Raphael E. Hviding

We report the discovery of ‘the Step’, a dense protocluster at z = 3 revealed by JWST/NIRISS Wide-Field Slitless Spectroscopy (WFSS) of the field around the luminous quasar and starburst SDSSJ165202.64+172852.3. The protocluster contains at least 18 member galaxies and exhibits signs of two substructures in the 3D position-velocity space – notably a distinctive step-like distribution, from which the nickname is derived. We propose that the Step may be undergoing a merger of two halos, which could also explain the unusually large velocity span (700–850 km/s) between galaxies along the densest line of sight. This sightline contains four (seven) galaxies within projected distances of 20 (70) kpc from the quasar. Within a 320×230 kpc ² region, we find ten galaxies (including the quasar), consistent with a remarkably dense protocluster core. We derive a galaxy overdensity of a factor of >100 in this core and > 15 in the full field (1 Mpc ² ), making it one of the densest structures known at this redshift, and a very massive halo of 13.1 < log M _halo / M _⊙ < 13.8, which suggests the structure could potentially evolve into a massive, Coma-like cluster at z = 0. Compared to the field, the star-forming galaxies in the Step appear to show slightly elevated levels of star formation activity. Yet, we find two quiescent galaxies (log M _⋆ / M _⊙ ∼ 10.2, 10.4 ± 0.8) with strong continuum breaks and no evidence of ongoing star formation. Compared to other protoclusters at z = 3, the Step structure exhibits broadly similar or slightly suppressed star-forming activity for its halo mass. We also identify two new active galactic nucleus (AGN) candidates based on optical line ratios and find tentative evidence of an enhanced AGN fraction (10–20%) with respect to the field. The protocluster constitutes one of an emerging number of maturing structures at 2 ⩽ z ⩽ 4 that host both star-forming and quenched galaxies. This redshift regime may thus be a key transitional epoch for cluster studies, where overdensities transition from being the most active sites of star formation in the early Universe towards shaping the massive passive ellipticals seen in the cores of today’s clusters.

Compact and red sources in the high-redshift ( z ∼ 5) Universe, known as “Little Red Dots” (LRDs), are among JWST’s most intriguing discoveries. These sources have broad Balmer emission lines, weak X-ray emission, and unique spectral energy distributions (SEDs) poorly fit by either stellar or active galactic nucleus (AGN) templates. Local LRD-like objects allow for detailed studies of the underlying physical processes with archival multiwavelength datasets unavailable in the high- z Universe. We show that the Sloan Digital Sky Survey (SDSS) ugriz filters at z ≈ 0.4, 0.8 overlap well with the JWST filters used to select LRDs at z ∼ 5. We use SDSS quasars to define a sample of ∼1300 Local Red Dots (LoRDs) that share the same photometric colors of LRDs. A subset of the LoRD sample selected to have V-shaped continua ( N = 244) show prominent higher-order Balmer absorption features and [Ne v ] λ 3426 emission, both of which would likely be missed in JWST/PRISM observations given the low spectral resolution. A composite SED of the LoRDs differs from a typical quasar SED in the rest-frame UV/optical, but the two agree with each other in the near-infrared. The LoRD SED matches well with a stack of LRDs and can be modeled either as a reddened AGN combined with a host galaxy, or as a reddened AGN combined with a host galaxy and a cool blackbody. Interestingly, the LoRDs are X-ray detected at a rate comparable to typical quasars. However, the probability that LoRDs and typical quasars would go undetected, if subject to the LRD X-ray upper limits, is >50%.

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.

The central engines of Little Red Dots (LRDs) may be “black hole stars” (BH*s), early stages of black hole growth characterized by dense gas envelopes. So far, the most direct evidence for BH*s comes from a handful of sources where the host galaxy is completely outshone as suggested by their remarkably steep Balmer breaks. Here we present a novel scheme to disentangle BH*s from their host galaxies assuming that the [OIII]5008Å line arises exclusively from the host. Using a sample of 98 LRDs ( z ≈ 2 − 9 ) with high quality NIRSpec/PRISM spectra, we demonstrate that the host-subtracted median stack displays a Balmer break > 2 × stronger than massive quiescent galaxies, with the rest-optical continuum resembling a blackbody-like SED ( K, erg s, au). We measure a steep Balmer decrement (H α /H β > 10 ) and numerous density-sensitive features (e.g., FeII, HeI, OI). These are hallmark signatures of dense gas envelopes, providing population-level evidence that BH*s indeed power LRDs. In the median LRD, BH*s account for ∼ 20 % of the UV emission, ∼ 50 % at the Balmer break, and ∼ 90 % at wavelengths longer than H α with the remainder arising from the host. BH*s preferentially reside in low-mass galaxies () undergoing recent starbursts, as evidenced by extreme emission line EWs (e.g., [OIII]5008Å ≈ 1100 Å, CIII] ≈ 12 Å), thereby favoring BH* origins linked to star-formation. We show V-shaped LRD selections are biased to high BH*/host fractions ( ≳ 60 % at 5500Å) – less dominant BH*s may be powering JWST’s blue broad-line AGN. We find BH*s are so commonplace and transient (duty cycle ∼ 1 % , lifetime ∼ 10 Myrs) that every massive black hole may have once shone as a BH*.

The nature of little red dots (LRDs) has largely been investigated through their continuum emission, with lines assumed to arise from a broad-line region. In this paper, we instead use recombination lines to infer the intrinsic properties of the central engine. Our analysis first reveals a tension between the ionizing properties implied from H α and He ii λ 4686. The high H α EWs require copious H-ionizing photons, more than the bluest active galactic nucleus (AGN) ionizing spectra can provide. In contrast, He ii emission is marginally detected, and its low EW is, at most, consistent with the softest AGN spectra. The low He ii /H β (∼10 ⁻² , average optical depth (>10) from gas/dust, and soft ionizing spectra with abundant H-ionizing photons, consistent with, e.g., a cold accretion disk or a composite of AGN and stars. The latter is an intriguing scenario since high hydrogen densities are highly conducive for star formation, and nuclear star clusters are found in the vicinity of local massive black holes. While previous studies have mostly focused on features dominated by the absorbing hydrogen cloud, the He ii -based diagnostic proposed here represents a crucial step toward understanding the central engine of LRDs.

Luminous quasars at z > 6 are key probes of early supermassive black hole (SMBH) growth, massive galaxy evolution, and intergalactic medium properties during cosmic reionization. However, their discovery is very challenging due to their scarcity and overwhelming contamination. Foreground ultracool dwarfs (UCDs) outnumber z > 6 quasars by 2–4 orders of magnitude. In this work, we leveraged the extensive coverage of DESI Legacy Survey DR10 to conduct a self-supervised search for quasars at z > 6, directly analyzing multiband optical images and minimizing the biases of the traditional catalog-driven color-color selection criteria. By applying a contrastive learning (CL) method followed by spectral energy distribution (SED) fitting prioritization, we identified 1139 high-priority quasar candidates, for which we expect a competitive 1:1 quasar-to-UCD ratio based on the literature samples. We spectroscopically confirm 16 new quasars at z = 5.94 − 6.45, achieving a 45% success rate. Remarkably, all 16 objects are relatively bright ( M ₁₄₅₀ < −25.5) quasars, including several with unusual properties such as narrow Ly [[INLINE10]] emission (FWHM ≲ 2600 km s ⁻¹ ), strong Ly [[INLINE13]] +N V emission with an equivalent width > 100 Å, and a mild observed-frame red near-infrared (NIR) continua ( z − J > 0.4). Notably, three of them would have been missed by traditional color–color selections. These results highlight the power of self-supervised machine learning, combined with SED fitting prioritization, to uncover rare, distant sources beyond the limitations of conventional techniques. Our approach offers a scalable and robust framework for data mining and can be readily extended to forthcoming wide-field surveys such as Rubin/LSST, 4MOST, Euclid , and Roman. These applications will advance the census of high-redshift quasars, potentially extend the redshift frontier, and improve constraints on SMBH formation and evolution in the first billion years of the Universe.

JWST’s “little red dots” (LRDs) are increasingly interpreted as active galactic nuclei (AGN) obscured by dense thermalized gas rather than dust as evidenced by their X-ray weakness, blackbody-like continua, and Balmer line profiles. Key questions are how LRDs connect to standard UV-luminous AGN, whether transitional phases exist, and whether they are observable. We present the “X-ray dot” (XRD), a compact source at z = 3.28 observed by the NIRSpec Wide Guaranteed Time Observation survey. The XRD exhibits LRD hallmarks: a blackbody-like ( T _eff ≃ 6400 K) red continuum, a faint but blue rest-UV excess, falling mid-IR emission, and broad Balmer lines (FWHM ∼ 2700–3200 km s ⁻¹ ). Unlike LRDs, however, it is remarkably X-ray luminous ( L _{2−10 keV} = 10 ^44.18 erg s ⁻¹ ) and has a continuum inflection that is blueward of the Balmer limit. We find that the red rest-optical and blue mid-IR continuum cannot be reproduced by standard dust-attenuated AGN models without invoking extremely steep extinction curves, nor can the weak mid-IR emission be reconciled with well-established X-ray–torus scaling relations. We therefore consider an alternative scenario: the XRD may be an LRD in transition, where the gas envelope dominates the optical continuum but optically thin sight lines allow X-rays to escape. The XRD may thus provide a physical link between LRDs and standard AGN, offering direct evidence that LRDs are powered by supermassive black holes and providing insight into their accretion properties.