Cristian Vignali

Context. Dual active galactic nuclei (DAGN) – systems of two simultaneously accreting supermassive black holes at kiloparsec-scale separations – are critical tracers of galaxy mergers and the progenitors of gravitational-wave sources targeted by pulsar timing arrays and the upcoming Laser Interferometer Space Antenna. Despite their importance, their radio properties remain largely unconstrained, particularly at high redshift, due to the limited combination of sensitivity and sky coverage of existing high-angular-resolution surveys. Aims. In this pilot project, we primarily aim to demonstrate the detectability of high-redshift DAGN at 144 MHz performing subarcsecond resolution imaging of the LOFAR Two-metre Sky Survey (LoTSS) data, and characterize the low-frequency radio properties of individual systems. Methods. We crossmatched a sample of 92 spectroscopically confirmed DAGN at z > 0.3 and projected separation r _p < 30 kpc with the LoTSS catalogs of data release (DR) 2 (resolution ∼ 6″), and reprocessed the international LOFAR telescope (ILT) LoTSS data for three sources spanning 0.59 < z < 2.39, achieving ∼0.3″ resolution at 144 MHz. We performed a multifrequency analysis and brightness temperature diagnostics to characterize the AGN nature and radio properties of each system. Results. We successfully produced high-quality ILT images for the three DAGN in the pilot sample. In one source at z = 1.49, we detected both AGN components at > 5 σ , making it one of the highest- z and faintest DAGN detected at radio frequencies. Brightness temperature measurements confirm that ILT 144 MHz observations can efficiently distinguish AGN-powered emission from star formation even in sub-milliJansky sources. In the other two sources, we detected only one AGN. In one of them, the new ILT imaging constrains the low-frequency radio spectrum of the compact steep spectrum source, indicating that its complex morphology is shaped by jet-interstellar medium interaction rather than by the companion galaxy. In addition, for the full 92 DAGN sample, we found a detection rate of ∼24% in both LoTSS DR2 and DR3, significantly higher than that of single AGN matched in selection (∼9–10%), suggesting an enhanced detectability of DAGN in LoTSS. Conclusions. Low-frequency, subarcsecond-resolution radio observations constitute a highly efficient and dust-unbiased pathway to detect, confirm, and characterize DAGN across cosmic time. The pilot sample presented here establishes the foundation for a systematic DAGN search in the upcoming all-sky ∼0.3″ resolution international LoTSS (ILoTSS) and provides an early benchmark for the future Square Kilometre Array (SKA) observations.

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.

Aims. The extreme brightness of high- z strongly lensed galaxies detected by Planck surveys, thanks to their exceptionally large gravitational magnifications, offers a unique opportunity to investigate their structure and kinematics during their active star formation phase in extraordinary detail. As a step in this direction, we present our analysis of NOEMA imaging and spectroscopic data for four lensed high- z galaxy candidates. Methods. We performed continuum and line imaging of the sources at 2 and 3 mm bands down to 1″.2 spatial resolution and 40 km s ⁻¹ spectral resolution, reconstructed and fitted the line profiles, and produced moment maps of the line emission to investigate the spatial distribution and kinematics of the molecular gas. We also carried out the gravitational lens modelling for one of the sources. Results. The continuum images showed multiple components for at least two of the sources, strongly supporting the strong lensing scenario. We detected two CO lines with high S/N values for all sources, at CO(3−2), CO(4−3), and CO(5−4) transitions; for one source, we also detected the [CI](1−0) line. We derived accurate spectroscopic redshifts 2.3 ≲ z ≲ 3.3, with 1σ uncertainties δ _z ≈ 10 ⁻⁴ in redshift. All four sources lie well above the CO line luminosity–linewidth relation for unlensed galaxies, providing independent confirmation of their strongly lensed nature. Three sources exhibit broad (FWHM ≳ 400 km s ⁻¹ ) double-peaked line profiles and substantial velocity gradients, while the fourth shows relatively narrow single-peaked lines and no detectable velocity gradients, possibly indicating a nearly face-on geometry or intrinsically simple kinematics.

Context. Ultra-fast broad absorption line outflows (uBALs; ~ 0.1−0.2 c ) represent some of the most extreme manifestations of quasar-driven winds. With velocities reaching a significant fraction of the speed of light, they are prime candidates for efficient feedback. However, their physical structure and location remain poorly constrained, particularly at high luminosities and redshifts. In this context, variability studies offer a valuable avenue for investigating the physical conditions and geometry of these extreme outflows. Aims. We investigated the variability of uBALs through their characterisation across different epochs and multiple ionic species. Our main goal was to constrain their physical properties, locations, and kinetic powers, and to evaluate their potential impact on the respective quasar host galaxies. Methods. We performed a multi-epoch analysis of three hyper-luminous quasars from the WISSH sample, namely WISSH53, WISSH56, and WISSH71, at redshifts z = 3.628, 4.101, and 3.567, respectively. We used new and archival spectra spanning up to ~23 years of monitoring in the observed frame. We modelled absorption associated with the C IV transition using Gaussian components and accounting for covering-factor variations. We also performed a conservative analysis of multi-ion profiles (P V , Ly α , N V , and Si IV ), considering the same uBAL velocity as C IV . We derived ionic column densities and ionisation parameters, which allowed us to estimate lower and upper limits on the distances and kinetic powers of the outflows. Results. The C IV uBALs in the three sources exhibit velocities up to ~0.2 c and show significant variability across epochs, likely driven by changes in the ionisation state and/or transverse gas motion. The coordinated analysis of the multiple troughs highlights the dominant mechanisms that cause variability and enables us to derive robust lower and upper limits of the physical and energetic properties of the outflows. Outflow distances are constrained from the broad-line region (BLR) radius (~1 pc) up to a few hundred parsecs, and the kinetic powers of individual troughs reach up to ~25% of the quasar bolometric luminosity in the most extreme cases. Our results demonstrate that extreme outflows such as uBALs can inject sufficient kinetic energy to affect the host medium.

Context . The X-ray emission of active galactic nuclei (AGNs) is commonly interpreted as arising from Comptonisation in a hot corona located above the accretion disc. In many sources, a prominent soft X-ray excess is also observed, often explained within the two-corona scenario, where a warm Comptonising layer produces the soft excess, while a hotter corona generates the hard X-ray continuum. However, the physical properties of these coronal components remain poorly constrained in very luminous quasars. Aims . We investigate the structure and physical properties of the X-ray-emitting coronae in two luminous radio-quiet quasars, PG 1407+265 ( z = 0.94) and RBS 229 ( z = 0.334), and test whether the two-corona scenario provides a consistent description of their broadband spectra. Methods . We performed a broadband spectral analysis combining XMM-Newton EPIC and Optical Monitor data with simultaneous NuSTAR observations and long-term Swift monitoring over months to years. The spectra were modelled using Comptonisation models to characterise the properties of both the warm and hot coronae. Results . Both quasars are X-ray-loud with respect to the standard α _OX – L _UV relation, with PG 1407+265 exhibiting the strongest deviation. Their broadband spectra are well reproduced by a two-corona configuration, consisting of a warm, optically thick Comptonising component that accounts for the soft excess and a hot corona responsible for the hard X-ray continuum. Both sources show high energy cut-offs in the range ∼70–80 keV. RBS 229 exhibits moderate reflection and a weak neutral Fe K α line, whereas no significant reflection features were detected in PG 1407+265. The soft excess is present in both quasars, but displays a markedly different behaviour for each: the warm-corona parameters in RBS 229 remain consistent across epochs, while PG 1407+265 displays strong variability, producing an extreme soft-excess episode extending up to ∼3 keV in the rest frame. Conclusions . These results provide new constraints on the properties of AGN coronae in the high-luminosity regime, which remains relatively poorly explored in observations. Both quasars show high energy cut-offs that are broadly consistent with the trends observed in terms of luminosity and accretion rates among AGN samples and safely below the runaway pair-production limit. No clear dependence of the warm-corona properties on the luminosity or the accretion rate was found, as both quasars show similar kT, τ , and soft-excess strengths despite their different regimes, with variations seen mainly in terms of the spectral slope and variability. The strong variability of the warm component in PG 1407+265, compared to the stability of RBS 229, supports a scenario in which the warm corona responds to accretion-flow changes, while remaining within a narrow range of physical conditions.

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.

Context. Models of supermassive black holes (SMBHs) and galaxy coevolution, simulations, and recent JWST observations suggest that the population of heavily obscured, Compton-thick (CTK) active galactic nuclei (AGNs) at high redshift might be underestimated by X-ray surveys. To retrieve a complete census of SMBHs it is therefore necessary to identify new and complementary methods to select these sources, for example by exploiting the radio band as radio waves are almost unaffected by obscuration. Aims. We aim to test the effectiveness of radio selection to discover heavily obscured AGNs, particularly at high z, and, in turn, measure their abundance for the first time from a radio perspective. Methods. We considered the radio sources detected in the J1030 field, which is one of the fields with the deepest combination of 1.4 GHz radio and X-ray observations publicly available. We defined a radio excess parameter as the ratio between the star formation rate (SFR) that would correspond to the observed radio luminosity and the one directly derived from the spectral-energy-distribution (SED) fitting, REX = SFR _1.4GHz /SFR _SED ^corr . We then selected sources with REX > 8.5 as radio-excess AGNs; this corresponds to a 3 σ excess above the median value (REX ~ 1). In this way, we find 145 radio-excess sources falling in the Chandra X-ray image footprint but without X-ray detection. Results. From the deep X-ray upper limits, we estimated a lower limit to the obscuration of each radio-excess AGN, finding on average log( N _H /cm ⁻² ) > 23.7. A CTK AGN scenario is also supported by the results of the X-ray stacking analysis performed on sources at z > 1.5, which revealed X-ray luminosities and hardness ratios compatible with very highly obscured AGNs. Finally, we computed the number density of these radio-selected CTK AGNs. While at z ~ 2 the radio number density agrees well with the CTK AGN predictions of different population synthesis models, at z ~ 3 the radio selection returns a CTK AGN number density ~2–3 times larger than what is predicted by the Cosmic X-ray Background (CXB) models and X-ray observations. This result supports the effectiveness of radio emission in selecting the most obscured sources, unraveling a population of AGNs potentially missed by X-ray surveys at z > 3; this paves the way to a synergistic use of the future radio and X-ray facilities such as the Square Kilometer Array Observatory ( SKAO ), NewAthena , and the Advance X-ray Imager Satellite ( AXIS ).

We present the first results from the WISSHFUL program, an XMM-Newton heritage program targeting luminous quasi-stellar objects (QSOs) at cosmic noon. We report on recent simultaneous XMM-Newton and NuSTAR observations of the super-Eddington accreting quasar WISSH13 at z=3.294, which provide the highest-quality broadband X-ray spectrum to date for a non-lensed QSO at this redshift. Physical modeling of the continuum reveals a soft photon index (Γ ∼ 2) and strong reflection ( R ~ 1.4-1.8), despite the weak narrow Fe emission, and a low high-energy cutoff ( E _cut ∼ 60-80 keV, kT _e = 15-20 keV, depending on the model adopted). Most notably, we detect two significant absorption features (at confidence levels of 96.7% and 98.9%, respectively) at ∼7.5 and ∼10 keV in the rest-frame, interpreted as a blueshifted blend of Fe XXV He α and Fe XXVI Ly α . These features indicate the presence of two kinematic components of a highly ionized, high-column ultrafast outflow (UFO) with a velocity of v _out ∼ 0.1 c and v _out ∼ 0.3 c , respectively. The slower wind is consistently detected in an archival 2017 XMM-Newton observation, whereas the faster wind is detected only in 2024. This stratified and variable wind exhibits extreme energetics, with a mass outflow rate of M _out ∼ 20 M _⊙ /yr (corresponding to 15% M _acc ) for each component, and a kinetic power on the order of ∼1 and ∼10% of the bolometric luminosity, respectively. While this represents one of the most powerful UFOs ever detected, its kinetic power is a similar fraction of the QSO’s bolometric luminosity compared to lower-redshift active galactic nuclei. We present several theoretical frameworks to explain the peculiar accretion and ejection properties of this remarkable QSO at cosmic noon.

Little red dots (LRDs) are compact, red sources discovered by JWST at high redshift ( z ≳ 4), marked by distinctive “V-shaped” spectral energy distributions (SEDs) and often interpreted as rapidly accreting active galactic nuclei (AGNs). Their true nature remains unclear though, and their evolutionary connection to their lower-redshift counterparts is still poorly constrained. Thus, we present WISEA J123635.56+621424.2 (here dubbed the Saguaro), a z = 2.0145 galaxy in GOODS-North, as a possible analog of high-redshift LRDs and a potential missing link in their evolutionary path toward lower-redshift systems. It features a compact LRD-like nucleus surrounded by a face-on spiral host. Its connections to LRDs include the following: (1) its nuclear spectrum shows a clear “V-shaped” SED, and (2) when redshifted to z = 7, surface brightness dimming makes the host undetectable, thus mimicking an LRD. This suggests that high-redshift LRDs may be embedded in extended hosts. To test this, we stack rest-frame UV images of 99 photometrically selected LRDs, revealing faint, diffuse emission. Stacking in redshift bins reveals mild radial growth, consistent with the expected galaxy size evolution. A simple analytic model confirms that surface brightness dimming alone can explain their compact appearance. Lastly, we show that the Saguaro is not unique by describing similar objects from the literature at z ≲ 3.5. Taken together, our results support a scenario in which LRDs may not be a distinct population, but could be the visible nuclei of galaxies undergoing a short-lived, (perhaps) AGN-dominated evolutionary phase, with their compact, red appearance driven largely by observational biases.