Galaxy protoclusters (PCs) at z > 2 are dense regions in which cold gas availability and elevated galaxy interaction rates trigger intense, often dust-obscured, star formation. These mechanisms are also expected to promote super-massive black hole (SMBH) growth, but this effect remains unclear, largely due to heterogeneous galaxy selections and active galactic nuclei (AGN) identification methods in previous studies. We quantitatively assess the impact of 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 field sample, and investigate the physical mechanisms driving any difference. We consider ALMA-detected DSFGs in sub-mm/mm continuum of seven PCs at 2 < z < 4.5, and construct a selection-matched control sample from the COSMOS survey. We statistically compare X-ray AGN incidence and host galaxy physical properties obtained through uniform spectral energy distribution fitting. We find a significant enhancement of X-ray AGN fraction in PCs by 2.7x (Poisson significance p = 3e-4). Similar values are found in two redshift bins: 2.7x at z = 2-3 (p = 0.003) and 2.6x at z = 3-4.5 (p = 0.03). PC and field DSFG samples are well matched in stellar mass, star-formation rate, and dust mass, ruling out selection effects or systematically higher host masses as the driver. Our results provide quantitative evidence that the dense PC environment enhances AGN incidence and SMBH growth in DSFGs beyond what host galaxy properties alone predict, likely through increased gas availability and interaction-driven fueling. This work is a first step toward a homogeneous assessment of environmental effects on SMBH growth across cosmic time.

We analyse the cluster content of the ∼ 4.5 deg^2 XMM-LSS and CDFS VIDEO fields which are expected to be partially covered by the upcoming MOONRISE survey. Using AMICO and WaZP photometric redshift-based cluster finders, we construct a sample of 519 cluster candidates detected by both finders in the redshift range z = 0.1-3, including 74 detections at z > 1.5. For all detections, we identify the Brightest Central Galaxy (BCG) and compute a list of probabilistic cluster memberships. Our photometric redshift measurements of the clusters agree well with spectroscopic redshifts from the literature, when available. From ancillary spectroscopic data, we assign z_spec measurements to 116 cluster candidates based on their spectroscopic members and to 204 based on their likely BCGs. We also show that candidates containing Radio-Loud members are efficiently recovered using the prior-based cluster finder PPM. We perform a preliminary analysis of the galaxy content of these candidates, focusing on the Red-Sequence components of their apparent Colour-Magnitude Diagram. By comparing with models of galaxy evolution, we show that this population is consistent with a model of passive evolution with a formation at high redshift, and is already in place at z = 1.5-2.0. Finally, our cluster sample is used to evaluate how these clusters would be detected and characterised, according to various MOONRISE strategies. We show that cluster spectroscopic confirmation and characterisation could be efficiently achieved up to z∼1.7 even with the shallowest survey strategy. This open unprecedented insight into the physical properties of high-redshift galaxy clusters and into galaxy formation in dense environments.

Context. Not all galaxies at cosmic noon (2 ≲ z ≲ 3) evolve in the same way. Particularly, it remains unclear how and to what extent the local environment – especially the extreme overdensities of protoclusters – affects the stellar mass assembly of its constituent galaxies at high redshifts. The imprint of these early processes is encoded in the galaxy stellar-mass function (SMF); comparing SMFs across environments therefore reveals differences in evolutionary history. Aims. We present the SMF of the Hyperion proto-supercluster at z ∼ 2.5, one of the largest and most massive protostructures in the early universe. This dataset yields the most statistically robust SMF of a single protostructure at z ≳ 2. By comparing the SMF of the overdense peaks within Hyperion to the coeval field, we begin to answer the question of how early, and how strongly, a dense environment tilts the balance in favor of massive galaxies Methods. Given that Hyperion resides in the field of the Cosmic Evolution Survey (COSMOS), we combined the extensive COSMOS2020 photometric catalog with ground-based spectroscopy and new grism spectroscopy from the Hubble Space Telescope (HST). The structure of Hyperion is defined based on a three-dimensional overdensity map, allowing us to place galaxies into (i) the highly overdense peaks of Hyperion, (ii) the less-overdense outskirts of Hyperion, or (iii) a coeval field. We performed 100 Monte Carlo realizations of the data to propagate redshift and stellar mass uncertainties, refitting galaxy properties in each realization. After constructing SMFs for the outskirts and peaks of Hyperion, we normalized them to that of the field to highlight differences in the underlying shape of the SMFs. Results. The overdense peaks of Hyperion host a striking excess of massive galaxies relative to the field: the number densities of log10(M*/M⊙)∼11 galaxies are ∼10× higher than the coeval field, whereas log10(M*/M⊙)∼9.5 galaxies are enhanced by only ∼3.5×. On the other hand, both the SMF of the outskirts of Hyperion and the SMF of Hyperion as a whole mirror the overall shape of the coeval field. Conclusions. Environmental effects that govern stellar mass growth are already well established by z ∼ 2.5. The densest regions of Hyperion host galaxies that have already experienced accelerated stellar mass growth. Furthermore, this impact is largely masked in the total SMF of Hyperion, highlighting the necessity of deep spectroscopic surveys when uncovering environmental trends at high redshifts. These findings imply that high-redshift protostructures begin sculpting the high-mass end of the SMF well before the epoch when local clusters experience widespread quenching, and may provide the appropriate laboratories for producing the elevated star formation observed at cosmic noon.

We present the Flagship galaxy mock, a simulated catalogue of billions of galaxies designed to support the scientific exploitation of the Euclid mission. Euclid is a medium-class mission of the European Space Agency optimised to determine the properties of dark matter and dark energy on the largest scales of the Universe. It probes structure formation over more than 10 billion years primarily from the combination of weak gravitational lensing and galaxy clustering data. The breath of Euclid's data will also foster a wide variety of scientific analyses. The Flagship simulation was developed to provide a realistic approximation to the galaxies that will be observed by Euclid and used in its scientific analyses. We ran a state-of-the-art N-body simulation with four trillion particles, producing a lightcone on the fly. From the dark matter particles, we produced a catalogue of 16 billion haloes in one octant of the sky in the lightcone up to redshift z=3. We then populated these haloes with mock galaxies using a halo occupation distribution and abundance matching approach, calibrating the free parameters of the galaxy mock against observed correlations and other basic galaxy properties. Modelled galaxy properties include luminosity and flux in several bands, redshifts, positions and velocities, spectral energy distributions, shapes and sizes, stellar masses, star formation rates, metallicities, emission line fluxes, and lensing properties. We selected a final sample of 3.4 billion galaxies with a magnitude cut of H_E<26, where we are complete. We have performed a comprehensive set of validation tests to check the similarity to observational data and theoretical models. In particular, our catalogue is able to closely reproduce the main characteristics of the weak lensing and galaxy clustering samples to be used in the mission's main cosmological analysis. (abridged)

We investigate the environmental dependence of galaxy properties at z ∼ 2.5 using the Ly α Tomography IMACS Survey (LATIS), which provides high-resolution three-dimensional maps of intergalactic medium (IGM) overdensity via Ly α forest tomography. Our analysis focuses on a UV-selected spectroscopic sample of 2185 galaxies from LATIS and a complementary set of 1157 galaxies from heterogeneous spectroscopic surveys in the COSMOS field. We compare these data sets to forward-modeled mock catalogs constructed from the IllustrisTNG300-1 simulation, incorporating realistic selection functions to match both LATIS and the literature sample. While the mass-complete simulation predicts strong environmental trends—more massive and quiescent galaxies preferentially occupy overdense regions—we find that such trends are significantly weaker or absent in the observed samples. The LATIS galaxies show no measurable correlation between specific star formation rate (sSFR) and IGM overdensity, a result reproduced by LATIS-like mock catalogs, confirming that UV selection systematically excludes passive and dusty galaxies in dense environments. The literature compilation, despite improved high-mass coverage, remains incomplete and affected by similar biases. We also analyze a mass-complete photometric sample from the COSMOS-Web catalog at z ∼ 2.5 and find no detectable sSFR–environment relation, a null result that our simulations indicate can be explained by photometric redshift uncertainties. In particular, we find no evidence for a reversal of the sSFR–density relation at cosmic noon. These results demonstrate that observed correlations can be heavily shaped by selection effects and caution against inferring physical trends from incomplete spectroscopic samples. Deeper, more representative spectroscopic surveys are needed to robustly characterize environmental effects at this epoch.

We forecast the expected population of active galactic nuclei (AGN) observable in the Euclid Wide Survey (EWS) and Euclid Deep Survey (EDS). Starting from an X-ray luminosity function (XLF) we generate volume-limited samples of the AGN expected in the survey footprints. Each AGN is assigned an SED appropriate for its X-ray luminosity and redshift, with perturbations sampled from empirical distributions. The photometric detectability of each AGN is assessed via mock observation of the assigned SED. We estimate 40 million AGN will be detectable in at least one band in the EWS and 0.24 million in the EDS, corresponding to surface densities of 2.8$\times$10$^{3}$ deg$^{-2}$ and 4.7$\times$10$^{3}$ deg$^{-2}$. Employing colour selection criteria on our simulated data we select a sample of 4.8$\times$10$^{6}$ (331 deg$^{-2}$) AGN in the EWS and 1.7$\times$10$^{4}$ (346 deg$^{-2}$) in the EDS, amounting to 10% and 8% of the AGN detectable in the EWS and EDS. Including ancillary Rubin/LSST bands improves the completeness and purity of AGN selection. These data roughly double the total number of selected AGN to comprise 21% and 15% of the detectable AGN in the EWS and EDS. The total expected sample of colour-selected AGN contains 6.0$\times$10$^{6}$ (74%) unobscured AGN and 2.1$\times$10$^{6}$ (26%) obscured AGN, covering $0.02 \leq z \lesssim 5.2$ and $43 \leq \log_{10} (L_{bol} / erg s^{-1}) \leq 47$. With this simple colour selection, expected surface densities are already comparable to the yield of modern X-ray and mid-infrared surveys of similar area. The relative uncertainty on our expectation for detectable AGN is 6.7% for the EWS and 12.5% for the EDS, driven by the uncertainty of the XLF.

The Charting Cluster Construction with VUDS and ORELSE (C3VO) survey is an ongoing imaging and spectroscopic campaign aiming to map out the growth of structure up to z similar to 5 and was born from the combination of the Visible Multi-Object Spectrograph Ultra Deep Survey and the Observations of Redshift Evolution in Large-Scale Environments (ORELSE) survey. As we previously accomplished with the ORELSE survey, we apply our technique known as Voronoi tessellation Monte Carlo (VMC) mapping to search for serendipitous galaxy overdensities at 2 < z < 5 in the three C3VO fields. We also apply the same technique to mock observations of simulated galaxies with properties derived from the GAlaxy Evolution and Assembly semianalytic model in order to judge the effectiveness of our search algorithm as a function of redshift, total mass, and fraction of spectroscopic redshifts. We find completeness and purity values of the order of 30%-50% for log(M-z=0/M-circle dot)>14 and 2 < z < 4, with a strong dependence on mass and redshift, with values as high as similar to 80% and similar to 70%, respectively, in the best-case scenario for log(M-z=0/M-circle dot)>14.5. In the C3VO fields, we were able to recover many of the previously known structures in the literature as well as find hundreds of new overdensity candidates, once again demonstrating the powerful capabilities of VMC mapping when applied to wide-field optical and infrared galaxy evolution surveys at ever higher redshifts.

We present the data release of the Ly α Tomography IMACS Survey (LATIS), one of the largest optical spectroscopic surveys of faint high-redshift galaxies. The survey provides 7408 optical spectra of candidate z ∼ 2–3 galaxies and QSOs in the Canada–France–Hawaii Telescope Legacy Survey D1, D2 (COSMOS), and D4 fields. The R ∼ 1000 spectra were obtained using the Inamori Magellan Areal Camera and Spectrograph (IMACS) at the Magellan Baade telescope, with typical integrations of 12 hr. From these spectra, we measured 5575 high-confidence spectroscopic redshifts, of which 4176 are at z > 1.7, thereby substantially increasing the number of public spectroscopic redshifts at z ≈ 2–3 in COSMOS and the other survey fields. The data release includes Ly α transmission fluctuations measured in 4.7 × 10 ^5 pixels, which were used to create 3D maps of the intergalactic medium (IGM) transmission spanning 1.65 deg ^2 and z = 2.2–2.8 at a resolution of 4 h ^−1 cMpc. These are the largest such maps to date and provide a novel tracer of large-scale structure in legacy fields. We also provide ancillary data, including mock surveys. The LATIS data will enable a variety of community studies of galaxy evolution, environments, and the IGM around cosmic noon.

The Near-Infrared Spectrometer and Photometer (NISP) on board the satellite provides multiband photometry and $R slitless grism spectroscopy in the 950--2020\,nm wavelength range. In this reference article, we illuminate the background of NISP's functional and calibration requirements, describe the instrument's integral components, and provide all its key properties. We also sketch the processes needed to understand how NISP operates and is calibrated as well as its technical potentials and limitations. Links to articles providing more details and the technical background are included. The NISP's 16 H2RG detectors with a plate scale of $ deliver a field of view of 0.57\,deg$^2$. In photometric mode, NISP reaches a limiting magnitude of sim \,24.5\,AB\,mag in three photometric exposures of about 100\,s in exposure time for point sources and with a S/N of five. For spectroscopy, NISP's point-source sensitivity is a SNR = 3.5 detection of an emission line with flux sim \,$2 $ integrated over two resolution elements of 13.4\ in 3times 560\,s grism exposures at 1.6\ (redshifted Halpha ). Our calibration includes on-ground and in-flight characterisation and monitoring of the pixel-based detector baseline, dark current, non-linearity, and sensitivity to guarantee a relative photometric accuracy better than 1.5 and a relative spectrophotometry better than 0.7. The wavelength calibration must be accurate to 5\ or better. The NISP is the state-of-the-art instrument in the near-infrared for all science beyond small areas available from HST and JWST -- and it represents an enormous advance from any existing instrumentation due to its combination of field size and high throughput of telescope and instrument. During six-year survey covering 14\,000\,deg$^2$ of extragalactic sky, NISP will be the backbone in determining distances of more than a billion galaxies. Its near-infrared data will become a rich reference imaging and spectroscopy data set for the coming decades.

The Ly α Tomography IMACS Survey (LATIS) has produced large 3D maps of the intergalactic medium (IGM), providing a new window on the cosmic web at z ∼ 2.5. A key advantage of Ly α tomography is that it enables the discovery of overdense regions without the need to detect their galaxy members in spectroscopic surveys, circumventing possible selection biases. We use these maps to identify 37 IGM-selected overdensities as regions of strong and spatially coherent Ly α absorption. Simulations indicate that 85% of these are protoclusters, defined as the progenitors of z = 0 halos with mass M _desc > 10 ^14 M _⊙ , and that nearly all of the rest are protogroups (10 ^13.5 < M _desc / M _⊙ < 10 ^14 ). We estimate the masses and space densities of the IGM-selected overdensities and show they are in accordance with mock surveys. We investigate the LATIS counterparts of some previously reported protoclusters, including the proto-supercluster Hyperion. We identify a new component of Hyperion beyond its previously known extent. We show that the Ly α transmission of the galaxy density peaks within Hyperion is consistent with a simple physical model (the fluctuating Gunn–Peterson approximation), suggesting that active galactic nucleus feedback or other processes have not affected the large-scale gas ionization within this structure as a whole. The LATIS catalog represents an order-of-magnitude increase in the number of IGM-selected protogroups and protoclusters and will enable new investigations of the connections between galaxies and their large-scale environments at cosmic noon.

The near-infrared calibration unit (NI-CU) on board NISP is the first astronomical calibration lamp based on LED to be operated in space. is a mission in ESA's Cosmic Vision 2015--2025 framework to explore the dark universe and provide a next-level characterisation of the nature of gravitation, dark matter, and dark energy. Calibrating photometric and spectrometric measurements of galaxies to better than 1.5 accuracy in a survey homogeneously mapping sim \,14\,000\,deg$^2$ of extragalactic sky requires a very detailed characterisation of NIR detector properties as well as constant monitoring of them in flight. To cover two of the main contributions -- relative pixel-to-pixel sensitivity and non-linearity characteristics -- and to support other calibration activities, NI-CU was designed to provide spatially approximately homogeneous ($<$\,12 variations) and temporally stable illumination (0.1--0.2 over 1200\,s) over the NISP detector plane with minimal power consumption and energy dissipation. NI-CU covers the spectral range sim \,nm -- at cryo-operating temperature -- at five fixed independent wavelengths to capture wavelength-dependent behaviour of the detectors, with fluence over a dynamic range of gtrsim \,100 from sim $. For this functionality, NI-CU is based on LED . We describe the rationale behind the decision and design process, the challenges in sourcing the right LED and the qualification process and lessons learned. We also provide a description of the completed NI-CU, its capabilities, and performance as well as its limits. NI-CU has been integrated into NISP and the satellite, and since launch in July 2023, it has started supporting survey operations.

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 deg^2 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 present the first results and catalogs from the HST-Hyperion survey. This survey has collected 50 orbits of WFC3/F160W imaging and WFC3/G141 grism spectroscopy in the most overdense regions of the Hyperion protosupercluster at z ∼ 2.45, which are analyzed in conjunction with the adjacent 56 orbits of WFC3/F140W imaging and WFC3/G141 grism spectroscopy from the 3D-HST survey. Sources were identified and spectra were extracted using grizli , which subsequently fit the combined grism data with object-matched photometric data from the COSMOS2020 catalog to obtain a redshift and best-fit spectral model. Each source was then visually inspected by multiple team members and quality flags were assigned. A total of 12,814 objects with m _HST ≤ 25.0 were inspected, of which 5629 (44%) have reliable redshifts from the grism data and are sensitive to emission lines at a level of ∼8.8 × 10 ^−18 erg s ^−1 cm ^−2 (1 σ ). A comparison with high-quality, ground-based spectroscopic redshifts yields a scatter of σ _NMAD = 0.0016. The resulting catalogs contain 125 confirmed members of the Hyperion structure within 2.40 < z < 2.53, with an additional 71 confirmed galaxies in projection within 2.35 < z < 2.65. The redshift, stellar population, and line flux catalogs, as well as all grism spectra, are publicly available.

We discuss the properties of a sample of z>6 bright (bolometric luminosity L_ bolo>10^46.25 erg/s) Quasars drawn from a realization of the GAlaxy Evolution and Assembly (GAEA) model coupled with the Planck Millennium Simulation. We focus on the properties and environment of host galaxies, and their evolution down to z=0, with the aim of assessing how well the bright high redshift QSOs population traces the progenitors of most massive haloes in the local Universe. Our results show that at z>6 bright QSOs live in a variety of environments, and that secular processes like disc instability are responsible for triggering roughly the same number of QSOs as galaxy mergers. Half of cubic (7.5 h^-1 cMpc size) mock fields built around these high-z QSOs include other active galaxies (with L_ bolo>10^44 erg/s) in sizeable number, the other host galaxies being relatively isolated. The large field-to-field variance in the the number of companions (both active and non-active) recently reported from JWST observations is fairly well reproduced by GAEA predictions. Descendants of host galaxies at z=0 cover a wide range of physical properties and environments with only a small fraction of the hosts of high-z QSOs ending up in massive galaxy clusters. Viceversa, GAEA predicts that only a small fraction of Bright Central Galaxies have a bright z>6 QSOs among their progenitors. Our results suggest that luminous high-z QSO loosely trace the progenitors of low-z galaxy clusters, and that additional information about the environment of high-z QSOs are required to identify the most promising proto-cluster candidates.

We investigate the consistency of intergalactic medium (IGM) tomography and galaxy surveys as tracers of the cosmic web and protoclusters at z ∼ 2.5. We use maps from the Ly α Tomography IMACS Survey (LATIS), which trace the distributions of Lyman-break galaxies (LBGs) and IGM Ly α absorption on ≃4 h ^−1 cMpc scales within the same large volume. Overall, the joint distribution of IGM absorption and LBG density is well constrained and accurately described by a simple physical model. However, we identify several exceptional locations exhibiting strong IGM absorption indicative of a massive protocluster, yet no coincident overdensity of LBGs. As discussed by Newman et al., whose results we revise using the complete LATIS survey data, these are candidate ultraviolet (UV)-dim protoclusters that may harbor distinct galaxy populations missed by rest-UV spectroscopic surveys. We present follow-up observations targeting one such candidate embedded within Antu, an extended region of IGM absorption at z = 2.685 that contains five IGM-selected protoclusters and has a total mass of 3 × 10 ^15 M _⊙ . Ly α emitters trace the overall structure of Antu but avoid the center of the candidate UV-dim protocluster, which also appears to contain no submillimeter-selected sources. A near-infrared spectroscopic galaxy census is needed to determine whether this large region is dominated by galaxies with reduced or absent star formation activity. This work adds to a growing and puzzling literature on discrepancies among different galaxy and IGM tracers, whose resolution promises to shed light on the early stages of environment-dependent galaxy evolution.

We carried out a study on the relationship between galaxy environments and their active galactic nucleus (AGN) activity at high redshifts (2.0 < z < 4.0). Specifically, we studied the AGN fraction in galaxies residing in a range of environments at these redshifts, from field galaxies to the densest regions of highly overdense peaks in the GOODS-S extragalactic field. Utilizing the extensive photometric and spectroscopic observations in this field, we measured the local overdensities ( σ δ ) and global overdensities across a broad a range of environments, including those in massive (M tot ≥ 10 14.8 M ⊙ ) protostructures. We employed a multiwavelength AGN catalog consisting of AGNs in nine different categories. Our analysis shows a higher AGN fraction (10.9 −2.3 +3.6 %) for galaxies in the highest local overdensity regions compared to the AGN fraction (1.9 −0.3 +0.4 %) in the corresponding coeval field galaxies (a ∼4 σ difference). This trend of increasing AGN fraction in denser environments relative to the field is present in all redshift bins. We also find this trend to be consistent across all five AGN categories that have a sufficient number of AGNs to make a meaningful comparison: the mid-infrared (MIR) spectral energy distribution (SED), MIR color, X-ray luminosity, X-ray-luminosity-to-radio-luminosity-ratio, and optical-spectroscopy. Our results also demonstrate a clear trend for higher (∼4×) AGN fractions in denser local overdensity environments for a given stellar mass. Additionally, we observe the same trend (though at a lower significance) with the global environment of galaxies, measured using a metric based on the projected distance of galaxies from their nearest massive ( M tot > 10 12.8 M ⊙ ) overdense ( σ δ > 5.0) peak, normalized with respect to the size of the peak. These findings indicate that the prevalence of AGN activity is highly dependent on the environment a host galaxy resides in, even at early times in the formation history of the Universe.

This paper presents the specification, design, and development of the Visible Camera (VIS) on the ESA Euclid mission. VIS is a large optical-band imager with a field of view of 0.54 deg^2 sampled at 0.1" with an array of 609 Megapixels and spatial resolution of 0.18". It will be used to survey approximately 14,000 deg^2 of extragalactic sky to measure the distortion of galaxies in the redshift range z=0.1-1.5 resulting from weak gravitational lensing, one of the two principal cosmology probes of Euclid. With photometric redshifts, the distribution of dark matter can be mapped in three dimensions, and, from how this has changed with look-back time, the nature of dark energy and theories of gravity can be constrained. The entire VIS focal plane will be transmitted to provide the largest images of the Universe from space to date, reaching m_AB>24.5 with S/N >10 in a single broad I_E~(r+i+z) band over a six year survey. The particularly challenging aspects of the instrument are the control and calibration of observational biases, which lead to stringent performance requirements and calibration regimes. With its combination of spatial resolution, calibration knowledge, depth, and area covering most of the extra-Galactic sky, VIS will also provide a legacy data set for many other fields. This paper discusses the rationale behind the VIS concept and describes the instrument design and development before reporting the pre-launch performance derived from ground calibrations and brief results from the in-orbit commissioning. VIS should reach fainter than m_AB=25 with S/N>10 for galaxies of full-width half-maximum of 0.3" in a 1.3" diameter aperture over the Wide Survey, and m_AB>26.4 for a Deep Survey that will cover more than 50 deg^2. The paper also describes how VIS works with the other Euclid components of survey, telescope, and science data processing to extract the cosmological information.

Galaxy proto-clusters are receiving an increased interest since most of the processes shaping the structure of clusters of galaxies and their galaxy population are happening at early stages of their formation. The Euclid Survey will provide a unique opportunity to discover a large number of proto-clusters over a large fraction of the sky (14 500 square degrees). In this paper, we explore the expected observational properties of proto-clusters in the Euclid Wide Survey by means of theoretical models and simulations. We provide an overview of the predicted proto-cluster extent, galaxy density profiles, mass-richness relations, abundance, and sky-filling as a function of redshift. Useful analytical approximations for the functions of these properties are provided. The focus is on the redshift range z= 1.5 to 4. We discuss in particular the density contrast with which proto-clusters can be observed against the background in the galaxy distribution if photometric galaxy redshifts are used as supplied by the ESA Euclid mission together with the ground-based photometric surveys. We show that the obtainable detection significance is sufficient to find large numbers of interesting proto-cluster candidates. For quantitative studies, additional spectroscopic follow-up is required to confirm the proto-clusters and establish their richness.

The WHT Enhanced Area Velocity Explorer (WEAVE) is a new, massively multiplexing spectrograph. This new instrument will be exploited to obtain high S/N spectra of $\sim$25000 galaxies at intermediate redshifts for the WEAVE Stellar Population Survey (WEAVE-StePS). We test machine learning methods for retrieving the key physical parameters of galaxies from WEAVE-StePS-like spectra using both photometric and spectroscopic information at various S/Ns and redshifts. We simulated $\sim$105000 galaxy spectra assuming SFH with an exponentially declining star formation rate, covering a wide range of ages, stellar metallicities, sSFRs, and dust extinctions. We then evaluated the ability of the random forest and KNN algorithms to correctly predict such parameters assuming no measurement errors. We checked how much the predictive ability deteriorates for different S/Ns and redshifts, finding that both algorithms still accurately estimate the ages and metallicities with low bias. The dispersion varies from 0.08-0.16 dex for ages and 0.11-0.25 dex for metallicity, depending on the redshift and S/N. For dust attenuation, we find a similarly low bias and dispersion. For the sSFR, we find a very good constraining power for star-forming galaxies, log sSFR$\gtrsim$ -11, where the bias is $\sim$ 0.01 dex and the dispersion is $\sim$ 0.10 dex. For more quiescent galaxies, with log sSFR$\lesssim$ -11, we find a higher bias, 0.61-0.86 dex, and a higher dispersion, $\sim$ 0.4 dex, for different S/Ns and redshifts. Generally, we find that the RF outperforms the KNN. Finally, the retrieved sSFR was used to successfully classify galaxies as part of the blue cloud, green valley, or red sequence. We demonstrate that machine learning algorithms can accurately estimate the physical parameters of simulated galaxies even at relatively low S/N=10 per angstrom spectra with available ancillary photometric information.

Context. How the quiescent galaxies evolve with redshift and the factors that impact their evolution are still debated. It is still unclear what the dominant mechanisms of passive galaxy growth are and what role is played by the environment in shaping their evolutionary paths over cosmic time. Aims. The population of quiescent galaxies is altered over time by several processes that can affect their mean properties. Our aim is to study the mass-size relation (MSR) of the quiescent population and to understand how the environment shapes the MSR at intermediate redshift. Methods. We used the VIMOS Public Extragalactic Redshift Survey (VIPERS), a large spectroscopic survey of similar to 90 000 galaxies in the redshift range 0.5 <= z <= 1.2. We selected a mass-complete sample of 4786 passive galaxies based on the NUVrK diagram and refined it using the D(n)4000 spectral index to study the MSR of the passive population over 0.5 <= z <= 0.9. The impact of the environment on the MSR and on the growth of the quiescent population is studied through the density contrast. Results. The slope and the intercept of the MSR, alpha = 0.62 +/- 0.04 and log(A) = 0.52 +/- 0.01, agree well with values from the literature at the same redshift. The intercept decreases with redshift, R-e(z) = 8.20 x (1 + z)(-1.70), while the slope remains roughly constant, and the same trend is observed in the low-density (LD) and high-density (HD) environments. Thanks to the largest spectroscopic sample at 0.5 <= z <= 0.9, these results are not prone to redshift uncertainties from photometric measurements. We find that the average size of the quiescent population in the LD and HD environments are identical within 3 sigma and this result is robust against a change in the definition of the LD and HD environments or a change in the selection of quiescent galaxies. In the LD and HD environments, similar to 30 and similar to 40% of the population have experienced a minor merger process between 0.5 <= z <= 0.9. However, minor mergers account only for 30-40% of the size evolution in this redshift range, the remaining evolution likely being due to the progenitor bias.