Piero Madau

We tested whether the so-called little red dots (LRDs) are the dust-reddened high-inclination counterparts of bluer compact broad-line active galactic nuclei, here referred to as little blue dots (LBDs), by modeling their relative number densities and luminosities. Using the observed ultraviolet (UV) luminosity function (LF) of broad-line active galactic nuclei (BLAGNs) at z ≳ 4 as the parent distribution, we forward-modeled the effects of the accretion rate, anisotropic emission, orientation, and dust obscuration within our super-Eddington unification framework. We show that a model with a geometrically thick accretion flow and a dusty circumnuclear cloud population reproduces the LRD LF over the luminosity range currently constrained by the James Webb Space Telescope. The predicted LRD-to-BLAGN ratio is strongly luminosity dependent, rising from ≃3% at M ₁₅₀₀ = −21 mag to a peak value of ∼20% near M ₁₅₀₀ ≃ −19. The model also predicts a larger apparent LRD fraction at rest-frame optical wavelengths, reaching 27% at M ₄₅₀₀ = −20 mag and 39% at M ₆₅₀₀ = −21. The fiducial solution adopted a baseline host extinction A _V ^host = 0.5 mag, a characteristic per-cloud nuclear extinction ⟨ A _V ^c ⟩ = 3.0 mag, and a gray R _V = 5.5 attenuation curve, and it yields a mean dust covering factor ⟨ C _dust ⟩≃0.23. These results can support an orientation-based unification of little dots and identify the LRD LF as a key demographic test of rapid accretion onto infant black holes at cosmic dawn. In our model, the UV continuum is dominated by the active nucleus; LRDs and LBDs are drawn from the same observed BLAGN UV LF, but attenuation makes UV-selected LRDs intrinsically brighter and their inferred black holes more massive at fixed observed M ₁₅₀₀ .

The nature of dark energy (DE) remains elusive, even though it constitutes the dominant energy-density component of the Universe and drives the late-time acceleration of cosmic expansion. By combining measurements of the expansion history from baryon acoustic oscillations, supernova surveys, and cosmic microwave background data, the Dark Energy Spectroscopic Instrument (DESI) Collaboration has inferred that the DE equation of state may evolve over time. The profound implications of a time-variable, “dynamical” DE (DDE) that departs from a cosmological constant motivate the need for independent observational tests. In this work, we use a focused set of cosmological hydrodynamical simulations to investigate how DDE affects the properties of the Lyman- α “forest” of absorption features produced by neutral hydrogen in the cosmic web. We find that DDE models consistent with the DESI constraints induce a spectral tilt in the forest transmitted flux power spectrum, imprinting a scale- and redshift-dependent signature relative to standard Λ CDM cosmologies. These models also yield higher intergalactic medium temperatures and reduced Lyman- α opacity compared to Λ CDM . We discuss the observational implications of these trends as potential avenues for independent confirmation of DDE.

We reassess the direct collapse black hole (DCBH) interpretation of UHZ1 (UNCOVER-26185), a gravitationally lensed galaxy at z _spec = 10.054. That interpretation rests on a hard (2–7 keV) X-ray excess detected with Chandra, attributed to a Compton-thick AGN with an inferred 2–10 keV luminosity of L _X,int ∼ 10 ⁴⁶ erg s ⁻¹ (Á. Bogdán et al. 2024). The resulting extreme X-ray to rest-frame optical–IR ratio was taken as the hallmark signature of an “outsize black hole galaxy” at cosmic dawn. We analyze the full 2.2 Ms Chandra imaging dataset—including 0.95 Ms of unpublished observations—and present new JWST/MIRI photometry at λ _obs > 5 μ m. Across the full range of plausible Chandra data reductions, the 2–7 keV excess at the position of UHZ1 reaches a significance of only 2.0–2.9 σ ; the originally reported 4.2–4.4 σ detection is sensitive to the specific astrometric alignment adopted and is not robustly reproducible. Moreover, the hard X-ray signal does not grow with the additional exposure, contrary to expectations for a steady source, indicating that any excess is not persistent. UHZ1 is also undetected in all nine MIRI imaging bands. Fitting red/obscured AGN SED templates to the tightest MIRI upper limit, we constrain the bolometric luminosity of any buried AGN to L _bol < 1.3 × 10 ⁴⁵ erg s ⁻¹ . These conclusions are further supported by independent JWST spectroscopy, which reveals no AGN signatures in the rest-frame UV or optical. Taken together, the multiwavelength data paint a consistent picture of UHZ1 as a low-mass, metal-poor, star-forming galaxy in the early Universe, with no compelling evidence for a luminous obscured AGN, regardless of its proposed formation channel.

We investigated whether little red dots (LRDs) are the dust-reddened, high-inclination counterparts of compact, blue broad-line AGNs (“little blue dots”; LBDs) powered by super-Eddington accretion. We modeled the central engine as a geometrically thick, radiation-pressure supported accretion flow whose funnel produces strongly anisotropic, intrinsically blue ionizing continua. This is coupled to an equatorially concentrated broad-line region (BLR) and dusty reprocessing clouds with a modest covering factor. Using inclination-dependent spectral energy distributions (SEDs) as input to CLOUDY, we show that the extreme broad H α equivalent widths (EWs) of JWST LRDs can be reproduced with global BLR covering factors of only C _BLR ≃ 0.1, fully consistent with standard type 1 active galactic nuclei (AGNs) and far below unity. Large Balmer EWs arise because self-shadowing suppresses the high-inclination optical continuum while the BLR is illuminated by an ionizing-rich extreme ultraviolet (EUV) SED. Weak high-ionization lines (e.g., He II λ 4686) follow from the orientation-dependent suppression of the hard extreme ultraviolet (XUV) and soft X-ray continuum toward equatorial directions, without requiring a fully enclosing gaseous “cocoon.” Applying a gray dust attenuation law with A _V ≃ 3 along high-inclination (LRD-selected) sightlines, our fiducial model reproduces the V-shaped UV–optical continua of LRDs and the large Balmer decrements. Strong Balmer breaks arise only along the most obscured sightlines. A compact equatorial dust structure with a modest global covering factor intercepts and reradiates only a small fraction of the bolometric luminosity, yielding a modest hot-dust bump and far-infrared and submillimeter output consistent with current measurements and limits and implying small dust masses. This single-framework model links LRD and LBD observables through orientation, predicting correlated trends in H α EW, Balmer decrement, Balmer break, high-ionization line strengths, and infrared emission.

We investigate the origin of the broad exponential wings seen in the H α profiles of many JWST-discovered little red dots (LRDs) and little blue dots (LBDs). Recent studies show that exponential broad-line profiles are not unique to LRDs, are common in LBDs too, and need not imply that electron scattering is the dominant broadening mechanism in every source. We model the broad Balmer emission with a virialized, radially stratified broad-line region (BLR), motivated by our unification picture, in which LRDs are dust-reddened, high-inclination counterparts of compact blue broad-line active galactic nuclei (AGNs). In this framework, the observed profile is the luminosity-weighted superposition of clouds spanning a range of radii, and hence virial velocities. We show that a stratified BLR reproduces the exponential-like wings observed in three representative LRDs and one LBD, without requiring electron scattering as the dominant broadening mechanism. For the LBD GS-3073, the same BLR – calibrated only to H α – predicts, parameter-free, that He II λ4686 and H β should be broader than H α , consistent with observations and offering an independent test of BLR ionization stratification. Our results support a picture in which wings and cores encode different physics: the wings arise primarily from virial BLR stratification, while the cores retain imprints of absorption and radiative transfer in dense gas. The fits further suggest that the cloud radial distribution peaks near the dust sublimation radius, with the exponential wings shaped by the inner, line-emitting BLR shells whose higher virial velocities produce the high-velocity tails. This offers a simple physical explanation for the exponential wings of little dots, without invoking exotic new components or scenarios.

We present a high-resolution cosmological zoom-in simulation of a group of field dwarf galaxies that includes on-the-fly radiative transfer (RT) and is evolved to z=0. Emission from stars is included according to age-dependent spectral energy distributions, and a redshift-dependent UV background is modelled by background particles and propagated through the simulation volume. The simulation also incorporates star formation, supernova (SN) feedback, a non-equilibrium primordial chemical network, and metal cooling and diffusion. We compare results from this simulation to previous simulations with the same initial conditions and largely similar galaxy formation physics, but without RT. The inclusion of RT results in the formation of eight additional faint dwarf galaxies with stellar masses of 10^ 4 , ̊m M_⊙ to several 5 , ̊m M_⊙ and only old stellar populations, similar to the observed ultra-faint dwarf galaxies. These eight faint dwarf galaxies formed before and during cosmic reionisation and were mostly quenched by z ∼ 3-4 when reionisation was complete. The simulated galaxies follow many observed scaling relations such as the stellar mass-halo mass relation, the mass-size relation, and the luminosity-velocity dispersion relation. However, the simulation appears to underpredict the stellar iron abundances for the faintest population with L_ , < 10^ ̊m L_⊙. For the more massive "classic" dwarf galaxies, radiative feedback suppresses star formation, making it less bursty and reducing explosive outflows. This consequently reduces the dark matter core sizes by a factor of two to three, rendering the core sizes (∼ 1 kpc) more consistent with observations. The distribution of neutral hydrogen in the circumgalactic medium (CGM) is ubiquitous, with a covering fraction of unity within R_ is very compact and declines sharply beyond the interstellar medium (ISM) scale. and have a more extended distribution, but their column densities are generally below the detection limit. Radiative feedback leads to smaller column densities of the metal ions, partly due to the reduction of total metal production, and partly because hard photons from the stellar radiation escape the ISM and further ionise the CGM. The abundance of is particularly sensitive to the latter effect. v 6 H I and in good agreement with observations. It is rather insensitive to radiative or SN feedback at z=0, but at z>5, it is much higher in the RT simulation. In contrast, the distribution of low-ionisation species such as Si II C IV O VI C IV

We revisit black hole mass estimates for high-redshift broad-line active galactic nuclei (AGNs) discovered with JWST by jointly analysing their broad emission lines and their systematic non-detections in deep Chandra imaging. Building upon a self-shadowed, super-Eddington accretion framework in which the corona undergoes efficient radiative over-cooling, we couple funnel-dependent Comptonisation physics with slim-disc spectral models and explore the resulting parameter space through a full Markov Chain Monte Carlo inference. Using a recently compiled sample of JWST high-redshift broad-line AGNs, we show that the observed X-ray weakness – manifested as extreme bolometric corrections, suppressed 2−10 keV luminosities, and non-detections in the 0.5−5 keV Chandra band – naturally arises when the corona is confined and radiatively over-cooled inside a narrow super-Eddington funnel. The combined broad line+X-ray analysis yields strongly bimodal posteriors: either very massive, very low-Eddington black holes (physically disfavoured), or a population of low-mass (∼10 ⁶ − 10 ⁷ M _⊙ ) black holes accreting at f _Edd ≫ 1. The latter solution is strongly preferred for nearly all objects and returns masses consistent with, or lower than, local M _BH − M _★ relations, mitigating the extreme mass ratios implied by single-epoch virial estimators. The predicted intrinsic spectra are redder and exhibit reduced hard-X-ray output but higher bolometric luminosities, implying bolometric corrections larger than those typical of the local AGN population, yet consistent with low-redshift highly accreting counterparts. These results support a picture in which many JWST broad-line AGNs are powered by rapidly growing, super-Eddington black holes whose suppressed coronal emission and self-shadowed broad-line region geometry combine to mimic overmassive black holes at z ≳ 6.

Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift broad-line active galactic nuclei (BLAGNs) characterized by moderate luminosities, weak X-ray emissions, and faint high-ionization lines. We propose that a subset of these BLAGNs, the so-called “little blue dots” (LBDs) are accreting at super-Eddington rates and use geometrically thick, non-advective disk models to investigate photon scattering and shadowing within the polar funnel. Our models predict extremely blue optical-UV continuum slopes and highly collimated radiation fields where isotropic-equivalent luminosities exceed the Eddington limit in the polar direction, while shadowing suppresses emission at higher inclinations. This “searchlight” configuration naturally generates a stratified ionization structure: coronal and high-excitation narrow lines are produced along the symmetry axis, while the equatorial broad-line region (BLR) remains shielded from the hardest ionizing photons. We show that the anisotropic illumination of the BLR explains the observed faintness of high-ionization lines despite strong Balmer emission. For M _BH = 10 ^7.5 –10 ⁸ M _⊙ black holes accreting at Eddington ratios ∼10, standard BLR conditions predict He II λ 4686/H β in the range of 0.08–0.28. Notably, because inherently blue disk spectra provide a much higher ratio of ionizing to optical photons than standard quasar composites, the observed large Balmer equivalent widths are matched with typical BLR covering factors without invoking enshrouded geometries. Taken together, these findings support the view that super-Eddington accretion flows, shaped by thick disk geometries, may naturally account for the ionizing SED and emission line diagnostics of high- z LBDs, while offering a plausible pathway to rapid black hole growth at cosmic dawn.

Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift, broad-line active galactic nuclei (AGNs), whose properties challenge standard models of black-hole growth and AGN emission. We analyzed a spectroscopic sample of 34 Type 1 AGNs from the JWST Advanced Deep Survey (JADES) survey, spanning redshifts 1.7 < z < 9, to constrain the physical nature of the accretion flows powering these sources with broad-line diagnostics statistically for the first time. At z > 5, we find a marked suppression of high-ionization emission lines (HeII, CIV, NV) relative to prominent broad H α and narrow [OIII] features. This contrast places strong constraints on the shape of the ionizing spectral energy distribution (SED) and on the physical conditions in the broad-line region (BLR). By comparing the observations to photoionization models based on SEDs of black holes accreting at sub-Eddington ratios, we show that standard AGN continua struggle to reproduce the observed broad-line ratios and equivalent widths (EWs) across a wide ionization parameter range. These results suggest the need for modified SEDs – either intrinsically softened due to super-Eddington accretion or radiative inefficiencies in the innermost disk, or externally filtered by intervening optically thick gas that absorbs or scatters the highest energy photons before they reach the BLR.

The black hole occupation fraction (f _occ) defines the fraction of galaxies that harbor central massive black holes (MBHs), irrespective of their accretion activity level. While it is widely accepted that f _occ is nearly 100% in local massive galaxies with stellar masses M _⋆ ≳ 10¹⁰ M _⊙, it is not yet clear whether MBHs are ubiquitous in less-massive galaxies. In this work, we present new constraints on f _occ based on over 20 yr of Chandra imaging data for 1606 galaxies within 50 Mpc. We employ a Bayesian model to simultaneously constrain f _occ and the specific accretion-rate distribution function, p(λ), where the specific accretion rate is defined as λ = L _X/M _⋆, where L _X is the MBH accretion luminosity in the 2–10 keV range. Notably, we find that p(λ) peaks around 1 0 28 erg s − 1 M ⊙ − 1 ; above this value, p(λ) decreases with increasing λ, following a power law that smoothly connects with the probability distribution of bona fide active galactic nuclei. We also find that the occupation fraction decreases dramatically with decreasing M _⋆: in high-mass galaxies (M _⋆ ≈ 10¹¹⁻¹² M _⊙), the occupation fraction is >93% (a 2σ lower limit), and then declines to 6 6 % − 7 % + 8 % (1σ errors) between M _⋆ ≈ 10⁹⁻¹⁰ M _⊙, and to 3 3 % − 9 % + 13 % in the dwarf galaxy regime between M _⋆ ≈ 10⁸⁻⁹ M _⊙. Our results have significant implications for the normalization of the MBH mass function over the mass range most relevant for tidal disruption events, extreme mass ratio inspirals, and MBH merger rates that upcoming facilities are poised to explore.