Conference Programme

Thursday, June 17 (11:30 - 16:30 UTC)




Poster session and science discussions


11:30 - 12:00

Session 6: The formation, evolution, star-formation and quenching of (proto)cluster galaxies

Chairs: Remco van Der Burg, Carlos De Breuck

12:00 - 13:45 

Environmental effects in the Spiderweb protocluster at z=2.16 DOI:10.5281/zenodo.4980094

Jose Manuel Perez-Martinez

12:00 - 12:15

We use multi-object near-IR spectroscopy with KMOS to investigate the role of the environment in the evolution of the gas properties of galaxies in the Spiderweb protocluster at z=2.16. Based on rest-frame optical emission lines, [NII]λ6584 and Hα, we measure star-formation rates (SFR), gas-phase oxygen abundances, and the gas kinematics of 40 protocluster members as a function of local density and global environment properties. Our results show that galaxies embedded in this structure display on average higher SFRs and metallicity values compared to their field counterparts at similar redshift, while our kinematic analysis yields relatively low gas velocity dispersion for most galaxies, arguing against strong gravitational interactions between protocluster members at this stage. We discuss the implications of these results on different scenarios of environmentally driven galaxy evolution during the early stages of massive cluster assembly.

CO luminosity function of the Spiderweb protocluster at z=2.16

Shuowen Jin

12:15 - 12:30

The CO luminosity function and molecular gas density have been already well explored for field galaxy samples, while these physical quantities in dense environments such as galaxy (proto)cluster, have not yet been characterized. We report a CO(1-0) survey of the Spiderweb protocluster at z=2.16 with 475 hrs of observations with the Australia Telescope Compact Array (ATCA), being part of the COALAS (CO ATCA Legacy Archive of Star-forming Galaxies) large program. This survey contains 13 individual pointings covering an area of 21 arcmin^2 and ±6500km/s range in velocity. We obtained a robust sample of 46 CO(1-0) detections physically related to the protocluster, constituting the largest sample of molecular gas measurements in high-z clusters to date. The CO emitters show an overdensity at z=2.12−2.21, suggesting a galaxy super-protocluster with ~120 cMpc size. We use these CO emitters to place the first constraint on the CO luminosity function and molecular gas density in an overdense environment. I will conclude with the discussion of the efficiency of such a molecular line survey.

Gas mass calibration in a z=2.5 protocluster DOI:10.5281/zenodo.5025819

Minju Lee

12:30 - 12:45

The evolution of cosmic gas content up to z=6 in general fields is now relatively well constrained, thanks to deep and/or wide surveys of dust and CO measurements. Compared to this, the gas measurements of rarer fields including high-z clusters and their progenitors, protoclusters, still suffer from not only the small number of statistics but also the unknown gas mass calibration of, especially more abundant, main-sequence galaxies. Probing these dense environments is important because they offer a unique window in understanding the formation of the earliest quenched population in the early universe. This talk discusses the gas-mass cross-calibration of star-forming main-sequence galaxies in a z=2.5 protocluster, using three different cold gas tracers of dust, CO(3-2), and the lowest atomic carbon transition [C I]^3P_1 −^3P_0 from Atacama Large Millimeter/submillimeter Array observations. The [CI]-based gas measurements are, in general, bridging the gap between the CO(3-2)-based and dust-based measurements. There is a hint that CO-based gas content could be overestimated if one takes the same recipe adopted in the field galaxies. Based on the gas-mass cross calibrations, we find that the mean gas fraction of protocluster galaxies is comparable to field galaxies for a stellar-mass range of log(Mstar/Msun) = [10.6, 11.3]. However, a stronger stellar-mass dependence in the gas depletion time scale is observed and this might be related to earlier quenching in dense environments.

Starbursts with suppressed gas velocity dispersion revealed in a forming cluster at z=2.51 DOI:10.5281/zenodo.4987064

Mengyuan Xiao

12:45 - 13:00

One of the most prominent features of galaxy clusters is the presence of a dominant population of massive ellipticals in their cores. While stellar archaeology suggests that these massive beasts assembled most of their stars in the early Universe via intense starbursts, detailed physical mechanisms and the role of dense environments in triggering the starburst activities remain unknown. Here we report ALMA high-resolution observations towards a forming galaxy cluster core with an intense starburst at z=2.51, which reveals the presence of dynamically cold disks with $V/\sigma > 10$ in both the two cluster starbursts. This is in great contrast with field normal star-forming and starburst galaxies at similar redshifts, which exhibit velocity dispersion at least 3 times larger. The only similar systems reported so far are two lensed starbursts at high redshifts, for which their environment remains unexplored. These findings challenge the common idea that high starbursts are accompanied by high-velocity dispersion, and suggest that the ubiquitous starburst activities in forming cluster cores likely originated from the suppressed velocity dispersion of their gas reservoir.

Rethinking satellite galaxy quenching with the GOGREEN survey DOI:10.5281/zenodo.4974014

Michael Balogh

13:00 - 13:10

I will present first results from the GOGREEN survey of 21 galaxy clusters at redshifts between 1 and 3. The survey was a five year imaging and spectroscopic campaign, with the foundation of a 440h Gemini Large and Long Program to obtain deep spectroscopy of low-mass galaxies in these systems. Its objective was to shed light on the galaxy evolution process by comparing satellite galaxies to their counterparts in the field, thus isolating the role of recent gas accretion from the cosmic web. We find that these cluster populations are remarkably similar to local clusters, in the sense that they have an excess of galaxies that have ceased star formation, relative to similar-mass galaxies in the field. However, unlike results at z=0, the stellar mass function of these galaxies does not show evidence for a significant population of recently transformed (“quenched”) galaxies, a result that is supported by measurements of star formation rates and mass-weighted ages. We conclude that these clusters host a significant population of galaxies that quenched within the protocluster environment at redshifts greater than 3, though no simple model or simulation considered so far is consistent with all the data.

Environmental quenching and galaxy structural evolution in the first cluster environments

Veronica Strazzullo

13:15 - 13:30

We present results from recent and ongoing investigations of galaxy populations in the central regions of a sample of five massive galaxy clusters at z~1.5 identified in the South Pole Telescope Sunyaev-Zel’dovich (SPT-SZ) survey. With a mass threshold of ~4 10^14 M⊙ and an area of 2500 square degrees, at z~1.5 SPT-SZ identifies the rarest, most massive clusters that first emerge from the cosmic web, unique structures where to study early environmental effects on the evolution of galaxies, at a cosmic time bridging active proto-cluster environments at z≳2 and largely quiescent z≲1 cluster cores. In the context of the many recent studies of galaxy populations in distant clusters, we present results focusing on environmental quenching and structural evolution. Albeit with some potentially significant variation, we find typically enhanced quiescent fractions with environmental quenching efficiencies of ~50-80%, suggesting that massive cluster core environments at z~1.5 are already efficient at suppressing star formation. The population of bulge-dominated galaxies is overall also enhanced with respect to the field (morphology-density relation), but the broad structural properties of quiescent and star-forming populations (separately) in the cluster cores are remarkably similar to those of field counterparts. These observations highlight the interplay between star formation suppression and structural evolution in early cluster environments, constraining processes and timescales.

Satellite quenching is not important in z>1 clusters: most quenching happens in the infall region DOI:10.5281/zenodo.4974454

Stephane Werner

13:30 - 13:45

The high quiescent galaxy fraction in redshift 1 clusters suggests that environmental quenching occurs on timescales as short as 1Gyr at high redshift. But recent studies reveal that most quiescent cluster members at this redshift have stellar ages greater than 3Gyr. We address this apparent contradiction by investigating whether the galaxies which fall into redshift 1.0 clusters are pre-processed, or form differently in the protocluster before they fall in. We investigate galaxies in the outskirts of 17 clusters with redshifts between 0.8 and 1.5 drawn from the GOGREEN survey. We find that infall galaxies are typically more massive and have a higher quenched fraction compared to a control sample of field galaxies. We prove that most quiescent cluster galaxies must have been quenched in the infall region, before they entered the cluster. We investigate this quenching in the infall region and find evidence that massive protocluster galaxies reside in dark matter haloes that are significantly more massive than those hosting similar stellar mass galaxies in the field. We interpret these results as evidence that high-redshift clusters are surrounded by massive haloes, which alters the galaxy properties before they become satellites. Since field galaxies do not have similar properties to cluster galaxy progenitors, we argue that using the excess of quenched cluster galaxies compared to the field leads to erroneous estimates of environmental quenching timescales at this redshift.



13:45 - 14:15

Session 6 (continued)

Chairs: Helmut Dannerbauer, Nina Hatch

14:15 - 16:00

Review talk: Star Formation and Quenching in High-redshift Cluster Galaxies

Allison Noble

14:15 - 14:35

The effect of the cluster environment on the star formation and quenching of high redshift galaxies has been elusive. However, technological capabilities and dedicated spectroscopic campaigns have opened a new window onto these processes within massive, virialized clusters at z~1 and all the way back to filamentary, extended protoclusters at z>2.  I will review progress we have made on these fronts through the lens of gas, mass, and stars within high-redshift (proto)cluster galaxies, focusing in particular on recent clues into environmental quenching from ALMA.

Revealing the star formation history of a mature galaxy cluster at z=2 DOI:10.5281/zenodo.4976415

Ariane Trudeau

14:35 - 14:50

At z=1.98, XLSSC 122 is a massive and mature galaxy cluster with 37 spectroscopically confirmed members. We employ 12 band PSF-matched photometry, WFC3 grism spectroscopy and BAGPIPES SED modelling to investigate the star formation histories, stellar mass, dust and metal content of the 21 confirmed red-sequence members. XLSSC 122 therefore provides a unique opportunity to explore massive galaxy evolution in dense environments at early times. The galaxy star formation history results can be placed in the context of the cluster accretion history, using cosmological simulations such as the MultiDark Planck 2 simulation. We demonstrate that red-sequence galaxies display a range of ages, from greater than 2 Gyrs for the BCG to 1 Gyr for the less massive satellite galaxies. Comparisons with simulated galaxy clusters show that these cluster members formed when XLSSC 122 had assembled 10 to 50% of its z=2 mass, a result that provides an important new view of the mass-scale associated with both the onset and cessation of star formation in this cluster.

The Role of Galaxy Clusters in Shaping the Size Growth and Quenching of Galaxies DOI:10.5281/zenodo.4976581

Jasleen Matharu

14:50 - 15:05

The cluster environment is known to be capable of altering the evolutionary paths of galaxies through a variety of mechanisms. High relative velocities of satellite galaxies make minor mergers between them rare. The presence of an intra-cluster medium makes it possible to rapidly quench star formation in satellite galaxies via gas stripping processes. Galaxy clusters are therefore excellent laboratories for testing which galaxy size growth mechanisms dominate and how the quiescent population of galaxies builds up with redshift. Using new results from the HST WFC3 and grism follow up to GCLASS – the largest spectroscopic survey conducted on 10 clusters at z~1 — I will demonstrate how BCG growth via minor mergers with compact cluster galaxies and the production of poststarburst galaxies in clusters can work simultaneously to maintain a stellar mass-size relation that is independent of environment at low redshifts. Last but not least, I will present the first spatially resolved H-alpha maps of cluster galaxies at z~1, and what these have uncovered about environmental quenching in some of the most extreme environments at this crucial epoch in the history of cosmic star formation.

1-min poster presentations


15:05 - 15:20

Building samples of distant cluster galaxies with ALMA DOI:10.5281/zenodo.4973025

Eelco van Kampen


Beyond z>1.5, galaxies in and near galaxy clusters are more actively forming stars than their local counterparts, most likely due to higher gas fractions at these earlier epochs. ALMA is and will be instrumental in quantifying this by measuring dust and molecular gas masses of these high-redshift sources. Results from successful ALMA programs targeting distant galaxy clusters show that there is indeed evidence for differences with local cluster galaxies, which will be highlighted. What is lacking is a large enough statistical sample of such sources, either from targeting specific areas of various clusters, or mapping out a whole cluster up to several times its virial radius and beyond. Strategies for obtaining such large samples of distant cluster galaxies are discussed, with the aim of probing a range of environments. Such strategies include exploitation of the growing ALMA archive.

Star Formation in SpARCS J0330: Bridging the Gap Between Protoclusters and Clusters DOI:10.5281/zenodo.4946372

Joseph Cairns


At z > 2, many studies have identified protoclusters - overdensities of galaxies spanning tens of Mpc in size and hosting some of the most vigorous star formation observed in the Universe to date. Conversely, galaxy clusters are typically observed at z < 1 and are massive, virialised and abundant in quiescent, elliptical galaxies. Current models of galaxy formation and evolution predict that protoclusters will evolve into z ~ 0 massive galaxy clusters, and so there must be some rapid, environmentally-driven quenching of star formation at z ~ 1 that transforms protoclusters into clusters. In order to understand this evolution, we must search for overdensities of galaxies at the onset of cluster formation at which time we can observe the full web of activity: the assembly of a collapsed cluster core feeding on the star-forming filamentary material that resides in the protocluster web. In this project, we present an analysis of SpARCS J0330, one of the most massive, high-z galaxy clusters discovered to date. By building up SEDs with up to 20-band photometry for the spectroscopically confirmed cluster members, as well as for the potential cluster members, we aim to map out distant, star-forming galaxies over a ~15Mpc region across the cluster, bridging the gap between studies of high-z protoclusters and local galaxy clusters, and providing critical insights into how galaxy clusters form out of the cosmic web.

Evolution of the Mass-Size relation in CARLA clusters

Anton Afanasiev


Clusters at redshifts 1.5≲z≲2 present unique medium to study the conversion of their galaxy population from blue starbursts into massive, mostly quiescent objects. The processes occurring in these times are crucial to understand the physics of the clusters in the local Universe. One particularly interesting question is the evolution of galactic size throughout cosmic time, and the dependence of that size on the mass. Here we present the results of the CARLA survey (Clusters Around Radio-Loud Agn) contributing to answering these questions. We study 16 most reliable structures from the CARLA survey ranging from z=1.35 to z=2 with one outlying cluster at z=2.8 using HST F160W and Spitzer channel 1 & 2 imaging. The IR imaging is used to trace the masses of individual galaxies while HST F160W images are fed to GALAPAGOS routine to determine effective radii. Based on these data we construct the mass-size relation for all the clusters (for both passive & active galaxies) and present our results: the percentage of compact galaxies and their possible evolution paths. We also study the dependence of the mass-size relation on some crucial cluster parameters. The more advanced understanding of clusters’ transformation through cosmic time is expected to appear with the launch of JWST and EUCLID, but the clues from studies like CARLA are essential to build up our paradigm of clusters’ evolution.

GOGREEN: The dynamics of clusters at z>1

Andrea Biviano


Using the new GOGREEN spectroscopic data-set (Balogh+21), we analyze the internal dynamics of 14 clusters in the redshift range 0.9-1.4 We determine their mass profile, the orbits of their member galaxies, and the pseudo phase-space density profile. Our results indicate that the clusters in our sample have already reached a high degree of dynamical relaxation. If clusters form at redshift about 2 this leaves only about 2 Gyr for clusters to reach their dynamical equilibrium configuration.

Resolving galaxies in cosmic infrared background peaks of the Planck survey DOI:10.5281/zenodo.5029489

Ruediger Kneissl


The Planck all-sky survey at mm/submm wavelengths has provided a census of the brightest peaks in the cosmic infrared background on angular scales of several arcminutes. Herschel follow-up of the Planck sample has shown that only a few of these sources are individually bright, lensing magnified galaxies, and that the majority are large overdensities of dusty star-forming galaxies at redshifts z~2-4. Reporting on follow-up results of this class of objects, I will highlight a detailed study of the proto-cluster candidate PLCK G073.4-57.5 with ALMA, which detected a large overdensity with 18 millimetre galaxies brighter than 0.3 mJy. Combining with submm and IR data photometric redshifts, infrared luminosities, star-formation rates (SFRs), stellar masses, dust temperatures, and dust masses were derived for two photometric redshift groups with SFRs of 10^3 Msun/yr each. Star-formation efficiencies and gas fractions were compared to the main sequence and average properties of a sample of cluster/proto-cluster members.

SCUBA-2 observations of the Spiderweb protocluster at z=2.2

Andrés Laza Ramos


Galaxy protoclusters are the high redshift predecessors of local galaxy clusters such as Virgo or Coma. The identification and detailed study of protocluster members is vital to understand the evolution process of these systems. This task could be complicated as many galaxies in these structures might be heavily obscured by dust, thus not visible in the optical but very bright in the submillimeter wavelength range. Here we present results from SCUBA-2 observations of a well-known galaxy cluster in formation at z=2, the Spiderweb protocluster, being part of the Radio Galaxy Environment Reference Survey (RAGERS). The main objective of this project is to search for cluster members at 850 micron. The selected submm sources will be correlated with an exquisite multi-wavelength dataset available for this field and their membership to this forming galaxy cluster will be verified. I conclude with a comparison of our work with previous APEX-LABOCA observations of the same structure.

An Optical/NIR Exploration of Proto-structure Environments at High-Redshift with Keck and VLT DOI:10.5281/zenodo.4908910

Brian Lemaux


The past decade has been witness to immense progress in the understanding of the early stages of cluster formation both from a theoretical and observational perspective. During this time, samples of forming clusters at higher redshift, termed “proto-clusters”, once comprised of heterogeneous mix of serendipitous detections or detections arising from dedicated searches around rare galaxy populations, have begun to compete with lower-redshift samples both in terms of numbers and in the homogeneity of the detection methods. Much of this progress has come from optical/near-infrared (NIR) imaging and spectroscopic campaigns designed to target large numbers of typical galaxies to exquisite depth. In this poster I will focus on observations from VIMOS on VLT and MOSFIRE/DEIMOS on Keck, which have uncovered a large number of “proto-structures"" at 2 < z < 5 that appear to resemble clusters and groups forming in the early universe. I will demonstrate the methods for finding, confirming, and characterizing proto-clusters and proto-groups in our sample. Several case studies of spectroscopically-confirmed massive proto-clusters with a diverse set of properties will be presented. I will finally present constraints on the relationship between star formation rate, stellar mass, and galaxy density at these redshifts.

Systematic Optical/NIR Discovery of Large Scale Structure at Intermediate and High Redshift DOI:10.5281/zenodo.4922030

Denise Hung


Voronoi tessellation Monte-Carlo (VMC) mapping is a powerful new technique for finding galaxy overdensities when combined with rich optical and near-infrared multi-band imaging and a wealth of spectroscopy. I will show how we were able to use this technique to measure precise systemic redshifts, estimate the total gravitating mass, and maintain high levels of purity and completeness even with moderate levels of spectroscopy across more than an order of magnitude in total structure mass at z~1 with the Observations of Redshift Evolution in Large-Scale Environments (ORELSE) survey. I will also show how we used the discovered structures to construct a cluster mass function and perform basic cosmological fitting that previously was only accomplished in the local universe for optical datasets. At higher redshifts (z>2), VMC mapping has already been integral to the detection and characterization of individual large-scale structures in the VIMOS Ultra Deep Survey (VUDS) and Charting Cluster Construction with VUDS and ORELSE (C3VO) survey. I will present preliminary results of new systematic structure searches at 2<z<5 both from the original VUDS survey and those enabled by recent spectroscopy obtained with Keck as part of the C3VO survey. Our successes with VMC mapping across a huge redshift baseline (0.5<z<5) and very different surveys attest to its potential to find structure in any number of current and future spectroscopically rich galaxy surveys in the high-redshift Universe.

A Protocluster at z=3.37 Around a Massive Quiescent Galaxy with an Elevated Quenched Fraction

Ian McConachie


The Massive Ancient Galaxies At z > 3 NEar-infrared (MAGAZ3NE) Survey is a near-infrared spectroscopic survey of high-redshift ultra-massive galaxies (UMGs; M_* > 10^11 M_solar) to determine how these extreme galaxies evolve. In this talk, I will discuss the some of our recent results on three spectroscopically confirmed protoclusters discovered around UMGs targeted by the MAGAZ3NE survey. Two of these protoclusters at z~3.37 contain bright, post-starburst UMGs and lie only ~35 comoving Mpc from each other, indicating that they could be progenitors of a massive z=0 cluster. Using deep photometric catalogs, one of the protoclusters is shown to have an elevated fraction of quiescent galaxies to the coeval field for galaxies with stellar mass M_* > 10^11 M_solar. This high quenched fraction provides a striking counterexample to the ubiquitousness of star-forming galaxy protoclusters in the early Universe and suggests that protoclusters may exist in a diversity of evolutionary states. Furthermore, the UMG in this protocluster is remarkable because it is among the most massive protocluster BCGs at this redshift and it has already quenched. The presence of such a massive quiescent galaxy in this protocluster when most UMGs are still assembling their stellar mass may indicate that massive galaxies in protoclusters rapidly form their stellar mass before those in lower-density environments.

Quiescent galaxies in clusters are older than those in the field at z>1: Implications for quenching DOI:10.5281/zenodo.4984934

Kristi Webb

15:20 - 15:35

Galaxies in dense clusters experience additional quenching processes compared to field galaxies, which are subject only to secular processes. Timescales of quenching processes can be used to constrain the physical mechanisms which suppress star formation. With the Gemini Observation of Galaxies in Rich Early Environments (GOGREEN) survey, we have collected a sample of ~300 spectra of quiescent galaxies (~200 in clusters) at z=1-1.5 -- when the star formation rate was twice as high as it is today. We explore the differences of the populations, as a function of both environment and mass, through modelling their star formation histories. We confirm that in general there is mass-dependent evolution, and add to this picture that at fixed mass galaxies in the field have overall longer star forming time scales and later formation times. We try to explain this age difference through two scenarios, i) galaxies in clusters formed earlier or ii) galaxies in clusters experience environmental quenching post-infall, and find that neither are sufficient (without preprocessing) in simultaneously predicting the observed age difference and quenched fractions. This is distinctly different from local clusters, for which quenching of recently accreted field galaxies plays an important role, particularly at low stellar masses. Our results suggest that the quenched population in galaxy clusters at redshifts above one has been driven by different physical processes than those at play locally.

Searching for kinematic signs of ram pressure stripping in z=1.6 clusters with resolved ALMA CO(2-1) DOI:10.5281/zenodo.4973997

William Cramer    

15:35 - 15:50

The evolution of the inter cluster medium over cosmic time, and its interaction with galaxies within those clusters, is key to understanding galaxy evolution and quenching processes. We present spatially resolved ALMA observations of 24 galaxies across three clusters at z=1.6, the largest such sample of cluster galaxies observed at this redshift. We find signs of interaction with the ICM, through the influence of ram pressure stripping, to be affecting the molecular gas in these galaxies. We find signs of enhanced molecular gas fractions, kinematic disturbances in the disk, and asymmetric head-tail morphologies, consistent with observations of ram pressure stripped galaxies at low redshift. We present preliminary results of our spatial and kinematic analysis of these galaxies, and compare the properties of galaxies in these clusters to low-redshift clusters in which the effects of ram pressure have been observed.

Discussion and summary of session

Allison Noble, Nina Hatch, Helmut Dannerbauer, Carlos De Breuck, Remco van der Burg

15:50 - 16:00

Poster session and science discussions


16:00 - 16:30

End of day



GCF 2021 channels on

This workshop is supported by ESO and the Smithsonian Astrophysical Observatory.

Keywords: merging clusters | high-z clusters | proto-clusters | detection, classification & comparison | ICM physics | AGN feedback | star formation | galaxy evolution | future facilities

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