I will review how clusters and protoclusters assemble in the framework of state-of the-art cosmological hydrodynamical simulations and semi-analytic models of galaxy formation. I will highlight the main successes and problems associated with theoretical models in reproducing the properties of clusters and protoclusters. I will describe the role of simulations in providing theoretical support to interpret the properties of up-coming samples from large multi-wavelength surveys.

Using the Magneticum simulations, one of the largest volumes covered by state-of-the-art hydrodynamical cosmological simulations, I study the formation of proto-clusters at redshifts z = 4, and show that such structures indeed are already virialized, as massive as inferred from the observations, and host the observed number of massive, fast rotating member galaxies. I will demonstrate that the dynamics of the member galaxies within the cluster is very similar to that inferred from observations, and that their rapid merging forms the cores of today's BCGs. I will further highlight that half of the gas reservoir of these proto-clusters is already in a hot phase, where the metal enrichment is just at the very early stage, while the cold, star-forming gas component of the member galaxies is already largely enriched to solar values. Finally, tracing those systems forward in time I will show that none of the possible mass indicators at such high redshift like virial mass, stellar mass, or star-formation rate, are good tracers to predict the present-day virial mass, with the exception of the number of member galaxies, with richness being a hint at a more massive outcome of the system at z=0. In fact, I find that only one of the simulated proto-cluster systems which show similar properties to the observed proto-clusters at z = 4, is among the top ten clusters at redshift z=0, with some not even reaching 10^15Msun, while the top 10 most massive clusters at z=0 are mostly not even above 10^13Msun at z=4.

About 2000 star-forming proto-cluster candidates from z=1.3 to z=3 were identified in the submm channels of the Planck survey. Complementary higher resolution IR and spectroscopic follow-up observations were conducted within the SPHERICS collaboration. They have recently allowed us to estimate star-formation rates for some of these candidates, showing that the high-redshift star-forming objects are key elements to trace the star-formation history of the Universe. In recent work, I have investigated whether the Planck-selected star-forming proto-clusters are actual progenitors of the massive clusters at z=0 by using the state-of-art hydrodynamical simulation. We focus our analysis on the most star-forming objects from z=1.3 to z=3 in the IllustrisTNG simulation. The star formation rate of proto-cluster candidates is estimated by taking into account the large aperture of Planck proto-cluster selection and the line-of-sight contamination. The redshift evolution of proto-cluster’s SFR in simulation seems in agreement with observational results. Focusing on the spectroscopically confirmed PHz G237.01+42.50 proto-cluster, we compared the characteristics of observed galaxy members with simulations. Finally by probing the merger history of these simulated high-z star-forming objects, we conclude that at least 70% of the Planck selected proto-clusters are progenitors of galaxy clusters at z=0.

We present VLT/MUSE spectroscopy, along with archival Gemini/GMOS spectroscopy, Magellan/Megacam imaging, and Chandra X-ray emission for SPT-CLJ0305-6225, a z=0.58 galaxy cluster. A large BCG-SZ centroid separation and a highly disturbed X-ray morphology classifies SPT-CLJ0307-6225 as a major merging cluster. We characterise the central regions of the two colliding structures, namely 0307-6225N and 0307-6225S. We find velocity derived masses of M_200,N=2.42±1.40×10^14 M_200,S=3.13±1.87×10^14 solar masses with a line-of-sight velocity difference between the two structures of|Δv|=342 km s-1. We model the merger using a Monte Carlo Merger Analysis Code, estimating a merging angle of 36 +14 −12 degrees with respect to the plane of the sky. Comparing with simulations of a merging system with a mass ratio of 1:3, we find that the best scenario is that of an outgoing merger, observed 0.96 +0.31 −0.18 Gyr after collision, which could be close to turnaround. Moreover, we find evidence that 0307-6225S suffered a previous merger.

Galaxy cluster mergers are a powerful laboratory for testing cosmological and astrophysical models. However, interpreting individual merging clusters depends crucially on their merger configuration, defined by the masses, velocities, impact parameters, and orientation of the merger axis with respect to the plane of the sky. In this work, we investigate the impact of merger parameters on the X-ray emitting intracluster medium and gravitational lensing maps using a suite of idealised simulations of binary cluster mergers performed using the GAMER-2 code. As a test case, we focus on modeling the Bullet Cluster-like merging system Abell 2146, in which deep Chandra X-ray and lensing observations revealed prominent merger shocks as well as the mass distribution and substructures associated with this merging cluster. We identify the most interesting parameter combinations and times and evaluate the effects of various merger parameters and viewing direction on the properties of merger shocks observed by deep Chandra and lensing observations. Due to the impact of the merger on the mass profiles, previous mass estimates from weak lensing are too high. The plane of the merger is tilted further from the plane of the sky than estimated previously, up to 30deg from the plane of the sky. We discuss the applicability of our results to multi-wavelength surveys of galaxy clusters.

Analyses of observations of galaxy clusters via their X-ray emission, gravitational lensing or Sunyaev–Zel’dovich (SZ) effect are often based on some parametrised cluster model for the distribution of the cluster dark matter and the thermodynamical properties of its intra-cluster medium (ICM). The accuracy and robustness of cluster parameters derived from studies at different wavelengths depend greatly on how well the model describes the physical properties of the cluster, and the assumptions made regarding the dynamical state of the cluster and its gas content. In this talk, I discuss a new method for modelling the physical properties of galaxy clusters. This technique allows for a 'freeform' reconstruction, but one for which the level of complexity is determined automatically by the observational data and may depend on position within the cluster. This is achieved by representing each independent cluster property as some interpolating or approximating function that is specified by a set of control points, or 'nodes', for which the number of nodes, together with their positions and amplitudes, are allowed to vary and are inferred in a Bayesian manner from the data.

Previous X-ray studies of the Perseus Cluster, consisting of 85 Suzaku pointings along eight azimuthal directions, revealed a particularly steep decrease in the projected temperature profile near the virial radius (～r200) towards the northwest (NW). To further explore this shock candidate, another 4 Suzaku observations on the NW edge of the Perseus Cluster have been obtained. These deeper data were designed to provide the best possible control of systematic uncertainties in the spectral analysis. Using the combined Suzaku observations, we have carefully investigated this interesting region by analyzing the spectra of various annuli and extracting projected thermodynamic profiles. We find that the projected temperature profile shows a break near r200, indicating a shock with M = 1.9+-0.3. Corresponding discontinuities are also found in the projected emission measure and the density profiles at the same location. This evidence of a shock front so far away from the cluster center is unprecedented, and may provide a first insight into the properties of large-scale virial shocks which shape the process of galaxy cluster growth.

We present the results of deep Chandra observations of ZwCl 2341+0000, which is a merging galaxy cluster hosting double radio relics. Previous study shows that half of the southern relic is associated with an X-ray surface brightness discontinuity while the other half not. The discontinuity was believed to be a shock front. Therefore it is a mysterious case of an only partial shock-relic connection. By using the 206.5 ks deep Chandra observations, we find that the previously reported southern edge is identified as a cold front based on the new temperature measurements. The actual location of the southern shock front is at the southeastern edge of the southern relic, which is unveiled by the radio spectral index map calculated using new GMRT 325 MHz and JVLA 1.5 GHz images. Meanwhile, the deep observations also serendipitously reveal several intriguing features, such as a unique cone shaped northern subcluster, a 400 kpc straight gas trail stripped from the cone apex, a peculiar high temperature region between the two subclusters that doesn't hold pressure equilibrium with the ambient gas. The northern subcluster is in a perfect cone shape, with a 500 kpc long cold front on each side. This type of conic subcluster has been predicted by simulations but is observed here for the first time. It represents a transition stage between a blunt body cold front and a slingshot cold front. The new findings improve the knowledge of the evolution of remnant core in head-on mergers.

The ASKAP-EMU survey is a deep wide-field radio continuum survey designed to cover the entire southern sky and a significant fraction of the northern sky up to +30deg. We report a discovery of a radio relic in the merging cluster SPT-CL J2023--5535 at z=0.23 from the ASKAP-EMU pilot 300 sq.deg survey (800 - 1088 MHz). The radio relic is located at the western edge of this radio halo stretched ~0.5 Mpc in the north-south orientation. The integrated spectral index of the radio relic within the narrow bandwidth is alpha (1088 MHz - 800 MHz) = - 0.76 +- 0.06. Our weak-lensing analysis of archival DECam data shows that the system is a massive M200 =(1.04 +- 0.36) 10^15 Msun) merging cluster composed of at least three subclusters. We suggest a scenario, wherein the radio features arise from the collision between the eastern and middle subclusters. Our discovery illustrates the effectiveness of the ASKAP-EMU survey in detecting diffuse emissions in galaxy clusters and when completed, the survey will greatly increase the number of merging cluster detections with diffuse radio emissions.

The formation of galaxy clusters is not a unique process. Due to the assembling by mergers and accretions, together with a continuous quest to reach dynamic equilibrium by virialization, it is more a sequence of unrelaxed/relaxed states. Measuring the current dynamical status of a specific cluster (or future cluster) is, thus, a hard job, and although there are many ways to treat the problem, a physically motivated method that is simple on its conception and robust on its results is still lacking. Our proposal is to quantify the degree of evolution of galaxy systems, from their observed global properties, by means of the evolution of their entropy. It is expected that the evolution towards relaxation corresponds to an increase in entropy. In the case of isolated systems, the virialization is a state of maximum entropy. When the system is not isolated, the sequence of mergers and accretions generate new entropy maxima. Since the thermodynamic behavior of self-gravitating systems is very peculiar, conventional thermodynamic expressions do not work entirely. In this work, we present a formalism that modifies some of such expressions to include gravitational systems. Specifically, entropy is first calculated as a function of the virial mass, velocity dispersion and volume. In a validation process we also use entropy measurements based on the information theory, considering the radial, azimuthal and line-of-sight distribution of galaxies in the cluster. This method was applied...

Abell 1240 is a merging galaxy cluster hosting prominent, symmetric double radio relics. To characterize its merging state, we provide the first weak-lensing detection of the dark matter distribution in the Abell 1240 field with Subaru/Suprime-Cam observations. Combining our new MMT/Hectospec observations with the spectroscopic redshifts from the literature allows us to investigate the cluster galaxy distribution as a proxy of the dark matter distribution. Both weak-lensing mass reconstruction and galaxy distribution appear to be elongated in the same direction as the X-ray surface brightness, and reveal that Abell 1240 consists of two subclusters separated by ~1 Mpc between the double relics. We find that Abell 1240 favors being head-on binary mergers of an approximate 2:1 mass ratio. With the radio relic priors, our weak-lensing results suggest that Abell 1240 may be observed in the returning phase of the merger after the first collision.

I will report on recent results based on radio and X-ray observations of two galaxy cluster pairs in a pre-merger phase: A1758N-A1758S (z=0.28) and RXCJ1825-CIZAJ1824 (z=0.06). In the former system, the deep LOFAR images reveal the presence of a giant bridge of radio emission connecting the two galaxy clusters on a scale of ~2 Mpc. This is the second large-scale radio bridge observed to date in a cluster pair. In contrast, no diffuse radio emission has been detected between RXCJ1825 and CIZAJ1824 possibly due to the lower energy dissipated by this pre-merging system (which is less massive than A1758N-A1758S). However, the new radio halo discovered in RXCJ1825 shows a low surface brightness extension likely generated during the interaction between the cluster and an infalling galaxy group (confirmed by optical and X-ray observations). The reported results represent the first investigation of non-thermal phenomena in clusters in the early phase of collision. The detection of radio bridges and extensions of radio halos demonstrate the existence of magnetic fields at large distances from the cluster center, with potential implications on the evolution of galaxies in cluster outskirts. In my talk, I will discuss the possible origin of these large-scale emissions and the prospects for future observations of pre-merging clusters.

We present an improved weak-lensing (WL) study of the high-z (z=0.87) merging galaxy cluster ACT-CL J0102-4915 (“El Gordo”), based on new wide-field Hubble Space Telescope (HST) imaging data. El Gordo is undergoing a merging event showing two distinctive mass clumps and radio relics. El Gordo is also known to be the most massive system at z > 0.6 and a rare cluster for its redshift in the current ΛCDM cosmology. The previous gravitational lensing studies revealed a clear bimodal mass structure and its high mass estimates. However, their mass measurements relied on extrapolation because the previous HST observation did not extend to the virial radius of the cluster. In this talk, we present a more accurate mass estimate of the cluster using WL analysis from new wide-field HST imaging data. The new imaging data cover the ~3.5 × ~3.5 Mpc region centered on the cluster and enable us to detect WL signals beyond the virial radius. While confirming the binary mass structure and ~2σ dissociation between the southeastern mass peak and the X-ray cool core, we find that the new data yield a ~24% lower mass for the entire system compared with our previous WL study. The mass ratio of the two subclusters is consistent with our previous WL study but significantly different from the previous strong lensing results. This discrepancy is attributed to the use of extrapolation in strong lensing studies. We discuss the rarity of the cluster in the ΛCDM paradigm and the merger scenario.

Cluster mergers are highly energetic events capable of drastically changing the observed properties of the intra-cluster medium (ICM) and therefore having a significant impact on cluster evolution. We present the results of a detailed Chandra X-ray study of the ICM of the AS0295 cluster, a hot (~9.5 keV), nearby (z=0.30), binary merging system. We construct temperature and surface brightness maps and use them to search for merger signatures like shocks and cold fronts. The Chandra images show a clearly disturbed morphology, elongated in the SE-NW direction, indicative of a cluster caught in the process of merging. The secondary cluster is clearly visible in the NW as an X-ray peak and shows the presence of cool gas (~ 6keV), while the primary cluster has a flatter gas distribution and its gas temperature does not deviate significantly from the mean temperature of the cluster. We find several merger features: a cold front ahead of the secondary, two possible shocks, and a plume of cool gas emerging from the primary. We discuss the comparison of our results with literature studies based on strong lensing and radio observations of the AS0295 cluster, as well as literature results of binary merger simulations of clusters. We propose for AS0295 a low-mass ratio, off-axis merging scenario, with the secondary close to the first apocenter.

Galaxy clusters constitute the building blocks of the large scale structure, and some interesting events arise during the collisions among them. For example, we can cite the intracluster medium gas (ICM) sloshing or, in more extreme events, the temporary ICM stripping of the cluster potential well. A combination of weak lensing + spectroscopy + hydrodynamical simulations has been proven powerful to describe the three cluster main components (sorted in ascending mass), galaxies, ICM and dark matter (DM), and therefore, understand the origin of those phenomena. In this talk, I'll discuss the interesting case of an ""invisible"" structure inducing a gas sloshing in the merging cluster A1644 (Monteiro-Oliveira et al. 2020; Doubrawa et al. 2020) and how the offset ICM - DM we currently observe in the interacting system A2034 (Moura, Machado & Monteiro-Oliveira et al. 2021) can be explained in agreement with the standard CDM instead of evoking non-gravitational interactions of DM.

According to large scale structure (LSS) formation scenarios, Galaxy Clusters (GCs) are thought to have formed hierarchically via mergers of smaller scale structures. Thermodynamic properties of the X-ray emitting intracluster medium are sensitive probes of these dynamical activities. In this talk, I will present the joint NuSTAR and XMM-Newton analysis of the GC A3395 (z = 0.0498) at the site where the cluster and the intercluster filament connecting A3395 and A3391 meet in projection. Although XMM-Newton detects a hot-spot in this location, which may point to accreted structures and/or internal dynamics, we find that NuSTAR does not agree with this result (possibly) due to its insensitivity to spatially varying foreground absorption that XMM-Newton suffers from. The interaction of cosmic web filaments and GCs, as we explore in our joint analysis of A3395, can help improve our understanding of the formation and growth of LSS.

To understand galaxy clusters and their formation one needs enough resolution to see their substructure. For low redshift clusters, X-ray observations have been able to provide this but, at the epoch of galaxy formation, redshift dimming means there are few photons and long integration times are needed. Cluster Observations with the Sunyaev Zel'dovich Effect (SZE) do not suffer this limitation, but SZE survey instruments have beams as large as typical clusters so are of limited use in investigating cluster physics.The 90 GHz MUSTANG2 camera on the Green Bank Telescope has been used to observe the SZE in over 40 galaxy clusters. It has a resolution of 10" and a sensitivity good enough to achieve map noise below 30 micro-Kelvin with a few hours of integration -- good enough to enable the study of substructure in high redshift clusters. In this presentation we will review some of our results, what we can learn from them, and the complimentary role these observations can play in the era of eROSITA.

Collisions between galaxy clusters are a frequent scenario in the hierarchical model of structures. Dissociative collisions provide an extreme environment of interaction between clusters of galaxies, where the properties of dark matter(DM) in relation to baryonic matter become evident. Investigating dissociative scenarios allows a deeper understanding of the behavior and dynamics of baryonic and non-baryonic matter in this context of collision. Abell 2034 (z= 0.114) is a bimodal system composed of a north and a south substructure, it has dissociative features observed in X-rays and gravitational lensing. Using N-body hydrodynamic simulations, we present a theoretical study based on the dissociative collision of Abell 2034, aiming to explore the effect that different relative concentrations between the clusters generate on the dynamics of the system. We investigated the relationship of the central density ratios with different levels of dissociation, where we analyzed nine models with different concentrations of the two components: intracluster gas and DM halo for each substructure. We found different degrees of dissociation that were quantified by the relative distance between the X-ray emission peak and the dark matter peaks, and also in terms of gas retention in each cluster. We found that the ratio of the gas central densities is more decisive than the ratio of dark matter central densities, in determining the level of dissociation for the parameters of this collision.

Galaxy clusters, occupying the most massive halos, serve as key laboratories for halo gas physics and galaxy evolution, hosting multiphase gas that contains clues as to how these environments have transformed over time. Accretion shocks are uniformly predicted in formation models of the intracluster medium (ICM), but their location (1-5 r200) varies from model to model, with important implications for infalling galaxies and gas physics in cluster outskirts. I am conducting a novel experiment to detect these accretion shocks through UV absorption spectroscopy of background quasars probing foreground galaxy clusters. The experiment works as follows: the accretion shock front heats infalling gas, which ionizes these gas clouds to a greater degree than photoionization in the intergalactic medium (IGM), and once the gas is exceedingly shock-heated, the neutral fraction should plummet, causing mappable depressions in the incidence of neutral hydrogen (H I) absorbers per unit redshift, dN/dz. Using HST/COS spectra, we select a statistical sample of quasar sightlines that probe 40 foreground galaxy clusters from within r200 to 5 r200 pairs, and quantify dN/dz as a function of projected clustocentric distance. My results reveal peaks around 1 r200 and 3 r200, with an increase in the amount of absorption lines per sightline detected around the outermost accretion shock, indicating a buildup of infalling H I clouds just outside the outer accretion shock.

Galaxy clusters are an important tool for cosmology, and their detection and characterization are key goals for current and future surveys. Using data from the Wide-field Infrared Survey Explorer (WISE), the Massive and Distant Clusters of WISE Survey (MaDCoWS) located 2,839 significant galaxy overdensities at redshifts 0.7≲z≲1.5, which included extensive follow-up imaging from the Spitzer Space Telescope to determine cluster richnesses. Concurrently, the Atacama Cosmology Telescope (ACT) has produced large area mm-wave maps in three frequency bands along with a large catalog of Sunyaev-Zeldovich (SZ) selected clusters, as part of its Data Release 5 (DR5). Using the maps and cluster catalog from DR5, we explore the scaling between SZ mass and cluster richness. We use complementary radio survey data from the Very Large Array, submillimeter data from Herschel, and ACT 224~GHz data to assess the impact of contaminating sources on the SZ signals. We then use a hierarchical Bayesian model to fit the mass-richness scaling relation. We find that MaDCoWS clusters have submillimeter contamination which is consistent with a gray-body spectrum, while the ACT clusters are consistent with no submillimeter emission on average. We find the best fit ACT SZ mass vs. MaDCoWS richness scaling relation has a slope of κ=1.84+0.15−0.14, where the slope is defined as M∝λ^(κ) where λ is the richness. Additionally, we find that the approximate level of in-fill of the ACT and MaDCoWS cluster SZ signals to be at the percent level. We conclude that the original MaDCoWS survey's selection function is not well defined, and, as such, reiterate the MaDCoWS collaboration's recommendation that the sample is suited for probing cluster and galaxy evolution, but not cosmological analyses.

The current consensus on the formation and evolution of the brightest cluster galaxies is that their stellar mass forms early (z≳4) in separate galaxies that then eventually assemble the main structure at late times (z≲1). However, advances in observational techniques have led to the discovery of protoclusters out to z∼7, suggesting that the late-assembly picture may not be fully complete. If these protoclusters assemble rapidly in the early universe, they should form the brightest cluster galaxies much earlier than suspected by the late-assembly picture. Using a combination of observationally constrained hydrodynamical and dark-matter-only simulations, we show that the stellar assembly time of a sub-set of the brightest cluster galaxies occurs at high redshifts (z>3) rather than at low redshifts (z<1), as is commonly thought. We find, using isolated non-cosmological hydrodynamical simulations, that highly overdense protoclusters assemble their stellar mass into brightest cluster galaxies within ∼1 Gyr of evolution -- producing massive blue elliptical galaxies at high redshifts (z≳1.5). We argue that there is a downsizing effect on the cluster scale wherein some of the brightest cluster galaxies in the cores of the most-massive clusters assemble earlier than those in lower-mass clusters. In those clusters with z=0 virial mass ⩾5e14 M⊙, we find that 9.8% have their cores assembly early.

The evolution of protoclusters at high redshift involves a complex interaction between primordial gas inflows and feedback driven outflows from star formation and AGN. This is hosted in the circumgalactic medium (CGM) of massive galaxies – effectively the primordial intra-cluster medium (ICM) - which is often underresolved in large scale simulations. Low resolution in the ICM can prevent simulations from capturing many interesting effects, including the penetration of primordial filaments into the ICM. This in turn has implications for the properties of the central galaxy of the halo, as more cold, metal-poor gas can reach it and fuel star formation and AGN activity. In Bennett & Sijacki 2020, we applied a new shock refinement scheme, where we boost resolution on-the-fly around shocks, to explore the evolution of the primordial ICM at high resolution. For these protoclusters, we can then make a number of predictions, including the distribution of HI and other ions, which can be compared to observations from instruments like COS and MUSE. With the larger FABLE simulation suite, we also investigate how ICM properties change as a function of time during major mergers, particularly focussing on the evolution of merger and AGN shocks, energy dissipation and non-thermal pressure support, as well as their interplay with the estimated hydrostatic mass bias. This can lead to interesting implications about future estimates of galaxy cluster mass from observations by Athena and ACT.

The mass and redshift of a cluster are its most fundamental properties. Linking observation to theory, these quantities allow us to use the evolution of cluster population over cosmic time to test models of structure formation. The dynamical state is a third, somewhat underappreciated, fundamental quantity, affecting both clyster detectability in various surveys and our ability to reconstruct the mass. In this talk I will give a short overview of cluster mass estimation methods and the link between the radial mass distribution and the dynamical state. I will also address the prospects for measurement of the mass and dynamical state into the proto-cluster regime.

I will present the resolved thermodynamic of the most distant cluster for which such a measurement has ever been performed, IDCSJ1426 at z=1.75, which turned out also to be the more precise measurement for every high redshift cluster thanks to our joint use of both X-ray and SZ data. Profiting from the largest ever redshift baseline, I determined the evolution of the thermodynamic profiles of this cluster down to z=0.07, our reference local comparison sample, with unprecedented precision over a 10 Gyr baseline. In the talk, I will also introduce a new definition of the evolutionary rate to effectively compare ancestors and descendants. It turned out to have the advantage of separating cluster evolution, dependence on mass, pseudo-evolution and to return a number with unique interpretation, unlike other definitions.

It is well known that about half of the baryons in the Universe must be in the galaxy clusters outskirts and in the form of hot and low-density filaments connecting galaxy clusters. Due to the low density, most of this filamentary plasma can not be detected by X-ray observatories. In particular cases of low redshift cluster pairs in the pre-merging phase, the Sunyaev Zel'dovich (SZ) effect can be used to observe the inter-cluster regions and detect the imprint of missing baryons. The Abell 399-401 (A399-401) system is the perfect laboratory to test our ability to detect filamentary structures via the SZ effect with <∼1′ angular resolution. This pair has been well studied at several frequencies: it exhibits double radio-halos, an excess of X-ray emission in the inter-cluster region and a synchrotron radio ‘ridge’ connecting the two clusters. Moreover, the Planck satellite provided the first SZ detection of the gas between A399-401 despite the poor angular resolution (∼10′) of its SZ map. We have used an Atacama Cosmology Telescope (ACT) and Planck satellite Compton-y map (1.65′ angular resolution) that combines ACT data from 2008 to 2019 with Planck maps and MUSTANG-2 at the Green Bank Telescope data (9′′ angular resolution) to study the A399-401 system in detail. We present the data analysis and results.

Detecting hot gas in the outskirts of galaxy clusters is one of the main goals of our next-generation X-ray and SZ telescopes. It is therefore of importance to understand how accretion and mergers shape the cluster’s atmosphere near and beyond the virial radius Rvir. In this talk, I will present our novel approach designed to explore the effects of mergers on the outskirts of clusters, whereby we simulate mergers between two self-similar clusters in an idealized cosmological context. Under this framework, I will discuss the formation and evolution of runaway merger shocks. They are considered as promising candidates for powering radio relics in many clusters. Eventually, these runaway shocks would inevitably overtake the accretion shock and drive long-living MA-shocks, which constitute a new boundary of the ICM that could travel up to a few Rvir into the intergalactic medium. In contrast to the gaseous atmosphere, the dark matter halo experiences a rapid contraction during the major merger and its splashback radius decreases, approaching the Rvir. Our simulations show that mergers could easily generate the MA-shock-splashback offset measured in cosmological simulations.

We present the direct detection of the splashback feature using the sample of massive galaxy clusters from the Local Cluster Substructure Survey. This feature is clearly detected in the stacked luminosity density profile from K-band magnitudes of spectroscopically confirmed cluster members. A Bayesian inference scheme ranked models including the splashback feature as more descriptive of the data with respect to models that do not allow for this transition. In addition, we exploited the extensive multi-wavelength LoCuSS dataset to test a wide range of proxies for the cluster formation history, finding the most significant dependence of the splashback feature location and scale according to the presence or absence of X-ray emitting galaxy groups in the cluster infall regions. This suggests a correlation between the properties of the cluster potential and its accretion rate and merger history. Clusters that are classified as old and dynamically inactive present stronger signatures of the splashback feature, with respect to younger, more active clusters. We are directly observing how fundamental dynamical properties of clusters reverberate across vastly different physical scales.

We have discovered a large-scale structure candidate surrounding the massive, spectroscopically confirmed galaxy cluster IDCS J1426.5+3508 at z=1.75 in Boötes. Using a sample of mid-infrared-bright Dust-Obscured galaxies (DOGs) as signposts, we have identified 14 (39) galaxy cluster/group-scale overdense nodes of photo-z selected galaxies that are significant at >= 4 (3) sigma level at z=1.75. Our pipeline has also rediscovered the IDCS cluster with greater than 4 sigma significance. The filamentary proto-supercluster-like structure is spectroscopically confirmed, with ~ 30 AGN at 1.72 <= z_spec <= 1.78 from the AGES survey spanning its 50 Mpc x 35 Mpc extent. This is an unprecedented example of a massive (X-ray/SZ-detected) high redshift galaxy cluster still embedded in its birth environment. We report the initial characterization of this remarkable multi-nodal structure and establish the opportunities for future space and ground-based multiwavelength studies of the galaxy populations and ICM dynamics across the supercluster.