The study of the formation of galaxy clusters is incomplete without connecting measurements of clusters at low redshifts to their progenitor structures in the early universe. New surveys are now finding significant numbers of these cluster progenitors at z > 2 with relative ease. While some of these are associated with powerful active galaxies (radio galaxies and quasars) that are good signposts for the progenitors of massive central cluster galaxies, the majority of structures being identified today are found based on large-scale overdensities of galaxies in deep, wide optical surveys, enhancements in the hydrogen optical depth as measured by tomographic studies of the intergalactic medium, and large spatial excesses of redshifted dust continuum emission from cluster-forming regions. These large structures of galaxies may furthermore have played an important role in the reionization of the universe, which can be tested by upcoming experiments. In this talk, I will review our current understanding of this rapidly growing field and the main questions we hope to answer during this decade.

The Lyman Alpha forest is currently one of the best probes of the neutral gas on large scales at high redshift (z ~ 2-3). The onset of large Lyman Alpha forest tomographic surveys, such as CLAMATO, allows for the study of the influence of different heating mechanisms on the large-scale absorption field. Using simulations, I will show that these 3D tomographic maps can be used to directly probe pre-heating in proto-clusters by looking at the distribution of Lyman Alpha transmission as a function of dark-matter density. This will help to constrain the various models of galaxy and AGN feedback in proto-clusters.

Lyman alpha emitting galaxies (LAEs) are excellent tracers of large scale structures and can be efficiently selected with narrowband technique at very high redshift. Many protoclusters have been discovered with LAEs at redshift up to 6.6. In this talk, I will present a protocluster at redshift 7 discovered by the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. We used the Dark Energy Camera on CTIO Blanco 4m telescope with a custom-made narrowband filter NB964 to search for LAEs at redshift 7. In the LAGER-COSMOS field, we discovered a protocluster with a significant overdensity of 5.1 and mass of 3.7e15 solar masses. It has been spectroscopically confirmed with Magellan Telescope observations. Furthermore, we found its member galaxies are able to provide sufficient ionizing photons to ionized the IGM within the protocluster. Note the average neutral hydrogen fraction of IGM is still 0.2--0.4 at redshift 7. Thus, this protocluster provides a unique natural laboratory to investigate the environment effect on the reionization and the structure assembling in the early universe.

At z>2, galaxies in proto-clusters (PCs) are actively star forming, while matured clusters at z<1 are dominated by quenched galaxies, suggesting that z~1.5 is a transition epoch of (proto-)clusters from star forming phase into quenching phase. To investigate galaxy evolution in this transition epoch, we search for PC cores at z~1.5. Here, a "core" is defined as the most massive halo in a given PC, where environmental effects are likely to work most effectively. Using a photo-z catalog of large and deep optical survey data with Subaru Hyper-Suprime Cam, we search for PC cores in a very wide field that reaches ~22deg2. Regarding galaxies more massive than ~2e11Msun as the central galaxies of PC cores, we detect more than 1000 core candidates. Then, we estimate their average halo mass by clustering analysis and find it to be ~3e13Msun. From a comparison with the Illustris TNG simulation, we confirm that these massive halos at z~1.5 are progenitors of present-day clusters comparable to or more massive than Fornax-type clusters. Classifying our galaxy sample into red and blue galaxies, we calculate the red fraction of the member galaxies of our PC core candidates. Although red and blue centrals have similar halo masses, only the red fraction around red centrals shows an excess compared to the field, while that around blue centrals shows no excess, suggesting a conformity effect. Combining with the Illustris TNG simulation, we discuss galaxy quenching in PC cores at z~1.5.

Using the Quasar Sightline and Galaxy Evolution (QSAGE) survey, we demonstrate that the UV brightest quasars at z = 1 - 2 live in overdense environments. This is based on an analysis of deep Hubble Space Telescope WFC3 grism spectroscopy of the galaxies along the lines-of-sight to UV luminous quasars in the redshift range z = 1 − 2. Of the 12 quasar fields studied, 8 display evidence for a galaxy overdensity at the redshift of the quasar. One of the overdensities, PG0117+213 at z = 1.50, has potentially 36 spectroscopically confirmed members, consisting of 19 with secure redshifts and 17 with single-line redshifts, within a cylinder of radius ∼ 700 kpc. Its halo mass is estimated to be log(M/Msol) = 14.7. This demonstrates that spectroscopic and narrow-band observations around distant UV bright quasars may be an excellent route for discovering protoclusters. Our findings agree with previous hints from statistical observations of the quasar population and theoretical works, as feedback regulated black hole growth predicts a correlation between quasar luminosity and halo mass.

We revise the three-dimensional (3D) overdensity field in a protocluster at z = 3.3, using new spectroscopic observations obtained from Keck/MOSFIRE and previous spectroscopic data obtained as part of the VIMOS-VLT Deep Survey (VVDS) and VUDS VIMOS Ultra-Deep Survey (VUDS). The protocluster is embedded in a large-scale overdensity structure around the original detection and has an estimated total mass of ~2.61e^15 solar masses. This proto-structure contains six clear density peaks connected by filaments. In this talk, I will focus on the two radio-detected AGN (RAGN) in this proto-structure, which have been confirmed by spectroscopic redshifts of their optical counterparts. The RAGN are hosted by the brightest and most massive galaxies, although their hosts show extreme differences in color, indicating that they are dominated by different stellar populations and have experienced different evolutionary paths. We find that the RAGN are not in the most locally dense parts of the proto-structure, but are fairly close to the centers of their parent peaks. I will then discuss the relation between RAGN and the environments of proto-structure.

High-z void galaxies, whose evolution has been driven almost completely free from galaxy mergers, are ideal targets to provide valuable insights into the role of the environment in galaxy evolution. However, a very wide galaxy survey with spectroscopic redshifts are required to find void regions, making it and there have been no studies beyond z>3. In this work, we develop a new deep learning method to select z~4 void galaxies from the g-dropout catalog produced by the HSC-SSP survey; called VoidNet. The VoidNet uses the sky distribution of galaxies and their (g-r) colors as a proxy for redshift despite the large uncertainty to characterize the three-dimensional spatial distribution of galaxies. We train the VoidNet by using Millennium simulation, and when setting a conservative threshold (recall = 0.1%), the VoidNet achieves 90% precision, which is about 20% better than 2D selection. This result shows that deep learning can provide better estimates of the large-scale structure of the universe even when using the photometric data. We are applying this same method to the identification of proto-clusters as well as voids to construct a significantly large sample.

Protoclusters are the densest and the most actively star-forming regions in the early universe. Therefore they are essential for understanding the cosmic star formation history. We study star formation and chemical abundance evolution in the protocluster region at z=0-10 using the cosmological SPH simulation code GADGET3-Osaka. In our simulation, the total star formation rate in the protocluster region reaches 3000 M⦿yr-1 at z=3, with the core region accounting for about half of the star formation in the protocluster at z~2. By studying the chemical abundance evolution, we find that the effect of Type II supernovae appears at z>3, while the effects of Type Ia supernovae and AGB stars appear at z~3. Furthermore, we also compare the chemical abundance of the gas in individual galaxies with observations and suggest that chemical evolution proceeds faster in overdense regions.

We investigate environmental dependence of star-forming galaxies at z=2.2 based on very deep narrow-band H-alpha imaging both in the field and protoclusters with Subaru/MOIRCS. We sampled 67 galaxies in the COSMOS field down to M*~10^8 Msun and to SFR~2 Msun/yr. Stellar mass and dust extinction were estimated by SED fitting using public data from ULTRA-VISTA, Hyper Supreme-Cam, Supreme-Cam, and Hubble Space Telescope. We compare those galaxies in the deep field with those in our deep protocluster data by Hayashi et al. (2012) and Shimakawa et al. (2018a,b), but applied the same technique and parameters to derive their physical quantities for a fair comparison. Although we see starbursting galaxies above the SFMS at low mass end (< 10^9.3 Msun) in protoclusters, no such trend is seen in the field environment. This suggests that some environmental effects may be at work in low mass galaxies in high density environments to enhance their star forming activities. We note that even deeper observations are needed for protoclusters in order to investigate the environmental effects in more detail. We will also discuss these results in the context of early environmental effects in protoclusters.

Various studies report the higher star formation rate of protocluster galaxies at z~2. It is crucial to investigate the picture at higher redshift. On the other hand, it was not impractical due to the lack of enough samples of the protoclusters. We estimate the rest-frame ultraviolet luminosity function of g-dropout galaxies in 177 protocluster candidates (PC UVLF) at z∼4 selected in the PDR1 of the Hyper Suprime-Cam Subaru Strategic Program. Comparing it with the UVLF of field galaxies at the same redshift, we find that the PC UVLF shows a significant excess toward the bright end. This study shows that the enhancement in star formation in overdense regions can generally be seen as early as at z∼4. We also estimate the star formation rate density in protocluster regions as 6-20% of the cosmic SFRD, based on the measured PC UVLF after correction for the selection incompleteness in our protocluster sample. This high value suggests that protoclusters make a non-negligible contribution to the cosmic SFRD at z∼4, as previously suggested by simulations. We will also show the result of the latest HSC-SSP data release.

In this work, we study the properties of the six optically dark galaxies detected in the 69 arcmin2 GOODS-ALMA 1.1mm continuum survey. While none of them are listed in the deepest H-band based CANDELS catalog in the GOODS-South field down to H = 28.16 AB, we were able to de-blend two of them from their bright neighbor and measure an H-band flux for them. We present the spectroscopic scan follow-up of five of the six sources with ALMA band 4. All are detected in the 2 mm continuum with signal-to-noise ratios higher than eight. One emission line is detected in AGS4 and AGS17, which we interpret in both cases as being due to the CO(6–5) line at zAGS4 = 3.556 and zAGS17 = 3.467, respectively. These redshifts match both the probability distribution of the photometric redshifts derived from the UV to near-IR SEDs and the far-IR SEDs for typical dust temperatures of galaxies at these redshifts. We present evidence that nearly 70% of the optically dark galaxies belong to the same overdensity of galaxies at z ∼ 3.5. overdensity The most massive one, AGS24 is the most massive galaxy without an active galactic nucleus (AGN) at z > 3 in the GOODS-ALMA field. It falls in the very center of the peak of the galaxy surface density, which suggests that the surrounding overdensity is a proto-cluster in the process of virialization and that AGS24 is the candidate progenitor of the future brightest cluster galaxy (BCG).

Understanding the build-up of stellar mass is one of the primary goals of extra-galactic astronomy. Various theoretical and observational studies find that star formation activity for galaxies in high density region peaks between redshift z=3-5, whereas for galaxies in the low density environment it peaks between redshift z=1-2. I will present the first measurements of formation histories of galaxies in a proto-cluster at z~2. We use Spectral Energy Distribution fitting module Prospector to measure the star formation histories. I will demonstrate how environment affects the stellar mass build-up in high stellar mass galaxies in our sample and how the relative star formation histories of galaxies at z=2 depends on their stellar masses. I will also present a comparison of star formation histories from Prospector to star formation histories from ILLUSTRISTNG cosmological simulations and discuss the role of mergers and their gas fractions in shaping the star formation histories of galaxies till z=2.

Do we see the famous transition from gas-rich with high SFR to gas-poor with high stellar masses and SZ hot gas protoclusters at around z=2, as predicted by simulations ? Many observations with different selections are piling up and show great variety. Here we use a biased sample towards high SFR (color-selected w/ Planck, Herschel, Spitzer, IRAM) and try to put constraints at z~2.

It has been hypothesised that submillimetre galaxies (SMGs) are the progenitors of local early-type cluster galaxies. If true, this would imply that SMGs should reside in galaxy cluster progenitors at high redshift. Whilst there are well-known examples of SMGs residing in protoclusters, these systems were selected for follow-up because of their high galaxy or SMG density. To explore the environments of SMGs in an unbiased way we have undertaken a narrowband VLT/HAWK-I study of H-alpha and [OIII] emitters around three ALMA-identified and spectroscopically-confirmed SMGs at z ∼ 2.3 and z ∼ 3.3, which were selected with no prior knowledge of their environments. On average, these SMGs reside in environments which are ∼2–4x overdense compared to the field. Our results suggest that SMGs do tend to reside in protocluster-like environments, supporting the claim that they likely evolve into the passive early-type galaxies observed in local clusters.

It is still unclear whether sub-mm galaxies (SMGs) are optimal tracers of protoclusters. Here I present the first systematic study of the Mpc-scale environments of SMGs. We have used the Poisson Probability Method (PPM) to search for protoclusters around 12 spectroscopically confirmed SMGs at z~2-5 located in the CANDELS/GOODS-N field. By using photometric redshift catalogs of different galaxy populations, we are able to detect overdensities around 11 out of 12 SMGs, i.e., 92% +/- 8%, six of them are even new discoveries. As the PPM enables a characterization of the protoclusters in terms of redshift, core size, and miscentering with respect to the SMG, I will present the physical properties of the detected overdensities. Based on our work, I will show the suitability of SMGs as excellent signposts of protoclusters. Finally, I will discuss applications of this work on future surveys with forthcoming facilities like Euclid, LSST, and WFIRST.

The study of very distant galaxy protoclusters (beyond z=4) has strongly developed in the past years. In particular, the range z=5-6 is considered as the epoch in which the progenitors of local galaxy clusters start to form. We present evidence of a rich protocluster at z=5.2 in the GOODS-N field containing an already known overdensity of galaxies around the well-studied submillimeter galaxy HDF850.1. We have selected candidate cluster members (Ly-alpha emitters and Lyman-break galaxies) from the optical 25 medium-band photometric catalog of the project SHARDS (Survey for High-z Absorption Red and Dead Sources). We have spectroscopically confirmed 10 new members via the Ly𝛼 line using OSIRIS at the GranTeCan (GTC), with a success rate of ~80%. Thus, we increased the number of confirmed members in this overdensity from 13 to 23 objects. Combining our dataset with the sample from the literature, we investigated the spatial distribution, the dynamical state and the clustering properties of this protocluster for the first time, revealing a complex structure. Furthermore, we studied the radio and far-infrared properties of its members in order to measure the fraction of obscured star formation. Considering that we observed only a part of this structure, we claim that it could be the richest protocluster beyond z=5. I will conclude, based on cosmological simulations, on the fate of this overdensity at z=0.

I will report on the discovery of a galaxy overdensity around the luminous quasar SDSS J1030+0524 at z=6.31. The structure is populated by at least six members, identified through dedicated spectroscopic campaigns at the Keck, VLT and LBT, with redshifts in the range 6.219-6.355. Assuming negligible peculiar velocities, this range corresponds to radial separations of ~5 physical Mpc from the QSO, that are comparable to the observed projected separations. The result is significant at more than 3.5sigma, and the overdensity level is at least 1.5-2 within the large volume probed (∼780 physical Mpc3). This is the first spectroscopic confirmation of a galaxy overdensity around a supermassive black hole in the first billion years of the Universe. Our finding lends support to the idea that the most distant and massive black holes form and grow within massive (> 1e12 Msun) dark matter halos in large scale structures, and that the absence of earlier detections of such systems was likely due to observational limitations. Prospects for future observations are discussed.

Obtaining large sample of galaxy groups and (proto-)cluster cores at redshift z>1.5 is crucial to better understand cluster formation and the role environment plays in shaping galaxy properties. While dozens of these structures have now been studied, a systematic search for such systems has proven challenging. Exploiting deep near-infrared selected photometric data is a promising approach. We designed a new group finder algorithm, the DElaunay TEssellation ClusTer IdentiFication with photo-z (DETECTIFz) to detect galaxy groups in photometric data. The algorithm uses the joint PDF of redshift and stellar mass of galaxies to identify groups as stellar-mass over-densities. Using data from three of the deepest near-infrared surveys: UKIDSS UDS, VIDEO and UltraVISTA, that were homogeneously reduced (the REFINE survey), we build a pure (>90%) sample of 448 candidate groups up to z=2.5 and study some of their properties. In this talk I present results from Sarron & Conselice (submitted). I will detail the DETECTIFz algorithm and show its performances on mock galaxy catalogues. I will show some properties of our group catalogue. In particular, we find that galaxies with (log) stellar mass between 10.25-11 Msun have higher quenched fractions in groups than in the field up to z=2.3, the difference growing with decreasing redshift.

We present HST, XMM and SZ observations of the high redshift galaxy cluster XLSSC 122. These observations confirm XLSSC 122 at z=1.98 and reveal a massive and remarkably mature galaxy cluster viewed 3.4 Gyr after the big bang. HST WFC3 spectroscopy identifies 37 members that are strongly bimodal in colour. A 100ksec XMM exposure reveals an intra-cluster medium that is hot (5 keV), metal-rich (0.3 solar) and contained within a massive potential well (M500c=0.6-1e14 Msolar). As a mature, very high redshift cluster XLSSC 122 provides an important perspective on the end of proto-cluster formation and the first emergence in the universe of present-day cluster properties, e.g. a metal rich intra-cluster medium and an old, quenched red sequence galaxy population. XLSSC 122 therefore offers an important waypoint in our understanding of the interplay between the evolution of large scale structure and the galaxy populations they host.

The sub-millimeter galaxy (SMG) population with high star formation rates of 200-2000 M☉/yr (compared to the Milky Way’s 1 M☉/yr) pose a challenge for cosmological simulations of how galaxies form and evolve, particularly in the first few billion years after the Big Bang. They are 1000 times more prevalent 10 billion years ago than today, highlighting a peak epoch of galaxy building and contributing significantly to the buildup of the Universe’s stars. In massive galaxy protoclusters in the early Universe, SMGs are often dominant. As such they can elucidate the formation modes of cluster cores, the assembly of the “Brightest Cluster Galaxies”, and the collapse of the cosmic web over large scales. I will present an ongoing survey with the South Pole Telescope which has uncovered protoclusters of galaxies at very early (z=4-7) epochs in the Universe, when the central core is collapsed and actively forming stars and the galaxy halos are actively merging. I will discuss what they teach us about galaxy growth, the establishment of the ICM, and the collapse of large scale structure in an evolving Universe.

Galaxy protoclusters offer us a glimpse back to the most active phase in the assembly of large-scale structure, when mergers and starbursts were at their peak. While this star formation is bright and easy to detect at millimetre wavelengths, the stars themselves are heavily obscured by dust at optical and infrared wavelengths, thus we know very little about the evolutionary state of the galaxies that make up these exciting systems. SPT2349-56 is perhaps the most extreme example, containing dozens of star-forming galaxies packed into a cubic megaparsec at z=4. We have now undertaken deep and extensive optical and infrared follow-up observations of this structure, measuring for the first time the stellar masses of a statistically significant number of z=4 protocluster galaxies. I will discuss our results, including the establishment of the galaxy main sequence in SPT2349-56, and a novel measurement of the z=4 galaxy cluster stellar mass function.