We introduce a suite of Lambda cold dark matter (ACDM) cosmological, hydrodynamical simulations that track the evolution of a large population of dwarf galaxies. The suite comprises zoom-in simulations of 25 spherical, underdense regions of r = 5 cMpc, selected to span approximate to 1 . 5 dex in mean enclosed density, covering voids to filamentary structures, whilst excluding haloes of Milky Way-mass or larger. The simulations achieve a mass resolution of similar to 10(5) M-circle dot with a galaxy formation model including cold, dense interstellar gas and whose subgrid stellar feedback efficiency reproduces the z = 0 galaxy stellar mass function. We investigate the impact of the cosmic environment on dwarf galaxy formation and evolution. We find that the 5 cMpc environment influences the normalization of the halo and galaxy mass functions, but does not significantly affect the stellar mass-halo mass (SMHM) relation and halo occupation fraction for galaxies with M* = 10(6)-10(9)M(circle dot). Instead, host halo concentration, estimated from DM-only counterparts, is more important: both the fraction of haloes hosting a resolved galaxy and the scatter about the SMHM relation correlate positively with concentration. Owing to halo assembly bias, concentration also influences galaxy formation times, such that at fixed halo mass more concentrated haloes host galaxies that are both older and more massive. The offset from the mean SMHM relation also anticorrelates with t90 , the time at which 90 per cent of a galaxy's stellar mass has assembled. These correlations between halo properties and galaxy star formation histories present testable predictions for forthcoming observational surveys.

The tidal stripping of satellite galaxies and the stellar detritus ejected during galaxy mergers builds up a diffuse stellar component in galaxy clusters known as the intracluster light (ICL). We investigate ICL assembly in cluster-mass haloes (M_178c∼10^14-10^15 M_⊙) using four different hydrodynamical simulations (Horizon-AGN, TNG100, The Three Hundred Gizmo-Simba 7K, and Hydrangea) under a homogenized ICL identification framework. For our fiducial ICL definition we obtain broadly consistent z≈0 ICL stellar mass fractions (∼0.1-0.2) and, by tracking the progenitors of z≈0 clusters back to z≳2, find no significant evolution in average ICL mass fractions. Alternative approaches for distinguishing the ICL from the central galaxy show the absolute ICL fraction to be highly sensitive to adopted definition, but we never find any significant inter-simulation discrepancies when implementing a consistent methodology to identify the ICL. Whether the average ICL mass fraction falls with increasing redshift or does not evolve is determined by the ICL definition adopted. By tracing z≈0 ICL stars back to their progenitor galaxies, we find that lower-mass satellites typically make slightly larger ICL contributions relative to their mass in every considered simulation, but which galaxies make the dominant contribution to the ICL is primarily controlled by the infalling satellite mass function. Most ICL stars sourced from satellite galaxies are therefore expected to originate from galaxies with infall stellar masses above ∼10^10 M_⊙ and largely within 10^10.5-10^11.5 M_⊙.

We present the calibration of stellar and active galactic nucleus (AGN) feedback in the subgrid model for the new COLIBRE hydrodynamical simulations of galaxy formation. COLIBRE directly simulates the multiphase interstellar medium and the evolution of dust grains, which is coupled to the chemistry. COLIBRE is calibrated at three resolutions: particle masses of m(gas )approximate to m(dm) similar to 10(7 )(m7), 10(6) (m6), and 10(5) M-circle dot (m5). To calibrate the COLIBRE feedback at m7 resolution, we run Latin hypercubes of approximate to 200 simulations that vary up to four subgrid parameters in cosmological volumes of ( 50 cMpc)(3). We train Gaussian process emulators on these simulations to predict the z = 0 galaxy stellar mass function (GSMF) and size-stellar mass relation (SSMR) as functions of the model parameters, which we then fit to observations. The trained emulators not only provide the best-fitting parameter values but also enable us to investigate how different aspects of the prescriptions for supernova and AGN feedback affect the predictions. In particular, we demonstrate that while the observed z = 0 GSMF and SSMR can be matched individually with a relatively simple supernova feedback model, simultaneously reproducing both necessitates a more sophisticated prescription. We show that the calibrated m7 COLIBRE model not only reproduces the calibration target observables, but also matches various other galaxy properties to which the model was not calibrated. Finally, we apply the calibrated m7 model to the m6 and m5 resolutions and, after slight manual adjustments of the subgrid parameters, achieve a similar level of agreement with the observed z = 0 GSMF and SSMR.

We investigate whether the intracluster light (ICL) can serve as a reliable tracer of the shape of the underlying dark matter (DM) haloes in galaxy clusters. Using the cosmological Hydrangea cluster simulations, we measure the 3D and projected shapes of both components with a shape tensor computed in concentric ellipsoidal shells, out to the virial radius R200c for each cluster. The ICL and DM are closely aligned, with their major axes typically offset from each other by 10 deg. Their axis ratios also match closely, with a typical difference of only approximate to 0.07 for both the major-to-minor and major-to-intermediate axes, the DM being slightly rounder than the ICL. These trends are consistent across 2D and 3D measurements and agree well with results from isophotal fitting of mock images. In detail, the axis ratio offset is sensitive to the method used to remove satellites, and may also depend on the choice of subgrid physics models. We demonstrate that the ICL traces the DM shape better than the distribution of satellite galaxies, which exhibits larger scatter in the axis ratio and misalignment angle and is overall more elliptical. Together, these results indicate that the ICL can act as a useful proxy for DM halo ellipticity and orientatio

We present the COLIBRE galaxy formation model and the COLIBRE suite of cosmological hydrodynamical simulations. COLIBRE includes new models for radiative cooling, dust grains, star formation, stellar mass loss, turbulent diffusion, pre-supernova stellar feedback, supernova feedback, supermassive black holes, and active galactic nucleus (AGN) feedback. The multiphase interstellar medium is explicitly modelled without a pressure floor. Hydrogen and helium are tracked in non-equilibrium, with their contributions to the free electron density included in metal-line cooling calculations. The chemical network is coupled to a dust model that tracks three grain species and two grain sizes. In addition to the fiducial thermally driven AGN feedback, a subset of simulations uses black hole spin-dependent hybrid jet/thermal AGN feedback. To suppress spurious transfer of energy from dark matter to stars, dark matter is supersampled by a factor 4, yielding similar dark matter and baryonic particle masses. The subgrid feedback model is calibrated to match the observed z approximate to 0 galaxy stellar mass function, galaxy sizes, and black hole masses in massive galaxies. The COLIBRE suite includes three resolutions, with particle masses of similar to 10(5), 10(6) , and 10(7)M(circle star) in cubic volumes of up to 100, 200, and 400 cMpc on a side, respectively. The largest runs use 136 billion ( 5 & times; 3008(3) ) particles. We describe the model, assess its strengths and limitations, and present both visual impressions and quantitative results. Comparisons with various low-redshift galaxy observations generally show very good numerical convergence and excellent agreement with the data.

We investigate the correlation between star formation rate (SFR) surface density and gas surface density [known as the Kennicutt-Schmidt (KS) relation] at kiloparsec (kpc) scales across cosmic time ( 0 <= z <= 8 ) for galaxies with stellar masses > 10(9) M-circle dot, using the COLIBRE state-of-the-art cosmological hydrodynamical simulations. These simulations feature on-the-fly non-equilibrium chemistry coupled to dust grain evolution and detailed radiative cooling down to ti 10 K, enabling direct predictions for the atomic (Hi) and molecular (H-2 ) KS relations. At z approximate to 0 , COLIBRE reproduces the observed (spatially resolved) KS relations for Hi and H-2 , including the associated scatter, which we predict to be significantly correlated with stellar surface density, local specific SFR (sSFR), and gas metallicity. We show that the Hi KS relation steepens for lower-mass galaxies, while the H2 KS relation shifts to higher normalization in galaxies with higher sSFRs. The H2 depletion time decreases by a factor of approximate to 20 from z = 0 to z = 8 , primarily due to the decreasing gas-phase metallicity. This results in less H2 and more H i being associated with a given SFR at higher redshift. We also find that galaxies with higher sSFRs have a larger molecular gas content and higher star formation efficiency per unit gas mass on kpc scales. The predicted evolution of the H-2 depletion time and its correlation with a galaxy's sSFR agree remarkably well with observations in a wide redshift range, 0 <= z <= 5 .

Recent theoretical work and targeted observational studies suggest that filaments are sites of galaxy preprocessing. The aim of the WISESize project is to directly probe galaxies over the full range of environments to quantify and characterize extrinsic galaxy quenching in the local universe. In this paper, we use GALFIT to measure the IR 12 mu m (R 12) and 3.4 mu m (R 3.4) effective radii of 603 late-type galaxies in and surrounding the Virgo cluster. We find that Virgo cluster galaxies show smaller star-forming disks relative to their field counterparts at the 2.5 sigma level, while filament galaxies show smaller star-forming disks to almost 1.5 sigma. Our data, therefore, show that cluster galaxies experience significant effects on their star-forming disks prior to their final quenching period. There is also tentative support for the hypothesis that galaxies are preprocessed in filamentary regions surrounding clusters. On the other hand, galaxies belonging to rich groups and poor groups do not differ significantly from those in the field. We additionally find hints of a positive correlation between stellar mass and size ratio for both rich group and filament galaxies, though the uncertainties on these data are consistent with no correlation. We compare our size measurements with the predictions from two variants of a state-of-the-art semi-analytic model (SAM), one which includes starvation and the other incorporating both starvation and ram pressure stripping (RPS). Our data appear to disfavor the SAM, which includes RPS for the rich group, filament, and cluster samples, which contributes to improved constraints for general models of galaxy quenching.

As the environment harbouring the majority of galaxies, filaments are thought to play a key role in the co-evolution of galaxies and the cosmic web. In this first part of a series to understand the link between galaxies and filaments through cosmological simulations, we address two major current obstacles on this path: the difficulty of a meaningful filament identification, and their poorly constrained properties and internal structure. We use the public EAGLE and TNG100 simulations to build physically motivated filament catalogues with the DisPerSE algorithm based on the dark matter (DM) field at redshifts z = 0 and z = 2, explicitly accounting for the multi-scale nature of filaments and carefully validating our results. Filament widths, lengths, and densities vary by factors of ≈5−100 in both simulations, highlighting the heterogeneous nature of filaments as a cosmic environment. All filaments are relatively thin, with overdensity profiles of galaxies, DM, and gas dropping to the cosmic mean within at most ≈3 Mpc from their spines. In contrast with groups and clusters, filament cores are highly substructure dominated, by as much as ≈80%. Filament gas maps reveal rich temperature and density structures that limit the applicability of simple cylindrically symmetric models. The EAGLE and TNG100 simulations both show that z = 2 filament spines are traced by overdense cool gas in pressure equilibrium with a > 10 times hotter envelope. However, significant differences in the details between their predicted gas property maps imply that individual simulations cannot yet describe the baryon structure of filaments with certainty. Finally, we compare our fiducial filament network to one constructed from galaxies. The two differ in many aspects, but the distance of a galaxy to its nearest galaxy-based filament still serves as a statistical proxy for its true environment.

Self-interacting dark matter (SIDM) has the potential to significantly influence galaxy formation in comparison to the cold, collisionless dark matter paradigm (CDM), resulting in observable effects. This study aims to elucidate this influence and to demonstrate that the stellar mass Tully-Fisher relation imposes robust constraints on the parameter space of velocity-dependent SIDM models. We present a new set of cosmological hydrodynamical simulations that include the SIDM scheme from the TangoSIDM project and the SWIFT-EAGLE galaxy formation model. Two cosmological simulations suites were generated: one (Reference model) which yields good agreement with the observed $z=0$ galaxy stellar mass function, galaxy mass-size relation, and stellar-to-halo mass relation; and another (WeakStellarFB model) in which the stellar feedback is less efficient, particularly for Milky Way-like systems. Both galaxy formation models were simulated under four dark matter cosmologies: CDM, SIDM with two different velocity-dependent cross sections, and SIDM with a constant cross section. While SIDM does not modify global galaxy properties such as stellar masses and star formation rates, it does make the galaxies more extended. In Milky Way-like galaxies, where baryons dominate the central gravitational potential, SIDM thermalises, causing dark matter to accumulate in the central regions. This accumulation results in density profiles that are steeper than those produced in CDM from adiabatic contraction. The enhanced dark matter density in the central regions of galaxies causes a deviation in the slope of the Tully-Fisher relation, which significantly diverges from the observational data. In contrast, the Tully-Fisher relation derived from CDM models aligns well with observations.

Context. As the most massive nodes of the cosmic web, galaxy clusters represent the best probes of structure formation. Over time, they grow by accreting and disrupting satellite galaxies, adding those stars to the brightest cluster galaxy (BCG) and the intra-cluster light (ICL). However, the formation pathways of galaxy clusters can vary significantly. Aims. To inform upcoming large surveys, we aim to identify observables that can distinguish galaxy cluster formation pathways. Methods. Using four different hydrodynamical simulations, Magneticum, TNG100 of IllustrisTNG, Horizon-AGN, and Hydrangea, we studied how the fraction of stellar mass in the BCG and ICL (f(ICL + BCG)) relates to the galaxy cluster mass assembly history. Results. For all simulations, f(ICL + BCG) is the best tracer for the time at which the cluster has accumulated 50% of its mass (z(form)), performing better than other typical dynamical tracers, such as the subhalo mass fraction, the halo mass, and the position offset of the cluster mass barycenter to the BCG. More relaxed clusters have a higher f(ICL + BCG), in rare cases up to 90% of all stellar mass, while dynamically active clusters have lower fractions, down to 20%, which we find to be independent of the exact implemented baryonic physics. We determine the average increase in f(ICL + BCG) from stripping and mergers to be between 3-4% per gigayear. Furthermore, f(ICL + BCG) is tightly traced by the stellar mass ratio between the BCG and both the second (M12) and fourth (M14) most massive cluster galaxy. The average galaxy cluster has assembled half of its halo mass by z(form) = 0.67 (about 6 gigayears ago), though individual histories vary significantly from z(form) = 0.06 to z(form) = 1.77 (0.8-10 gigayears ago). Conclusions. As all four cosmological simulations consistently find that f(ICL + BCG) is an excellent tracer of the cluster dynamical state, upcoming surveys can leverage measurements of f(ICL + BCG) to statistically quantify the assembly of the most massive structures through cosmic time.

We introduce the Bias-free Extragalactic Analysis for Cosmic Origins with NIRCam (BEACON) survey, a JWST Cycle 2 program allocated up to 600 pure-parallel hours of observations. BEACON explores high-latitude areas of the sky with JWST/NIRCam over ∼100 independent sight lines, totaling ∼0.3 deg ^2 , reaching a median F444W depth of ≈28.2 AB mag (5 σ ). Based on existing JWST observations in legacy fields, we estimate that BEACON will photometrically identify 25–150 galaxies at z > 10 and 500–1000 at z ∼ 7–10 uniquely enabled by an efficient multiple filter configuration spanning 0.9–5.0 μ m. The expected sample size of z > 10 galaxies will allow us to obtain robust number density estimates and to discriminate between different models of early star formation. In this paper, we present an overview of the survey design and initial results using the first 19 fields. We present 129 galaxy candidates at z ≳7 identified in those fields, including 11 galaxies at z ≳10 and several UV-luminous ( M _UV < −21 mag) galaxies at z ∼ 8. The number densities of z < 13 galaxies inferred from the initial fields are overall consistent with those in the literature. Despite reaching a considerably large volume (∼10 ^5 Mpc ^3 ), however, we find no galaxy candidates at z > 13, providing us with a complimentary insight into early galaxy evolution with minimal cosmic variance. We publish imaging and catalog data products for these initial fields. Upon survey completion, all BEACON data will be coherently processed and distributed to the community along with catalogs for redshift and other physical quantities.

We develop and present the Descriptive Parametric Model (DPM), a tool for generating profiles of gaseous haloes (pressure, electron density, and metallicity) as functions of radius, halo mass, and redshift. The model assumes single-phase, spherically symmetric, volume-filling warm/hot gas. The DPM framework enables mock observations of the circumgalactic medium (CGM), group haloes, and clusters across a number of wavebands including X-ray, sub-millimetre/millimetre, radio, and ultraviolet (UV). We introduce three model families calibrated to reproduce cluster profiles while having different extrapolations to the CGM-(i) self-similar haloes, (ii) a reduced gas model for lower halo masses, and (iii) a model with shallower radial slopes at lower masses. We demonstrate how our z = 0 . 0-0 . 6 models perform when applied to stacked and individual X-ray emission profiles, measurements of the thermal and kinetic Sunyaev-Zel'dovich effect, electron dispersion measures from fast radio bursts, O VI absorption, and UV-derived pressures. Our investigation supports models that remove baryons from haloes more effectively and have shallower profiles at lower halo mass. We discuss biases and systematics when modelling observables using consistent hot gaseous halo models for all wavebands explored. We release the DPMHALO code to encourage the use of our framework and new formulations in future investigations. Included with the DPMHALO distribution is a set of recent observations that allow the reproduction of most plots in this paper.

Cosmological simulations are essential for inferring cosmological and galaxy population properties based on forward-modelling, but this typically requires finding the population of (sub)haloes and galaxies that they contain. The properties of said populations vary depending on the algorithm used to find them, which is concerning as it may bias key statistics. We compare how the predicted (sub)halo mass functions, satellite radial distributions, and correlation functions vary across algorithms in the dark-matter-only and hydrodynamical versions of the FLAMINGO simulations. We test three representative approaches to finding subhaloes: grouping particles in configuration- (SUBFIND), phase- (ROCKSTAR and VELOCIRAPTOR) and history-space (HBT-HERONS). We also present HBT-herons, a new version of the HBT+ SUBHALO finder that improves the tracking of subhaloes. We find 10 per cent-level differences in the M-200c mass function, reflecting different field halo definitions and occasional miscentring. The bound mass functions can differ by 75 per cent at the high mass end, even when using the maximum circular velocity as a mass proxy. The number of well-resolved subhaloes differs by up to 20 per cent near R-200c, reflecting differences in the assignment of mass to subhaloes and their identification. The predictions of different subhalo finders increasingly diverge towards the centres of the host haloes. The performance of most subhalo finders does not improve with the resolution of the simulation and is worse for hydrodynamical than for dark-matter-only simulations. We conclude that HBT-HERONS is the preferred choice of subhalo finder due to its low computational cost, self-consistently made and robust merger trees, and robust subhalo identification capabilities.

As the environment harbouring the majority of galaxies, filaments are thought to play a key role in the co-evolution of galaxies and the cosmic web. In this first part of a series to understand the link between galaxies and filaments through cosmological simulations, we address two major current obstacles on this path: the difficulty of meaningful filament identification, and their poorly constrained properties and internal structure. We use the public EAGLE and TNG100 simulations to build physically motivated filament catalogues with the DisPerSE algorithm, based on the dark matter (DM) field at redshift z = 0 and z = 2, explicitly accounting for the multi-scale nature of filaments and with careful validation of results. Filament widths, lengths, and densities vary by factors ~5-100 in both simulations, highlighting the heterogeneous nature of filaments as a cosmic environment. All filaments are relatively thin, with overdensity profiles of galaxies, DM, and gas dropping to the cosmic mean within <3 Mpc from their spines. Contrary to groups and clusters, filament cores are highly substructure dominated, by as much as ~80 per cent. Filament gas maps reveal rich temperature and density structures that limit the applicability of simple cylindrically symmetric models. EAGLE and TNG100 agree that z = 2 filament spines are traced by overdense cool gas in pressure equilibrium with a >10x hotter envelope. However, significant differences in detail between their predicted gas property maps imply that individual simulations cannot yet describe the baryon structure of filaments with certainty. Finally, we compare our fiducial filament network to one constructed from galaxies. The two differ in many aspects, but the distance of a galaxy to its nearest galaxy-based filament still serves as a statistical proxy for its true environment.

ABSTRACT The warm-hot plasma in cosmic web filaments is thought to comprise a large fraction of the gas in the local Universe. So far, the search for this gas has focused on mapping its emission, or detecting its absorption signatures against bright, point-like sources. Future, non-dispersive, high-spectral resolution X-ray detectors will, for the first time, enable absorption studies against extended objects. Here, we use the Hydrangea cosmological hydrodynamical simulations to predict the expected properties of intergalactic gas in and around massive galaxy clusters, and investigate the prospects of detecting it in absorption against the bright cores of nearby, massive, relaxed galaxy clusters. We probed a total of 138 projections from the simulation volumes, finding 16 directions with a total column density $N_{{\rm O\, {\small VII}}} > 10^{14.5}$ cm−2. The strongest absorbers are typically shifted by ±1000 km s−1 with respect to the rest frame of the cluster they are nearest to. Realistic mock observations with future micro-calorimeters, such as the Athena X-ray Integral Field Unit or the proposed Line Emission Mapper (LEM) X-ray probe, show that the detection of cosmic web filaments in ${\rm O\, {\small VII}}$ and ${\rm O\, {\small VIII}}$ absorption against galaxy cluster cores will be feasible. An ${\rm O\, {\small VII}}$ detection with a 5σ significance can be achieved in 10–250 ks with Athena for most of the galaxy clusters considered. The ${\rm O\, {\small VIII}}$ detection becomes feasible only with a spectral resolution of around 1 eV, comparable to that envisioned for LEM.

Numerical simulations have become one of the key tools used by theorists in all the fields of astrophysics and cosmology. The development of modern tools that target the largest existing computing systems and exploit state-of-the-art numerical methods and algorithms is thus crucial. In this paper, we introduce the fully open-source highly-parallel, versatile, and modular coupled hydrodynamics, gravity, cosmology, and galaxy-formation code SWIFT. The software package exploits hybrid shared- and distributed-memory task-based parallelism, asynchronous communications, and domain-decomposition algorithms based on balancing the workload, rather than the data, to efficiently exploit modern high-performance computing cluster architectures. Gravity is solved for using a fast-multipole-method, optionally coupled to a particle mesh solver in Fourier space to handle periodic volumes. For gas evolution, multiple modern flavours of Smoothed Particle Hydrodynamics are implemented. SWIFT also evolves neutrinos using a state-of-the-art particle-based method. Two complementary networks of sub-grid models for galaxy formation as well as extensions to simulate planetary physics are also released as part of the code. An extensive set of output options, including snapshots, light-cones, power spectra, and a coupling to structure finders are also included. We describe the overall code architecture, summarise the consistency and accuracy tests that were performed, and demonstrate the excellent weak-scaling performance of the code using a representative cosmological hydrodynamical problem with ≈300 billion particles. The code is released to the community alongside extensive documentation for both users and developers, a large selection of example test problems, and a suite of tools to aid in the analysis of large simulations run with SWIFT.

Many quiescent galaxies discovered in the early Universe by JWST raise fundamental questions on when and how these galaxies became and stayed quenched. Making use of the latest version of the semianalytic model GAEA that provides good agreement with the observed quenched fractions up to z ∼ 3, we make predictions for the expected fractions of quiescent galaxies up to z ∼ 7 and analyze the main quenching mechanism. We find that in a simulated box of 685 Mpc on a side, the first quenched massive ( M _⋆ ∼ 10 ^11 M _⊙ ), Milky Way–mass, and low-mass ( M _⋆ ∼ 10 ^9.5 M _⊙ ) galaxies appear at z ∼ 4.5, z ∼ 6.2, and before z = 7, respectively. Most quenched galaxies identified at early redshifts remain quenched for more than 1 Gyr. Independently of galaxy stellar mass, the dominant quenching mechanism at high redshift is accretion disk feedback (quasar winds) from a central massive black hole, which is triggered by mergers in massive and Milky Way–mass galaxies and by disk instabilities in low-mass galaxies. Environmental stripping becomes increasingly more important at lower redshift.

Many $z=1.5$ galaxies with a stellar mass ($M_{\star}$) $\geq 10^{10}\,\mathrm{M}_\odot$ are already quenched in both galaxy clusters ($>50$ per cent) and the field ($>20$ per cent), with clusters having a higher quenched fraction at all stellar masses compared to the field. A puzzling issue is that these massive quenched galaxies have stellar populations of similar age in both clusters and the field. This suggests that, despite the higher quenched fraction in clusters, the dominant quenching mechanism for massive galaxies is similar in both environments. In this work, we use data from the cosmological hydrodynamic simulations Hydrangea and EAGLE to test whether the excess quenched fraction of massive galaxies in $z = 1.5$ clusters results from fundamental differences in their halo properties compared to the field. We find that (i) at $10^{10} \leq$ $M_{\star}/\,\mathrm{M}_\odot\leq 10^{11}$, quenched fractions in the redshift range $1.5 < z < 3.5$ are consistently higher for galaxies with higher peak maximum circular velocity of the dark matter halo ($v_{\mathrm{max, peak}}$), and (ii) the distribution of $v_{\mathrm{max, peak}}$ is strongly biased towards higher values for cluster satellites compared to the field centrals. Due to this difference in the halo properties of cluster and field galaxies, secular processes alone may account for (most of) the environmental excess of massive quenched galaxies in high-redshift (proto) clusters. Taken at face value, our results challenge a fundamental assumption of popular quenching models, namely that clusters are assembled from an unbiased subset of infalling field galaxies. If confirmed, this would imply that such models must necessarily fail at high redshift, as indicated by recent observations.

We have identified 189 candidate z > 1.3 protoclusters and clusters in the LSST Deep Drilling Fields. This sample will enable the measurement of the metal enrichment and star formation history of clusters during their early assembly period through the direct measurement of the rate of supernovae identified through the LSST. The protocluster sample was selected from galaxy overdensities in a Spitzer/IRAC colour-selected sample using criteria that were optimized for protocluster purity using a realistic light-cone. Our tests reveal that 60-80 per cent of the identified candidates are likely to be genuine protoclusters or clusters, which is corroborated by a similar to 4 sigma stacked X-ray signal from these structures. We provide photometric redshift estimates for 47 candidates which exhibit strong peaks in the photo-z distribution of their candidate members. However, the lack of a photo-z peak does not mean a candidate is not genuine, since we find a stacked X-ray signal of similar significance from both the candidates that exhibit photo-z peaks and those that do not. Tests on the light-cone reveal that our pursuit of a pure sample of protoclusters results in that sample being highly incomplete (similar to 4 per cent) and heavily biased towards larger, richer, more massive, and more centrally concentrated protoclusters than the total protocluster population. Most (similar to 75 per cent) of the selected protoclusters are likely to have a maximum collapsed halo mass of between 10(13) and 10(14) M-circle dot, with only similar to 25 per cent likely to be collapsed clusters above 10(14) M-circle dot. However, the aforementioned bias ensures our sample is similar to 50 per cent complete for structures that have already collapsed into clusters more massive than 10(14) M-circle dot.

The diffuse stellar component of galaxy clusters made up of intergalactic stars is termed the intracluster light (ICL). Although there is a developing understanding of the mechanisms by which the ICL is formed, no strong consensus has yet been reached on which objects the stars of the ICL are primarily sourced from. We investigate the assembly of the ICL starting approximately 10 Gyr before z=0 in 11 galaxy clusters (halo masses between similar to 1 x 10(14) and similar to 7 x 10(14) M-circle dot at z = 0 in the Horizon-AGN simulation. By tracking the stars of galaxies that fall into these clusters past cluster infall, we are able to link almost all of the z approximate to 0 ICL back to progenitor objects. Satellite stripping, mergers, and pre-processing are all found to make significant contributions to the ICL, but any contribution from in situ star formation directly into the ICL appears negligible. Even after compensating for resolution effects, we find that approximately 90 per cent of the stacked ICL of the 11 clusters that is not pre-processed should come from galaxies infalling with stellar masses above 10(9) M-circle dot, with roughly half coming from infalling galaxies with stellar masses within half a dex of 10(11) M-circle dot. The fact that the ICL appears largely sourced from such massive objects suggests that the ICL assembly of any individual cluster may be principally stochastic.