Chiara Mazzucchelli

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 investigate the multiphase circumgalactic medium during the epoch of reionization ( z > 6) by cross-correlating cool- C II and warm-ionized C IV absorption systems with star-forming [O III ] emitters. JWST/NIRCam wide-field slitless spectroscopy from the EIGER survey was combined with medium- and high-resolution optical and NIR spectra of six background quasars, including new VLT/X-Shooter observations of PSO J159-02. We analyzed the relation between 16 C II and 14 C IV absorbers (log( N ) > 13.0) and 136 galaxies within an impact parameter of R _⊥ ≤ 1000 pkpc and a line-of-sight separation of Δ v ≤ 500 km s ⁻¹ . We detected a statistically significant excess of both ions around galaxies compared to a randomized background. The C IV covering fraction remains enhanced up to ∼1 pMpc, whereas C II drops to the background level beyond ∼0.5 pMpc, demonstrating that the warm-ionized phase is more spatially extended than cooler gas. Jointly, the 3D galaxy-absorber spatial clustering is significantly weaker than the galaxy-galaxy autocorrelation. This provides direct physical evidence that early carbon enrichment is not confined to the virial radius of massive star-forming systems; rather, a substantial fraction of these metals permeates the diffuse intergalactic medium or is injected by a widespread population of faint undetected dwarf galaxies. Finally, we note a rapid radial decline of the C IV covering fraction compared with lower-redshift samples at z < 2 and z ∼ 3 − 4, which indicates an evolving circumgalactic medium ionization structure where early metals predominantly reside in lower ionization states. In conclusion, we determine a conservative lower limit for the observed carbon mass of M _{C  II +C  IV} ≥ 2.8 × 10 ⁶ M _⊙ within 300 pkpc.

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.

Context . Active galactic nuclei (AGNs) play a crucial role in galaxy evolution, but they are a minority of extragalactic sources with diverse spectral energy distributions (SEDs), which depend on their means of selection. Upcoming large-scale surveys such as LSST will identify many AGNs, but analysis tools are not optimized for them. The limited number of photometric bands in these surveys impacts the calculation of photometric redshifts for AGNs, which are essential for scientific advancement. Aims . We use LePHARE to demonstrate the impact that a limited number of bands and erroneous assumptions have on the determination of the photometric redshifts of AGNs. We conducted tests on six AGN samples selected using X-ray, radio, infrared, variability, color, and spectroscopic criteria in the COSMOS field, using photometry from HSC-CLAUDS, which is closest in depth and wavelength coverage to LSST. Methods . We present the LSST pipeline for LePHARE within the redshift assessment infrastructure layers (RAIL), facilitating comparison between SED fitting and machine-learning algorithms. Results . AGNs that appear as point-like sources in optical data are assigned highly unreliable photometric redshifts if they are processed using galaxy templates. Additionally, shallow all-sky surveys (such as eROSITA, WISE, and ZTF) miss many AGNs. As a result, these “hidden” AGNs are often misidentified as galaxies in public survey data, leading to incorrect photometric redshifts. Conclusions . We provide the configurations that are suggested for each type of AGN alongside measures of expected performance as a function of redshift, magnitude, and selection. To facilitate studies with a panchromatic view of AGNs, we also released photometric redshifts and posterior distributions for all AGN sources identified in the COSMOS field using the six criteria, based on 28-band photometry.

Doppler boosting allows blazars to be detected out to high z , making them promising probes of the intergalactic medium through the 21 cm forest. We report 0.144−11 GHz observations of the most distant known blazar, VLASS J041009.05−013919.88 at z ∼ 7, obtained with the upgraded Giant Metrewave Radio Telescope (uGMRT), the LOw Frequency ARray (LOFAR), and the Very Large Array (VLA). The first uGMRT epoch (300−820 MHz, 2023 April) revealed an inverted radio spectrum that, combined with earlier (2021–2022) VLA data (1.5–11 GHz), unveiled a double-peaked spectrum potentially indicative of multiepoch jet activity. A second uGMRT campaign (2023 August), simultaneous with new VLA observations (1.5–11 GHz), instead revealed a flat low-frequency and a peaked high-frequency spectrum, ruling out this interpretation. While limited by the two uGMRT epochs, variability analysis favors intrinsic jet processes, indicating a highly relativistic, closely aligned jet ( θ < 3°, δ > 19.3, Γ > 9.7). The equipartition magnetic field (>1 mG) exceeds the equivalent cosmic microwave background field at z ∼ 7 (0.2 mG), indicating that synchrotron losses dominate. The inferred Doppler boosting ( δ > 19.3) implies that J0410−0139 is intrinsically radio-weak. As a blazar, it traces a much larger parent population of radio quasars at z ∼ 7, detectable only with deep (≤ μ Jy) next-generation radio observations. LOFAR 144 MHz observations (2024 April and July) yielded ∼2 mJy, well below the ∼8 mJy predicted from uGMRT epochs, confirming strong variability at rest-frame ∼1 GHz. Low-frequency monitoring will be crucial for identifying high radio intensity states suitable for future 21 cm forest studies.

We present a detailed multiwavelength analysis of two gravitationally lensed z > 6 quasar candidates: J0109–5424 and P170+20. Using JWST/NIRSpec near-infrared (NIR) data from the Aether survey, we demonstrate that J0109–5424 is in fact a FeLoBAL quasar at z = 2.07, exhibiting strong iron broad absorption lines that mimic the Lyman break. From the Pa β emission line, we estimate a black hole mass of 3.9 × 10 ⁸ M _⊙ and a bolometric luminosity of L _bol = 7.7 × 10 ⁴⁵ erg s ⁻¹ for this quasar. Follow-up HST/ACS Wide Field Channel imaging of P170+20 with the F555W filter, probing emission below the Lyman limit of the quasar, reveals a compact source located 0 . ″ 09 ± 0 . ″ 04 from the quasar position. Point spread function modeling and subtraction yield no evidence for extended emission. This is inconsistent with a foreground galaxy acting as a lens. Furthermore, we obtain Gemini South/Flamingos-2 and LBT/LUCI NIR observations to complement the existing optical Gemini North/GMOS spectrum of P170+20. Our spectral analysis shows that the observed spectral shape of P170+20 is consistent with a composite quasar and M dwarf spectrum. However, this interpretation remains uncertain, as a chance alignment between an ultracool dwarf and a quasar within 0 . ″ 1 would be unlikely. Overall, our findings establish J0109–5424 as an intermediate redshift source and do not favor the lensed nature of P170+20, although this possibility cannot be conclusively ruled out, highlighting the challenges of selecting lensed quasars at high redshift.

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.

We present James Webb Space Telescope (JWST) NIRSpec integral field unit (IFU) observations of six galaxies at z ∼ 6, obtained as part of the Aether project (General Observers program 5645). The targets were originally identified by the Subaru High- z Exploration of Low-Luminosity Quasars (SHELLQs) survey as candidate obscured quasars with luminous (≳10 ⁴³ erg s ⁻¹ ) but narrow (≲500 km s ⁻¹ ) Ly α emission. Two objects exhibit a broad component in their Balmer lines (FWHM > 3000 km s ⁻¹ ), indicating the presence of active galactic nuclei (AGNs), while the remaining four show similar profiles in permitted and forbidden lines. Combining these data with similar SHELLQs objects reported previously, we find that the presence of broad lines is strongly correlated with Ly α luminosity ( L _{Ly α} ); the inferred AGN fraction is >77% and L _{Ly α} = 10 ⁴⁴ erg s ⁻¹ , respectively. Dust-extinction corrections inferred from the Balmer decrement would imply unrealistically high Ly α luminosities, suggesting that the line-emitting gas consists of multiple zones. The IFU data reveal diverse spatial structures. The AGN hosts are compact, whereas the other galaxies show extended ionized gas on scales up to 10 kpc and star formation rates of 60–600 M _⊙ yr ⁻¹ . One of the extended objects exhibits a signature of rotation, while most of the others show little ordered kinematics, with velocity widths (FWHM) up to 200–300 km s ⁻¹ . These objects populate the intermediate luminosity regime between classical luminous quasars and the low-luminosity AGNs discovered by JWST, including little red dots, potentially linking the two populations.

We present a systematic study of the environments of 25 luminous quasars at z > 6.5 from the ASPIRE program. Using JWST/NIRCam wide-field slitless spectroscopy data, we identified 487 galaxies at 5.3 ≲ z ≲ 7.0 exhibiting [O iii ] emission. Among these, 122 [O iii ] emitters lie within ∣Δ v _los ∣ < 1000 km s ⁻¹ of the quasars, corresponding to a ∼9.4-fold enhancement relative to the average galaxy density at other redshifts. Furthermore, we identified 16 [C ii ]-emitting galaxies at the quasar redshifts from Atacama Large Millimeter/submillimeter Array (ALMA) mosaic observations. A cross-correlation function analysis between quasars and [O iii ]+[C ii ] emitters yields a cross-correlation length of r 0 QG = 8.6 8 − 0.55 + 0.51 h − 1 cMpc and an autocorrelation of r 0 QQ = 15.7 6 − 2.70 + 2.48 h − 1 cMpc , indicating that z ∼ 7 quasars reside in dark matter halos with M halo = 1 0 12.2 7 − 0.26 + 0.21 M ⊙ and have a quasar lifetime of t Q = 1 0 7.05 − 1.01 + 0.95 yr . Notably, the number of [O iii ]-emitting galaxies at quasar redshifts varies significantly from field to field, ranging from 0 to 20, highlighting a diverse quasar environment. Remarkably, seven quasars trace significant galaxy overdensities (i.e., protoclusters), with δ _gal > 5 within a volume of V ∼ 500 cMpc ³ . We also find that ∣Δ v _los ∣ increases rapidly toward smaller galaxy–quasar separations in protocluster fields, consistent with galaxy kinematics around extremely massive halos in cosmological simulations. By combining JWST and ALMA data, we reveal the complex and diverse environments of these early quasars, providing robust evidence that the earliest luminous quasars are effective tracers of galaxy overdensities, albeit with substantial field-to-field variation.

Context. The identification of high-redshift quasars ( z > 4.5) is critical for studying the early Universe, supermassive-black-hole growth, and cosmic reionization. Most known high-redshift quasars are located in the northern hemisphere, leaving the southern sky largely unexplored. Aims. As part of the 4-meter Multi-Object Spectroscopic Telescope (4MOST) and Chilean AGN/Galaxy Extragalactic Survey (ChANGES) S1604 survey, we aim to create a large catalog of high-redshift quasar candidates in the southern hemisphere using multiwavelength photometry and spectral-energy-distribution (SED) fitting, with the goal of spectroscopic follow-up with 4MOST. Methods. We constructed a multiband photometric catalog by combining optical data from the Dark Energy Camera Local Volume Exploration Survey (DELVE DR2) and Dark Energy Camera Legacy Survey (DECaLS DR10), near-infrared data from the VISTA Hemisphere Survey (VHS DR5) with an additional field of the VISTA Kilo-degree Infrared Galaxy Public Survey (VIKING), mid-infrared data from the Wide-field Infrared Survey Explorer (AllWISE), and optical astrometry from Gaia DR3. After applying morphological and color-based cuts to remove contaminants such as brown dwarfs and red galaxies, we performed a custom-made SED fitting using quasar and brown-dwarf templates. Statistical outputs including χ ² , the Bayesian information criterion (BIC), and F _test were used to rank and select candidates. Results. Our final catalog contains 6104 high-redshift quasar candidates within the redshift range of 4.5 < z < 7. These sources have detections in seven or more photometric bands and satisfy our SED-based statistical selection criteria (e.g., BIC > 0 and F _test > 10). Initial spectroscopic validation using the New Technology Telescope (NTT) with the ESO Faint Object Spectrograph and Camera v.2 (EFOSC2) and the Palomar Observatory Hale Telescope with the Next Generation Palomar Spectrograph (NGPS) confirmed three high-redshift quasars at z > 5 out of six observed candidates.