Nicola Menci

The James Webb Space Telescope (JWST) has revealed an unexpected excess of UV-bright galaxies at z > 10, unaccounted for by extrapolations from pre-JWST observations and theoretical models. Understanding the physical properties and star formation histories (SFHs) of high-redshift systems is key to distinguishing between the different proposed scenarios and assessing their implications for the galaxy formation and evolution picture. We identified and analysed a sample of 2420 robust candidates at z ∼ 7 − 14 drawn from the ASTRODEEP-JWST dataset over ∼0.2 deg ² , and modelled their properties with non-parametric SFHs to derive the specific star formation rate (sSFR) and stellar population properties. We find that the median sSFR and M/L remain roughly constant across the probed redshift range. We show that this result is robust against potential systematics unless a hidden population of dust-enshrouded starbursts, undetectable in the current data, exists at these redshifts. In any case, the absence of observed high-sSFR systems at the highest redshifts suggests that any dust-free starburst phase must be short-lived. The observed sSFR evolution is in tension with most (though not all) theoretical models, making it a key quantity for discriminating among competing scenarios. The sample shows a wide range of physical conditions and galaxy classes, including systems with low sSFRs and high mass-to-light ratios (M/L) up to z ∼ 10, indicative of already-evolved galaxies only a few hundred megayears after the Big Bang, and different degrees of dust attenuation. We finally reconstructed the assembly histories of two sub-samples, namely the highest-M/L galaxies at z ∼ 7 − 8, which appear to have formed the bulk of their stars at least 500 Myr before observation, implying progenitors observable as UV-bright sources at z > 20, and the full sample of z > 11 galaxies, which formed through stochastic SFHs, remaining UV-faint for most of their early evolution, before undergoing recent (∼50 Myr old) episodes of major growth.

Context. Recent results from combined cosmological probes indicate that the dark energy component of the Universe could be dynamical. The simplest explanation envisages the presence of a quintessence field rolling into a potential, where the dark energy density parameter Ω _DE = Ω _Λ + Ω _x results from the contribution of the ground-state energy, Ω _Λ , and the scalar field energy, Ω _x . Provided that Ω _DE ≈ 0.7, negative values of Ω _Λ can be consistent with current measurements from cosmological probes and could help explain the large abundance of bright galaxies observed by the JWST at z > 10, largely exceeding the pre-JWST expectations in a Λ cold dark matter universe. Aims. We aim to explore the extent to which such a scenario can also account for the early presence of massive black holes (BHs) with masses of M _BH ≳ 10 ⁷ M _⊙ observed at z ≳ 8 and for the large over-abundance of active galactic nuclei (AGNs) with respect to pre-JWST expectations. Our aim is not to provide a detailed description of BH growth, but rather to compute the maximal BH growth that can occur in cosmological models with negative Ω _Λ under the simple assumption of Eddington-limited accretion onto initial light BH seeds with masses of M _seed ∼ 10 ² M _⊙ that originated from Pop III stars. Methods. To this aim, we developed a simple analytic framework to connect the growth of dark matter halos to the maximal growth of BHs within the above assumptions. Results. We show such models can account for present observations assuming values of Ω _Λ ≈ −1, simultaneously boosting both galaxy and AGN number counts without invoking any additional physics. This would allow us to trace the observed excess of bright and massive galaxies and the early formation of massive BHs and the abundance of AGNs to the same cosmological origin.

Aims. We aim to characterize the X-ray-to-optical/near-infrared(NIR) broad-band emission of luminous quasars (QSOs) in the first gigayear (Gyr) of cosmic evolution in order to decipher whether or not they exhibit differences compared to the lower-z QSO population. Our goal is also to provide a reliable and uniform catalog of derivable properties for these objects (from fitting their spectral energy distribution), such as bolometric and monochromatic luminosities, Eddington ratios, dust extinction, and the strength of the hot dust emission. Methods. We gathered all available photometry –from XMM-Newton proprietary data in X-rays to rest-frame NIR wavelengths– for the 18 QSOs in the HYPERION samples (6.0 ≤ z ≤ 7.5). For sources lacking uniform NIR coverage, we conducted NIR observations in the J, H, and K bands. To increase the statistical robustness of our analysis across the UV-to-NIR region, we add 36 additional sources to our sample from the E-XQR-30 sample with 5.7 ≲ z ≲ 6.6. We characterized the X-ray/UV emission of each QSO using average SEDs from luminous Type 1 sources and calculated bolometric and monochromatic luminosities. Finally, we constructed a mean SED extending from the X-rays to the NIR bands. Results. We find that the UV-optical emission of these QSOs can be modeled with templates of z ∼ 2 luminous QSOs. We observe that the bolometric luminosities derived while adopting some bolometric corrections at 3000 Å (BC_3000 Å) largely used in the literature are slightly overestimated, by 0.13 dex, as they also include reprocessed IR emission. We estimate a revised value of BC_3000 Å = 3.3, which can be used to derive L_bol in z ≥ 6 QSOs. We provide a subsample of 11 QSOs with rest-frame NIR photometry; these show a broad range of hot dust emission strength, with two sources exhibiting low levels of emission. Despite potential observational biases arising from nonuniform photometric coverage and selection biases, we produce an X-ray-to-NIR mean SED for QSOs at z ≳ 6 that is a good match to templates of lower-redshift, luminous QSOs up to the UV–optical range, with a slightly enhanced contribution from hot dust in the NIR.

Recent observations by JWST yield a large abundance of luminous galaxies at z ≳ 10 compared to that expected in the ΛCDM scenario based on extrapolations of the star formation efficiency measured at lower redshifts. While several astrophysical processes can be responsible for such observations, here we explore to what extent such an effect can be rooted in the assumed dark energy (DE) sector of the current cosmological model. This is motivated by recent results from different cosmological probes combined with the last data release of the Dark Energy Spectroscopic Instrument, which indicate a tension in the DE sector of the concordance ΛCDM model. We have considered the effect of assuming a DE characterized by a negative Λ as the ground state of a quintessence field on the galaxy luminosity function at high redshifts. We find that such models naturally affect the galaxy UV luminosities in the redshift range 10 ≲ z ≲ 15 needed to match the JWST observations, and with the value of Ω_Λ = [−0.6, −0.3] remarkably consistent with that required by independent cosmological probes. A sharp prediction of such models is the steep decline of the abundance of bright galaxies in the redshift range 15 ≲ z ≲ 16.

One of the main open questions in the field of luminous (L_bol > 10⁴⁷ erg s⁻¹) quasars (QSOs) at z ≳ 6 is the rapid formation (< 1 Gyr) of their supermassive black holes (SMBHs). For this work we analysed the relation between the X-ray properties and other properties describing the physics and growth of both the accretion disc and the SMBH in QSOs at the Epoch of Reionization (EoR). The sample consists of 21 z > 6 QSOs, which includes 16 sources from the rapidly grown QSOs from the HYPERION sample and five other luminous QSOs with available high-quality archival X-ray data. We discovered a strong and statistically significant (> 3σ) relation between the X-ray continuum photon index (Γ) and the C IV disc wind velocity (v_C․IV) in z > 6 luminous QSOs, whereby the higher the v_C․IV, the steeper the Γ. This relation suggests a link between the disc–corona configuration and the kinematics of disc winds. Furthermore, we find evidence at > 2 − 3σ level that Γ and v_C․IV are correlated to the growth rate history of the SMBH. Although additional data are needed to confirm it, this result may suggest that, in luminous z > 6 QSOs, the SMBH predominantly grows via fast accretion rather than via initial high seed BH mass.

Early observations with the James Webb Space Telescope (JWST) have revealed the existence of an unexpectedly large abundance of extremely massive galaxies at redshiftsz≳ 5: these are in tension with the predictions not only of the standard ΛCDM cosmology, but also with those of a wide class of dynamical dark energy (DE) models, and are generally in better agreement with models characterized by a phantom behaviour. Here we consider a model, inspired by string theory and the ubiquity of anti-de Sitter vacua therein, featuring an evolving DE component with positive energy density on top of a negative cosmological constant, argued in an earlier exploratory analysis to potentially be able to explain the JWST observations. We perform a robust comparison of this model against JWST data, considering both photometric observations from the CEERS program, and spectroscopic observations from the FRESCO survey. We show that the model is able to accommodate the JWST observations, with a consistency probability of up to 98%, even in the presence of an evolving component with a quintessence-like behaviour (easier to accommodate theoretically compared to phantom DE), while remaining consistent with standard low-redshift probes. Our results showcase the potential of measurements of high-redshift galaxy abundances in tests of fundamental physics, and their complementarity with standard cosmological probes.

The existence of luminous quasars (QSOs) at the epoch of reionization (EoR; i.e., z > 6) powered by well-grown supermassive black holes (SMBHs) with masses ≳10⁹ M_⊙ challenges models of early SMBH formation and growth. To shed light on the nature of these sources, we started a multiwavelength program based on a sample of 18 HYPerluminous quasars at the Epoch of ReionizatION (HYPERION). These are the luminous QSOs whose SMBHs must have had the most rapid mass growth during the first gigayear of the Universe and therefore acquired the largest mass at their respective epochs. In this paper, we present the HYPERION sample and report results from the first year of the planned three years of observations of the 2.4 Ms XMM-Newton Multi-Year Heritage program on which HYPERION is based. The goal of this program is to accurately characterise the X-ray nuclear properties of QSOs at the EoR. Through a joint X-ray spectral analysis of ten sources, covering the rest-frame ∼2 − 50 keV energy range, we report a steep average photon index (Γ ≈ 2.4 ± 0.1). No absorption is required at levels of 10²¹ − 10²² cm⁻². The measured average Γ is inconsistent at ≥4σ level with the canonical value (Γ = 1.8 − 2) measured in QSOs at z < 6. Such a steep spectral slope is also significantly steeper than that reported in lower-z analog QSOs with similar luminosity or accretion rate, suggesting genuine redshift evolution. Alternatively, we can interpret this result as suggesting the presence of a very low energy cutoff E_cut ≈ 20 keV on a standard Γ = 1.9 power-law, the likes of which is rarely reported at lower z. We also report mild indications that, on average, HYPERION QSOs show higher levels of coronal soft X-rays at 2 keV compared to the accretion disk UV at 2500 Å than expected for lower-z AGN in the high-luminosity regime. We speculate that either a redshift-dependent coupling between the X-ray corona and accretion disk or intrinsically different coronal properties account for the steepness of the X-ray spectral slope, especially in the presence of powerful winds. The reported steep slopes, if confirmed in lower-luminosity regimes, may have an important impact on the design of next-generation X-ray facilities and future surveys designed to investigate the early Universe.

To solve the still unsolved and fundamental problem of the role of active galactic nucleus (AGN) feedback in the shaping of galaxies, we implement eda new physical treatment of AGN-driven winds into our semi-analytic model of galaxy formation. With each galaxy in our model, we associated solutions for the outflow expansion and the mass outflow rates in different directions, depending on the AGN luminosity, on the circular velocity of the host halo and on the gas content of the considered galaxy. We also assigned an effective radius to each galaxy that we derived from energy conservation during merger events, and a stellar velocity dispersion that we self-consistently computed via Jeans modeling. We derived all the main scaling relations between the black hole (BH) mass and the stellar mass of the host galaxy and of the bulge, the velocity dispersion, the host halo dark matter mass, and the star formation efficiency. We find that our improved AGN feedback mostly controls the dispersion around the relations, but it plays a subdominant role in shaping slopes and/or normalizations of the scaling relations. The models agree better with the available data when possible limited-resolution selection biases are included. The model does not indicate that any more fundamental galactic property is linked to BH mass. The velocity dispersion plays a similar role as stellar mass, which disagrees with current data. In line with other independent studies carried out on comprehensive semi-analytic and hydrodynamic galaxy-BH evolution models, our current results signal either that the current cosmological models of galaxy formation are inadequate in their reproduction of the local scaling relations in terms of both shape and residuals, and/or they indicate that the local sample of dynamically measured BHs is only incompletely known.

We report the detection of a high density of redshift z ≈ 10 galaxies behind the foreground cluster A2744, selected from imaging data obtained recently with NIRCam on board JWST by three programs—GLASS-JWST, UNCOVER, and DDT#2756. To ensure robust estimates of the lensing magnification μ, we use an improved version of our model that exploits the first epoch of NIRCam images and newly obtained MUSE spectra and avoids regions with μ > 5 where the uncertainty may be higher. We detect seven bright z ≈ 10 galaxies with demagnified rest frame −22 ≲ M _UV ≲ −19 mag, over an area of ∼37 arcmin². Taking into account photometric incompleteness and the effects of lensing on luminosity and cosmological volume, we find that the density of z ≈ 10 galaxies in the field is about 10× (3×) larger than the average at M _UV ≈ −21 ( −20) mag reported so far. The density is even higher when considering only the GLASS-JWST data, which are the deepest and the least affected by magnification and incompleteness. The GLASS-JWST field contains five out of seven galaxies, distributed along an apparent filamentary structure of 2 Mpc in projected length, and includes a close pair of candidates with M _UV < −20 mag having a projected separation of only 16 kpc. These findings suggest the presence of a z ≈ 10 overdensity in the field. In addition to providing excellent targets for efficient spectroscopic follow-up observations, our study confirms the high density of bright galaxies observed in early JWST observations but calls for multiple surveys along independent lines of sight to achieve an unbiased estimate of their average density and a first estimate of their clustering.

Despite significant efforts in recent years, the physical processes responsible for the formation of passive galaxies through cosmic time remain unclear. The shape and evolution of the stellar mass function (SMF) give an insight into these mechanisms. Taking advantage of the CANDELS and the deep Hubble Frontier Fields (HFF) programs, we estimated the SMF of total, star-forming, and passive galaxies from z = 0.25 to z = 2.75 to unprecedented depth and we focus on the latter population. The density of passive galaxies underwent a significant evolution over the last 11 Gyr. They account for 60% of the total mass in the nearby universe against ∼20% observed at z ∼ 2.5. The inclusion of the HFF program allows us to detect, for the first time at z > 1.5, the characteristic upturn in the SMF of passive galaxies at low masses, usually associated with environmental quenching. We observe two separate populations of passive galaxies evolving on different timescales: roughly half of the high-mass systems were already quenched at high redshift, while low-mass passive galaxies are gradually building up over the redshift range probed. In the framework of environmental quenching at low masses, we interpret this finding as evidence of an increasing role of the environment in the buildup of passive galaxies as a function of time. Finally, we compared our findings with a set of theoretical predictions. Despite good agreement in some redshift and mass intervals, none of the models are able to fully reproduce the observations. This calls for further investigation into the involved physical mechanisms, both theoretically and observationally, especially with the brand-new JWST data.