical models incorporating dust evolution in cosmological simulations.

Context. Galaxy evolution emerges from the balance between cosmic gas accretion, fueling star formation, and supernova feedback, regulating metal enrichment of the interstellar medium. Hence, the relation between stellar mass (M_⋆) and gas metallicity (Z_g) is fundamental to understanding the physics of galaxies. High-quality spectroscopic JWST data enable accurate measurements of both M_⋆ and Z_g up to redshift z ≃ 10. Aims. Our aims are to understand (i) the nature of the observed mass-metallicity relation (MZR), (ii) its connection with the star formation rate (SFR), (iii) the role played by SFR stochasticity (flickering), and (iv) how it is regulated by stellar feedback. Methods. We compared the MZR obtained by the JADES, CEERS, and UNCOVER surveys, which comprise about 180 galaxies at z ≃ 3 − 10 with 10⁶ M_⊙ ≲ M_⋆ ≲ 10¹⁰ M_⊙, with ≃200 simulated galaxies in the same mass range from the SERRA high-resolution (≃20 pc) suite of cosmological radiation-hydrodynamic simulations. To interpret the MZR, we developed a minimal, physically motivated model of galaxy evolution that includes: cosmic accretion, possibly modulated with an amplitude A₁₀₀ on 100 Myr timescales; a time delay, t_d, between SFR and supernova feedback; and SN-driven outflows with a varying mass loading factor, ϵ_SN, which is normalized to the FIRE simulations predictions for ϵ_SN = 1. Results. Using our minimal model, we find the observed “mean” MZR is reproduced for relatively inefficient outflows (ϵ_SN = 1/4), in line with findings from JADES. Matching the observed MZR “dispersion” across the full stellar mass range requires a delay time, t_d = 20 Myr, in addition to a significant modulation (A₁₀₀ = 1/3) of the accretion rate. Successful models are characterized by relatively low flickering (σ_SFR ≃ 0.2), corresponding to a metallicity dispersion of σ_Z ≃ 0.2. Such values are close but slightly lower than predicted from SERRA (σ_SFR ≃ 0.24, σ_Z ≃ 0.3), clarifying why SERRA shows a flatter trend with respect to the observations and some tension, especially at M_⋆ ≃ 10¹⁰ M_⊙. Conclusions. The MZR appears to be very sensitive to SFR stochasticity. The minimal model predicts that high root mean square values (σ_SFR ≃ 0.5) result in a “chemical chaos” (i.e. σ_Z ≃ 1.4), virtually destroying the observed MZR. As a consequence, invoking a highly stochastic SFR (σ_SFR ≃ 0.8) to explain the overabundance of bright, super-early galaxies would lead to inconsistencies with the observed MZR.

We investigated the intrinsic and observational properties of z ≳ 6 galaxies that host the coalescence of massive black holes (MBHs; M_BH ∼ 10⁵⁻⁶ M_⊙) giving rise to gravitational waves (GWs) detectable with the Laser Interferometer Space Antenna (LISA). We adopted a zoom-in cosmological hydrodynamical simulation of galaxy formation and black hole (BH) co-evolution, based on the GADGET-3 code, zoomed-in on an M_h ∼ 10¹² M_⊙ dark matter halo at z = 6, which hosts a fast accreting (Ṁ ∼ 35 M_⊙ yr⁻¹) super-massive black hole (SMBH; M_BH ∼ 10⁹ M_⊙) and a star-forming galaxy (SFR ∼ 100 M_⊙ yr⁻¹). Tracing the SMBH’s formation backwards in time, we identified the merger events associated with its formation and selected those that are detectable with LISA. Among these LISA-detectable events (LDEs), we selected those–based on their intrinsic properties (Ṁ, SFR, gas metallicity, and dust mass)–that were expected to be bright in one or more electromagnetic (EM) bands, such as rest-frame X-ray, UV and far-infrared (FIR). After considering the effect of delay due to dynamical friction in the MBH coalescence, we further restricted our selection to those LDEs that are still occurring at z ≳ 6. We find that ∼20–30% of the LDEs and their host galaxies are also detectable with EM telescopes. We post-processed these events with dust radiative transfer calculations to make accurate predictions about their spectral energy distributions (SEDs) and continuum maps in the JWST to ALMA wavelength range. We compared the spectra arising from galaxies hosting the merging MBHs with those arising from the active galactic nuclei (AGN) powered by single accreting BHs. We find that identifying an LDE from the continuum SEDs is impossible because of the absence of specific imprints from the merging MBHs. Finally, we computed the profile of the H_α line arising from LDEs, taking into account both the contribution from their star-forming regions and the accreting MBHs. We find that the presence of two accreting MBHs would be difficult to infer even if both MBHs accrete at super-Eddington rates (λ_EDD ∼ 5 − 10). We conclude that the combined detection of GW and EM signals from z ≳ 6 MBHs is challenging (if not impossible) not only because of the poor sky-localization (∼10 deg²) provided by LISA, but also because the loudest GW emitters (M_BH ∼ 10⁵⁻⁶ M_⊙) are not massive enough to leave significant signatures (e.g. extended wings) in the emission lines arising from the broad line region.

The presence of supermassive black holes (SMBHs, M _• ∼ 10^6-10 M _⊙) in the first cosmic Gyr (z ≳ 6) challenges current models of BH formation and evolution. We propose a novel mechanism for the formation of early SMBH seeds based on primordial black holes (PBHs). We assume a non-Gaussian primordial power spectrum as expected in inflationary models; these scenarios predict that PBHs are initially clustered and preferentially formed in the high-σ fluctuations of the large-scale density field, out of which dark matter (DM) halos are originated. Our model accounts for (i) PBH accretion and feedback, (ii) DM halo growth, and (iii) gas dynamical friction. PBHs lose angular momentum due to gas dynamical friction, sink into a dense core, where BH binaries form and undergo a runaway merger, eventually leading to the formation of a single, massive seed. This mechanism starts at z ∼ 20-40 in rare halos (M_h ∼ 10⁷ M _⊙ corresponding to ∼ 5-7σ fluctuations), and provides massive (∼ 10^4-5 M _⊙) seeds by z ∼ 10-30. We derive a physically-motivated seeding prescription that provides the mass of the seed, M _seed(z) = 3.1 × 10⁵ M _⊙[(1 + z)/10]^-1.2, and seeded halo, M_h (z) = 2 × 10⁹ M _⊙[(1 + z)/10]^-2 e ^-0.05z as a function of redshift. This seeding mechanism requires that only a small fraction of DM is constituted by PBHs, namely f _PBH ∼ 3 × 10^-6. We find that z ∼ 6 - 7 quasars can be explained with 6 × 10⁴ M _⊙ seeds planted at z ∼ 32, and growing at sub-Eddington rates, 〈λ_E〉 ∼ 0.55. The same scenario reproduces the BH mass of GNz11 at z = 10.6, while UHZ1 (z = 10.1) and GHZ9 (z = 10) data favour instead slightly later (z ∼ 20-25), more massive (10⁵ M _⊙), and efficiently accreting (〈λ_E〉 ≃ 0.9) seeds. During the runaway phase of the proposed seed formation process, PBH-PBH mergers are expected to copiously emit gravitational waves. These predictions can be tested through future Einstein Telescope observations and used to constrain inflationary models.