The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015–2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg² of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

We compare the set of local galaxies having dynamically measured black holes with a large, unbiased sample of galaxies extracted from the Sloan Digital Sky Survey. We confirm earlier work showing that the majority of black hole hosts have significantly higher velocity dispersions σ than local galaxies of similar stellar mass. We use Monte Carlo simulations to illustrate the effect on black hole scaling relations if this bias arises from the requirement that the black hole sphere of influence must be resolved to measure black hole masses with spatially resolved kinematics. We find that this selection effect artificially increases the normalization of the M_bh–σ relation by a factor of at least ∼3; the bias for the M_bh–M_star relation is even larger. Our Monte Carlo simulations and analysis of the residuals from scaling relations both indicate that σ is more fundamental than M_star or effective radius. In particular, the M_bh–M_star relation is mostly a consequence of the M_bh–σ and σ–M_star relations, and is heavily biased by up to a factor of 50 at small masses. This helps resolve the discrepancy between dynamically based black hole–galaxy scaling relations versus those of active galaxies. Our simulations also disfavour broad distributions of black hole masses at fixed σ. Correcting for this bias suggests that the calibration factor used to estimate black hole masses in active galaxies should be reduced to values of f_vir ∼ 1. Black hole mass densities should also be proportionally smaller, perhaps implying significantly higher radiative efficiencies/black hole spins. Reducing black hole masses also reduces the gravitational wave signal expected from black hole mergers.

We investigate the cosmic evolution of the ratio between black hole (BH) mass (M _BH) and host galaxy total stellar mass (M _stellar) out to z ∼ 2.5 for a sample of 100 X-ray-selected moderate-luminosity, broad-line active galactic nuclei (AGNs) in the Chandra-COSMOS Legacy Survey. By taking advantage of the deep multiwavelength photometry and spectroscopy in the COSMOS field, we measure in a uniform way the galaxy total stellar mass using an spectral energy distribution decomposition technique and the BH mass based on broad emission line measurements and single-epoch virial estimates. Our sample of AGN host galaxies has total stellar masses of 10¹⁰⁻¹² M _⊙, and BH masses of 10^7.0–9.5 M _⊙. Combining our sample with the relatively bright AGN samples from the literature, we find no significant evolution of the M _BH–M _stellar relation with the BH-to-host total stellar mass ratio of M _BH/M _stellar ∼ 0.3% at all redshifts probed. We conclude that the average BH-to-host stellar mass ratio appears to be consistent with the local value within the uncertainties, suggesting a lack of evolution of the M _BH–M _stellar relation up to z ∼ 2.5.

Understanding the formation and growth of supermassive black holes (SMBHs) at high redshift represents a major challenge for theoretical models. In this work, we investigate the early evolution of the first SMBHs by constraining their distribution in mass and luminosity at z > 4. In particular, we focus on the poorly explored low-mass end of the nuclear black hole (BH) distribution down to z ≃ 4, and explore its connection with the nature of the first BH seeds and the processes governing their mass growth. To this aim, we have developed CAT (Cosmic Archaeology Tool), a new semi-analytic model that describes the formation of the first stars and BHs in a self-consistent way and follows the co-evolution of nuclear BHs and their host galaxies for a representative population at z > 4. We find that current observational constraints favour models where the growth of BH seeds is Eddington limited and occurs at the Bondi–Hoyle–Lyttleton rate or where super-Eddington accretion occurs via a slim disc during gas-rich galaxy mergers. The main difference between these two model variants lies at the low end of the predicted mass and luminosity functions at 4 ≤ z ≤ 6, where a clear gap appears in the first model, reflecting the stunted growth of light BH seeds formed as remnants of the first stars. Detecting this signature will be extremely challenging even for the future generation of space observatories, such as JWST, Athena, and Lynx.

Early observations with JWST have led to the discovery of an unexpectedly large density (stellar-mass density ρ _* ≈ 10⁶ M _⊙ Mpc⁻³) of massive galaxies (stellar masses M _* ≥ 10^10.5 M _⊙) at extremely high redshifts z ≈ 10. While such a result is based on early measurements that are still affected by uncertainties currently under consideration by several observational groups, its confirmation would have a strong impact on cosmology. Here we show that—under the most conservative assumptions and independently of the baryon physics involved in galaxy formation—such galaxy abundance is not only in tension with the standard ΛCDM cosmology but provides extremely tight constraints on the expansion history of the universe and on the growth factors corresponding to a wide class of Dynamical Dark Energy (DDE) models. Adopting a parameterization w = w ₀ + w _a (1 − a) for the evolution of the DDE equation of the state parameter w with the expansion factor a, we derive constraints on combinations of (w ₀, w _a ) that rule out with confidence level >2σ a major portion of the parameter space (w ₀, w _a ) allowed (or even favored) by existing cosmological probes.