Our Universe evolves through the hierarchical growth of structure through mergers and accretion. These processes have a pivotal impact on the subsequent evolution of the baryons and thus of the galaxies that are hosted within these growing large-scale structures. The hot intra-cluster medium (ICM) which pervades galaxy clusters provides a fundamental link between the small scale processes such as feeding active-galactic nuclei, and the total effect of the halo. I will review our recent advances and potential next steps in understanding the evolution of the ICM and its links to galaxy evolution.

Radio relics, Mpc-sized elongated diffuse radio emissions found at the outskirts of merging galaxy clusters, are believed to trace the position of the shock and thus have been used to interpret the merger histories. However, recent simulations (e.g. ZuHone et al. 2020) have shown that the fossil jets of AGN can be redistributed to mimic radio relics, whose positions in this case may become different from those of the shocks. In this study, we present a possible case for such an AGN-originated radio relic. We observed the merging galaxy cluster Abell 514 with upgraded GMRT and found three remarkable AGN jet tails that may have undergone multiple reorientations. Using multi-frequency data, we have performed spectral analysis across the AGN tails and examined their correlations with the X-ray emission.

Building a map of cluster evolution requires clusters spanning a range of redshifts, masses, and dynamical and evolutionary states. One way to build this sample is with radio AGNs, which have long been used to trace clusters at higher redshift. Specifically, bent, double-lobed radio sources are an effective tracer of clusters. Bent radio sources have a characteristic “C” shape, which is created when ram pressure from a dense gaseous medium, like the ICM, bends the radio lobes. Using the high-z Clusters Occupied by Bent Radio AGN (COBRA) survey, we identify a large sample of high-z cluster/protocluster candidates hosting bent AGNs. Using optical and IR images, we find strong red sequence populations in 39 cluster candidates at 0.35 < z < 2.2. In these clusters, we probe the build-up of the red sequence to determine how our sample of low- and high-mass systems compares to other studies and if BCGs preferentially host high-z bent AGNs. By correlating the distribution of red sequence galaxies in each cluster with the bending angle, we find that richer clusters host narrower bent AGNs. If ICM density scales with richness, the appearance of narrower bent AGNs may indicate which clusters have a denser or earlier forming ICM. Lastly, using VLA FIRST, VLASS, and LOFAR observations, we measure spectral indices for our high-z bent AGNs to further determine what causes the morphology of these bent radio sources and tease potential differences in the ICM distribution in our clusters.

I present the physical properties of a large-scale structure at z=1.7 that is populated by star-forming galaxies and is assembling around a powerful FRII. Based on ALMA observations of the CO(2-1) transition, we recently discovered three new gas-rich galaxies, in addition to a large molecular gas reservoir (M_H2~2 x 10^11 M_sun) around the FRII host galaxy. We also detect 3 mm continuum emission in the core and in one hotspot of the radio galaxy. Under simple assumptions, we show that the system will evolve into a ≳ 10^14 M_sun cluster at z=0 and that the FRII is the likely progenitor of the future BCG. Extended X-ray emission has been detected around the FRII. Remarkably, four of the protocluster members lie in an arc-like shape around the main X-ray extended emission. We propose that such emission originates from an expanding bubble of gas shock heated by the FRII jet, that is promoting the star formation on nearby galaxies by compression of their ISM. If confirmed, this would be the first evidence of positive AGN feedback on multiple galaxies on hundreds-kpc scales. I also present new LOFAR (150 MHz) and JVLA (1.4 GHz) observations of the FRII, which reveal extended radio emission around its lobes, likely linked to the diffuse X-rays. Exploiting the JVLA and LOFAR data we built the spectral index map that reveals signatures of re-acceleration of the plasma in the outskirts of the lobes, possibly induced by interactions with the ICM.

ClJ1449+0856, a galaxy cluster at z=2, is an exemplary case to investigate everything from galaxy evolution in dense environments, to early-time energetic feedback in clusters, and the physics of the high-redshift intra-cluster medium (ICM). Despite its early epoch, ClJ1449 has now been detected in both diffuse X-ray emission and through the thermal Sunyaev Zel’Dovich effect - the lowest mass single SZ detection to date, confirming its virialised nature. In this talk, I will summarise the picture of high-redshift galaxy and cluster evolution emerging from ClJ1449 so far, and present our latest discoveries. Having uncovered a population of highly-excited, merging, star-forming galaxies in ClJ1449, we turn our attention to the excess radio emission at 3GHz. We discover multiple faint radio-jet emission sites in the cluster core - quite unlike the picture in local galaxy clusters, where Active Galactic Nuclei (AGN) activity is confined to a single central galaxy. I will discuss the implications of this kinetic feedback for energy injection into the ICM, gas accretion, and the infrared-radio correlation in dense environments. Finally, I introduce our future goal to quantify the cold ICM in ClJ1449.

From a combination of very deep optical, near-IR (HST, Vista, Spitzer) and sub-mm (ALMA) data, we present a comprehensive picture of the highest redshift galaxy-group to have an observationally characterised halo, RO-1001 at z=2.91. There has been direct evidence of a massive Ly-alpha detected cold gas reservoir in this galaxy-cluster progenitor, being fed by accretion streams. This provides enough fuel for the extreme star formation in the three spectroscopically-confirmed primary massive galaxies inside the group that feature a total rate of ~1250 Msun/yr. However, based on a detailed photometric study, possibly within the same environment also exists an extremely old quiescent galaxy passively evolving for ~1.7 Gyr (mass-weighted stellar age). Such conflicting characteristics of similarly massive galaxies (≥10^11 Mstar) within the same group raises the question: HOW? We make a one-to-one comparison to answer this, using the quiescent galaxy and one of its star-forming counterparts. Adding another plot-twist is the ALMA detection of diffuse dust with a net flux of ~3 mJy, distributed over the intergalactic medium within the core of RO-1001. This, we propose, could be an additional channel for cooling the surrounding gas: through IR radiation from collisionally excited dust. This would further deepen the mystery of the existence of the passive galaxy, while also initiating a discussion on this cooling mode that is usually not considered in studies of such 'protoclusters'.

In the past decade, we have discovered thousands of new galaxy clusters via the Sunyaev Zel'dovich effect. These massive clusters span a huge range of redshift, from z=0 to z=2, providing an opportunity to study galaxy cluster evolution in unprecedented detail. In this talk I will summarize the results of a massive (>10Ms) observing campaign with Chandra and XMM-Newton to follow up a large fraction of these newly-discovered systems in the X-ray. These new data have shed new light on the evolution of metal enrichment, dynamical state, cooling state, AGN populations, and BCG growth, among other things, over the past 10 Gyr. I will finish with a brief look ahead, to how SZ and X-ray surveys can work together in the near future to extend such studies into the regime of protoclusters (z=2-3).

The study of Stellar Formation Rates has long been at the heart of galactic/extragalactic astrophysics. Recent galaxy cluster surveys, such as the South Pole Telescope Survey, have revealed that high redshift (z>1) galaxy clusters, unlike their local counterparts, are regions of intense star formation. Previous studies suggest the buildup of stellar mass through positive feedback mechanisms such as major mergers and ram pressure stripping; however our recent investigation of the massive galaxy cluster SpARCS104922.6+564032.5 reveals a new and unexpected mechanism -- an unrestrained cooling flow. More precisely, the cluster stands out as harboring a still assembling brightest cluster galaxy (BCG) undergoing extreme stellar formation at z=1.7091 (~850 M☉/yr). Here, we present 170 ks (~50 hours) of new Chandra observations. Using several techniques for calculating galactic substructure and proxies of cooling flows, we develop a more coherent image of the mechanism responsible for the rampant stellar formation of the BCG. Our results show the presence of a strong cooling flow cospatial with the region of intense star formation ~50 kpc from the BCG indicating a lack of AGN feedback in the system -- in direct contrast to local counterparts. Moreover, the lack of a mechanism to suppress star formation appears to be providing the ideal environment for a buildup of intracluster light. This demonstrates a novel mechanism for the formation of stellar mass in galaxy clusters.

The large gas reservoirs and high rate of galaxy interaction in protoclusters are expected to enhance star-formation activity and trigger luminous SMBH accretion in the host galaxies. We investigated the AGN content of the DRC protocluster at z = 4. Both the total SFR (>7400 Msun/yr) and gas mass (M_H2>1e12 Msun) are among the highest for known protoclusters, suggesting that DRC is undergoing a phase of extremely fast galaxy growth, fed by a large reservoir of cold gas, and not yet quenched by AGN feedback. I will present the results of Chandra observations (139 ks) of DRC. We detected obscured X-ray emission from the two most gas-rich members of the DRC, i.e. DRC-1 and DRC-2. Both of them are resolved into multiple interacting clumps in ALMA and HST observations. Moreover, X-ray stacking analysis reveals that SMBH accretion is likely also taking place in other DRC galaxies that are not detected individually by Chandra. These results point toward the presence of a strong link between large gas reservoirs, galaxy interactions, and luminous and obscured nuclear activity in protocluster environments. In particular, DRC-2 is found to host a Compton-thick (N_H > 1e24 cm^-2) AGN candidate, with luminosity (L_X~3e45 erg/s) comparable to the most luminous QSOs known at all cosmic times. DRC-2 is likely powering a nearby Lya blob detected with MUSE, possibly through photoionization or shock-induced ionization due to a massive outflow launched by the powerful AGN in DRC-2.

Galaxy clusters are known to harbour magnetic fields. The extent of the influence of the intra-cluster magnetic field on cluster member galaxies remains an unresolved question. Intra-cluster magnetic fields can be observed at the density contact discontinuity formed by cool and dense plasma running into hot ambient plasma, and the discontinuity exists in the central region of a merging galaxy cluster Abell 3376. Here we report on unambiguous evidence of an interaction between relativistic electrons and intra-cluster magnetic fields from MeerKAT observations of a radio galaxy MRC 0600-399 possessing bent jets. Contrary to typical bent jets, the jet shows a 90-degree bend at the contact discontinuity, and the collimated jet further extends over 100 kpc from the bend point. The spectral index flattens downstream of the bend point, indicating cosmic-ray re-acceleration. High-resolution numerical simulations reveal that the ordered magnetic field along the discontinuity, at which the intra-cluster magnetic field can be compressed and amplified, plays a significant role in the change in the direction of the jet propagation. The overall morphology of the bent jet bears remarkable similarities with the simulations, which greatly strengthens our understanding of the interaction between relativistic electrons and intra-cluster magnetic fields.

In my talk I will present recent results from a multi-wavelength analysis of the SPT-CL J2106-5844. Among the most massive galaxy clusters ever observed at a redshift z>1, multiple studies have shown that the system is undergoing a major merger. Observations of the Sunyaev-Zeldovich (SZ) effect by the Atacama Large Millimeter Array allowed us to identify two distinct components to the intracluster medium, more or less cospatial with mass sub-structures found in the independent weak-lensing reconstruction of the cluster mass. On the other hand, pilot data from the Evolutionary Map of the Universe survey, in combination with observations by the Australia Telescope Compact Array, highlighted the presence of an extended radio structure, potentially associated with runaway cooling of gas and enhanced star formation activity within the cluster core. Although many questions are still unresolved, this study demonstrated that the inclusion of SZ information into a nearly panchromatic picture of galaxy clusters can provide paramount insights into the dynamical and astrophysical processes operating in the intracluster medium at epochs central for the assembly of galaxy clusters.

Luminous z~4 quasars (QSOs) are the most strongly clustered population known in the early universe, and they should thus reside in massive dark matter halos surrounded by large overdensities of galaxies. Most QSO environment studies at z>~4 have been aimed at detecting overdensities of optical-selected galaxies around QSOs, but to-date such detections have been strongly elusive, revealing contradictory results. This might be the result of the low number statistic and incomplete sampling of the galaxy population around QSOs. Here I present an ALMA and VLT survey of 17 QSO fields at z~4 aimed to statistically trace simultaneously the overdensities of optical and dusty galaxy populations around QSOs. Optical observations show only mild galaxy (LAE) overdensities (~1.4x), in contradiction to the significant overdensities (~18x) of CO(4-3) emitters in these fields revealed by ALMA. We quantify this by measuring the QSO-galaxy cross-correlation function and find that dusty galaxies are ~7 times more strongly clustered around QSOs compared to LAEs. Our results suggest that galaxies near QSOs may contain excess dust, which makes them invisible in the optical, partially explaining the discrepancy that we found between the observed and expected number of LAEs in QSO fields. Our results show strong evidence that QSOs trace massive structures in the early universe and demonstrate the importance of multi-wavelength studies for a complete characterization of protoclusters.

Massive star-forming galaxies and proto-clusters at high redshift, z>2, are thought to be fed by narrow streams of cold, ~10^4K, gas from cosmic web filaments. However, the interaction of these cold streams with the ambient hot CGM is poorly understood. In particular, the observational signatures of this interaction and of cold streams more broadly, the thermal and morphological state of the gas that eventually reaches the central galaxy, and its effect on galaxy evolution, are all open questions. I will present the latest results from a systematic study of this interaction, using a combination of analytic models, idealized high-resolution numerical simulations, and cosmological simulations. We study the effects of hydrodynamics, radiative cooling, self-gravity, the halo potential, and magnetic fields, separately and in tandem, in order to gain insight into stream evolution in different limits. We find that while hydrodynamic instabilities can disrupt streams in the CGM, these are stabilized by cooling, gravity, and MHD. Radiative cooling in the turbulent mixing layer between the stream and the CGM is observable in Lyman alpha, and can explain several observed Lyman alpha blobs. Self-gravity in the streams can lead to star-formation in the CGM at redshifts z>~4. MHD effects lead to magnetically dominated interface regions which appear out of thermal pressure equilibrium and impact the phase structure of the high-z CGM.