Division J - Abstracts


Semi-Analytic Model Predictions of the Galaxy Population in Proto-clusters

Contini, Emanuele

We investigate the galaxy population in simulated proto-cluster regions using a semi-analytic model of galaxy formation, coupled to merger trees extracted from N-body simulations. We select the most massive clusters at redshift z=0 from our set of simulations, and follow their main progenitors back in time. The analysis shows that proto-cluster regions are dominated by central galaxies and their number decreases with time as many become satellites, clustering around the central object. In agreement with observations, we find an increasing velocity dispersion with cosmic time, the increase being faster for satellites. The analysis shows that proto-clusters are very extended regions, >20 Mpc at z>1. The fraction of galaxies in proto-cluster regions that are not progenitor of cluster galaxies varies with redshift, stellar mass and area considered. It is about 20-30 per cent for galaxies with stellar mass ~ 10^9 M_sun, while negligible for the most massive galaxies considered. Nevertheless, these objects have properties similar to those of progenitors. We investigate the building-up of the passive-sequence in clusters, and find that their progenitors are on average always active at any redshift of interest of proto-clusters. The main mechanism which quenches their star formation is the removal of the hot gas reservoir at the time of accretion. The later galaxies are accreted (become satellite), and the more the cold gas available, the longer the time spent as active. Central galaxies are active over all redshift range considered, although a non-negligible fraction of them become passive at redshift z<1, due to strong feedback from Active Galactic Nuclei.

The First Black Holes in the Cosmic Dark Ages

Pacucci, Fabio

The first black hole seeds formed when the Universe was younger than 500 Myr old and they played an important role in the growth of early (z=7) supermassive black holes. While much progress has been made in understanding their formation and growth, their observational signatures remain largely unexplored. As a result, we are yet to detect these sources. We present a novel photometric method to identify black hole seed candidates in deep multi-wavelength surveys. The method relies on infrared and X-ray observations and selects the only two objects with a robust X-ray detection found in the CANDELS/GOODS-S survey with a photometric redshift z>6. Moreover, the case of the z=6.6 Lyman alpha emitter CR7 is discussed. We show how its observational features are compatible with our black hole seed model and we present the results of our variability and spectroscopic studies with the HST and Keck. To date, these objects represent the most promising black hole seed candidates, possibly formed via the direct collapse black hole scenario. While this result is based on the best photometric observations of high-z sources available to date, additional insights are expected from deeper spectroscopic and X-ray data. For this reason, we explore the role that JWST will play in the detection of the first black holes and their host galaxies in the Universe.

Properties of a Virgo-like progenitor at z˜2

Nanayakkara, Themiya

Driven by the development of sensitive medium band imaging surveys, we have been able to obtain accurate photometric redshifts of galaxies to identify highly dense regions of galaxies at high redshift. MOSFIRE follow up of one such cluster identified by the ZFOURGE survey (Spitler et al., 2012) spectroscopically confirmed a large extended structure at z˜2 (Yuan et al., 2014), which was shown by simulations to form into a Virgo like cluster at z˜0.We used near-infra-red spectroscopy from MOSFIRE to obtain rest-frame optical emission lines to study star-formation, metallicity, ionization conditions, Initial mass function, and dynamics of this system. In my talk I will present a summary of the results we have obtained on this structure as a part of the ZFIRE survey and address the role of environment in the early universe in determining galaxy stellar population and inter-stellar-medium conditions.   

TolTEC extragalactic surveys of cluster and protocluster environments

Aretxaga, Itziar

TolTEC a large-format (~7000 detector) imaging polarimeter will conduct two legacy extragalactic surveys at 1.1, 1.4 and 2.1mm in 2019 at the 50m Large Millimeter Telescope. The Ultra-Deep Survey of Star-forming Galaxies (~1 sq.deg, r.m.s. ~0.025mJy at 1.1mm) is a confusion-limited survey which ties the entire Luminous Infrared Galaxy population from redshifts 2 to 10 directly to their optical counterparts and addresses the question of how do massive galaxies build up metals and stellar mass over cosmic time. The Large Scale Structure Survey (~100 sq. deg, r.m.s.~0.25 mJy at 1.1mm) probes the relationships between the spatial distribution of star forming galaxies and large scale structure and provides a detailed view of clusters and their substructure via the Sunyaev-Zeldovich (SZ) effect. We show the feasibility studies for cluster and protocluster environments derived from these surveys scaled from the existing 1.1mm AzTEC LMT surveys: abundance, detectability and follow up.

Molecular gas in two companion cluster galaxies at z = 1.2

Castignani, Gianluca

Probing the star formation (SF) history of distant cluster galaxies is essential to evaluate the effect of dense environment in shaping galaxy properties we observe today. We exploit NOEMA interferometer observations to study the molecular gas properties of two companion star-forming galaxies, with a projected angular separation of 6 kpc, and belonging to a galaxy cluster within the Irac Shallow Cluster Survey, at a redshift z=1.2, i.e., ~2 Gyr after the cosmic SF density peak. We describe the first CO detection from 1 < z < 1.4 star forming cluster galaxies with no reported evidence of AGN. We exploit NOEMA observations at ~3mm to detect CO(2-1) line emission from the two selected galaxies, unresolved by our observations. Based on the CO(2-1) spectrum we estimate a total molecular gas mass M(H2)=2.2e+10Msun, for the two blended sources, where a Galactic CO-to-H2 conversion is used. Assuming that the two sources equally contribute to the observe CO(2-1) our analysis yields a SF rate of ~78 Msun/yr, a (short) depletion time scale of ~0.14 Gyr, and a molecular gas fraction of ~15%, for each of two sources. We also provide a new more precise measurement of an unknown weighted mean of the redshifts of the two galaxies, z=1.163+/-0.001. Our results are in overall agreement with those of other distant cluster galaxies and with model predictions for main sequence field galaxies at similar redshifts. We suggest a late assembly of the two cluster members, via infall of gas as well as a scenario where the two galaxies are close to their interaction.


Beyond the Milky Way: galaxies in the Ophiuchus cluster

Durret, Florence

Ophiuchus is the second brightest cluster in X-rays in the nearbyuniverse, but due to its low Galactic latitude its optical propertiesremain poorly known.  Based on a deep image of the Ophiuchus clusterin the r' band obtained at the CFHT with MegaCam, we have applied aniterative process to subtract the contribution of the numerous starsthat pollute the image, and obtained a photometric catalogue of 2818galaxies fully complete at r'=20.5 mag. We use this catalogue toderive the cluster Galaxy Luminosity Function (GLF) in several zones(defined with density maps), and for a region covering exactly thesame physical size as that in which the GLF of the Coma cluster wasstudied. The GLF fits are much better fit when a Gaussian is added tothe usual Schechter function, to account for the excess of very brightgalaxies. Compared to Coma, Ophiuchus shows a strong excess of brightgalaxies.  The properties of the two nearby very massive clustersOphiuchus and Coma are quite comparable, though they seem embedded invery different large scale environments. Our interpretation is thatOphiuchus has built up long ago, as confirmed by its relaxed state,while Coma is still in the process of forming. Therefore, comparingcluster properties, even at low redshift, can give informations oncluster formation.

Infrared Weak-Lensing Analysis of the Emerging Galaxy Cluster SPARCSJ1049+56 at z=1.7

Finner, Kyle

The hierarchical formation model of structure in the universe suggests that galaxy clusters are assembled through a series of mergers. In turn, it is expected that galaxy clusters in the early universe are actively forming and dynamically young. Located at a high redshift of 1.7, SpARCSJ1049+56 offers a unique look into the galaxy cluster formation process. This cluster has been shown to be rich in cluster galaxies and have intense star formation in the brightest cluster galaxy. At such high redshift, the cluster pushes a weak-lensing analysis beyond the regime of the optical spectrum into that of the infrared. Equipped with deep Hubble Space Telescope WFC3 UVIS and IR observations, we present a weak-lensing characterization of SpARCSJ1049+56. As few IR weak-lensing studies have been performed, we discuss the details of point spread function modeling and galaxy shape measurement for an IR weak-lensing procedure and the systematics that come with the territory. Through a careful analysis, the mass distribution of this young galaxy cluster is mapped and compared to the galaxy distribution. The mass of the cluster is quantified by a mass-concentration relation and compared to the mass derived from galaxy velocity dispersion. Further tests are performed to ponder the accuracy of using a mass-concentration relation for galaxy clusters in an early state of formation.

K-CLASH: The field and cluster environment at the epoch of the build-up of the red sequence

Vaughan, Sam

Integral Field Spectroscopy (IFS) surveys are opening new windows on the nature of galaxies at z ~1—2 (e.g. KROSS, the KMOS Cluster Survey, and KMOS3D) and in the local Universe (e.g. MaNGA, SAMI, CALIFA, ATLAS3D), comparing the spatially-resolved, rest-frame optical properties of star forming galaxies at each epoch. They reveal a stark difference between galaxies 10 Gyr ago at the peak of cosmic star formation rate density that tend to be turbulent, highly star-forming gaseous disks, and the comparatively quiescent, ordered spirals we see in the local Universe. Further, in clusters, over the same period we witness a build-up of the “red sequence” of galaxies, with the fraction of S0 galaxies increasing. But whilst comparisons between these two extremes have revealed much about the nature and evolution of galaxies, there is an absence of equivalent studies at intermediate times. It is therefore not clear which processes are driving the changes we see between galaxies in the distant past and the present day, nor at what point these changes occurred. The K-CLASH survey is bridging this gap by using the K-band Multi-Object Spectrograph on the VLT to study the spatially resolved gas kinematics and chemistry of galaxies in 4 CLASH clusters (Postman+12) at 0.4 < z < 0.6, and in field galaxies along the same lines of sight. In combination with thousands of pre-existing IFS observations of galaxies at z ~ 1—2 and at z ~ 0, K-CLASH will allow us to determine the the role of environment in the build-up of the red sequence and the formation of fast-rotator, early-type galaxies, as well as catching the galaxy population in transition from turbulent, highly star-forming disks to the familiar late-type galaxies we observe today. I will describe the K-CLASH survey and highlight some key early science results, including measurements of their ionised gas kinematics, and finish with a discussion of what we may learn about the evolution of galaxies over time in either environment.

Mapping cluster evolution with Hubble Frontier Fields & BUFFALO observations

Jauzac, Mathilde

Massive galaxy clusters make the best, and most efficient locations to observe and trace the mass assembly processes of the Cosmic Web. Residing at the vertices of it (Bond et al. 1996), they grow by steady accretion of matter from the surroundings, as well as by discrete mergers with nearby groups and clusters. Supported by simulations, this scenario predictions regarding the total mass content and distribution of infalling substructures remain largely untested.Recently, some of the most massive and disturbed clusters have been the centre of attention thanks to the \emph{Hubble Frontier Fields} (HFF) initiative, which constitutes the largest commitment ever of \emph{Hubble Space Telescope} (HST) time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. These clusters were chosen for their strong lens properties, and are all highly disturbed objects, showing major and minor merging on-going processes, making them ideal target to trace the Cosmic Web assembly.While combining strong and weak-lensing regimes to map the total mass with X-rays observations of the hot gas and spectroscopy of cluster galaxies to look at their direction of motion, we can thus study the dynamical scenarios in place within these massive galaxy clusters, and trace the sub-structures engaged in these processes. I will present the latest results we obtained on the HFF clusters when compared with state-of-the-art numerical simulations of the substructure infall rates, mass growth of these cosmic beasts as a function of cosmic time. I will also discuss the different caveats present on both the observing and simulation sides.Finally, I will present what comes next, with the upcoming BUFFALO large HST programme, the 'spatial extension' of the HFF scheduled to start in July 2018.

Quantifying the suppression of the (un)-obscured star formation in galaxy cluster cores at 0.2 < z < 0.9

Rodriguez-Munoz, Lucia

We present the quantification of the star formation (SF) in the inner cores (R=0.3R200) of 24 massive galaxy clusters at 0.2 < z < 0.9 observed by the Herschel Lensing Survey and the Cluster Lensing and Supernova survey with Hubble. The multi-wavelength photometry (rest-frame UV-to-far-IR) provided by these programmes allows us to identify and accurately characterize star-forming cluster members, as well as quantifying their SF as traced by the direct emission of young stars and the emission of dust heated by the SF processes. We apply the same methodology to consistently select and analyze comparable field samples from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We compare the fraction (F) of color-selected star-forming galaxies (SFGs), and their average star formation rates (SFR) and average specific star formation rates (sSFR=SFR/M*, where M* is the stellar mass), in stellar mass-limited (log M*/MSun > 10) samples of cluster and field galaxies. We find that in the inner regions of intermediate-z clusters, the SF is suppressed in terms of both the fraction of SFGs and the rate at which they turn gas into stars. Clusters display F values on average a factor ~2 smaller than the field, a factor ~5 smaller if we consider only mid- or far-IR-detected galaxies. Furthermore, F appears to evolve faster with z in the clusters than in the field for both galaxy subsamples. In cluster cores SFGs present average SFR and sSFR typically ~0.3 dex smaller than in the field along the whole z range covered. Our results globally favor environmental quenching scenarios with a dominant contribution of slow physical processes in cluster galaxies (such as, e.g., strangulation), although they could also be compatible with other rapid quenching processes (such as, e.g., ram-pressure stripping) turning off infalling galaxies forming stars very actively.This work will be published in Rodriguez-Muñoz et al. 2018 (submitted to MNRAS).

Physical properties of ub-mm galaxy proto-clusters at peak epoch of galaxy and cluster formation

Cooray, Asantha

During this talk for the mini-symposium "Build of galaxy clusters" I will present my group's work on sub-mm galaxy proto-clusters, including spectroscopic and imaging observations with Keck, Spitzer, and Hubble. These include measurements of stellar masses, halo masses, gas content, AGN activity, and other properties of proto-clusters identified with Herschel maps, including lensed Herschel sources, at z=2-4.

Identifying Distant Galaxy Clusters Using Dust-obscured galaxies as Signposts at Redshift > 1.3

Saha, Ripon

Recent studies of high-redshift galaxy clusters have revealed a high concentration of star formation and AGN activity in cluster cores at z >1.4. This suggests that dusty, highly star-forming and/or active high-redshift galaxies, such as Ultra-Luminous Infrared Galaxies (ULIRGs) or Dust-obscured galaxies (DOGs), might be employed to detect high redshift galaxy clusters. In order to test this proposition, we compared the distribution of ~2600 DOGs at z~2 with a sample of 50 distant (z >1.3) galaxy clusters from the IRAC Shallow Cluster Survey (ISCS). The sky surface density of DOGs in 0.5 arcmin radius apertures around the ISCS clusters is found to be 0.46 +- 0.12 arcmin^-2. This detection is overdense by a factor of 5.3+-1.4 relative to the surface density of randomly distributed objects (0.087 arcmin^-2), corresponding to a detection significance of 3-sigma. This result indicates that DOGs preferentially reside in rich environments, often within 250 kpc of a galaxy cluster. This physical association has motivated us to employ DOGs as signposts to detect new distant clusters. We developed and tested a pipeline to isolate and rank our detections, and using it have found 360 new cluster candidates (overdensities) above a 3-sigma detection significance. This pipeline has re-discovered 80% of all the ISCS clusters lying at z >=1.3, including IDCS J1426.5+3508 at z=1.75, the most massive galaxy cluster known at z > 1.5. To characterize the sample, we calculated the two-point angular autocorrelation function of the cluster candidates in order to measure their clustering strength. The correlation length is found to be ~18 Mpc/h, which demonstrates conclusively that the sample has the mass scale of galaxy clusters. We conclude that distant, highly star-forming galaxies selected in the mid- or far-IR with Spitzer or Herschel are sensitive signposts of massive cluster and protoclusters halos at the highest redshifts.

Epoch of Merger-Driven Star Formation and AGN in High Redshift Clusters

Brodwin, Mark

Over the last few years, studies of distant galaxy clusters have found compelling evidence for a major epoch of merger-driven star formation and AGN fueling in z > 1 galaxy clusters.  I will briefly review the observations from the IRAC Shallow and Distant Cluster Surveys (ISCS/IDCS) that contributed to this new paradigm, and discuss new observations that extend it in both redshift (to z = 1.75) and mass (to M > 10^15 Msun) in the IDCS, SPT and other surveys.  As a case study I'll highlight the z = 1.75 IR-selected cluster IDCS J1426.5+3508 that, with a mass of 4 x 10^14 Msun, is the most massive cluster known at z > 1.5.  This extremely distant cluster has high levels of both star formation and AGN activity, yet is a relaxed, self-similar, cool-core cluster apparently in hydrostatic equilibrium.

Galaxy mergers in intermediate redshift clusters

Skelton, Rosalind

In the hierarchical model of galaxy formation, structures build up over time through the merging of smaller subunits. Mergers play an important part in the growth of galaxies and interactions between galaxies affect the level of star formation, with the potential to both enhance and reduce it. In today's massive clusters, mergers are thought to occur rarely, yet they often host massive early-types that are likely to have built up through dry mergers at earlier times. In this work we measure the close pair fraction in clusters at intermediate redshifts (z~0.5) using deep data from the Hubble Frontier Fields. We examine the distribution and properties of close pairs, and the evolution of the merger rate in clusters, considering not only the brightest cluster galaxy but the broader cluster environment.

Molecular gas reservoirs of galaxies in a galxy cluser at z=1.46

Hayashi, Masao

We would like to present molecular gas reservoirs of eighteen galaxies associated with one of the distant X-ray galaxy clusters, XMMXCS J2215.9-1738 cluster at z=1.46. Dust continuum emission at 870 micron and CO(2-1) emission line are detected from 8 and 17 member galaxies within a cluster-centric radius of R_200 from the ALMA data. This is one of the largest sample of CO detections in such a high-z cluster. The molecular gas masses derived from the CO and/or dust continuum luminosities show that the fraction of molecular gas mass and the depletion time scale for the cluster galaxies are larger than expected from the scaling relations of molecular gas for star-forming galaxies in general fields at similar redshifts. The galaxies closer to the cluster center seem to show a larger deviation from the scaling relations. We speculate that the environment of galaxy cluster helps feed the gas through inflow to the member galaxies and also reduce the efficiency of star formation. On the other hand, there is no galaxy with CO(2-1) detected in the very center of the cluster. The stacked spectrum of 12 quiescent galaxies with M_stellar~1E+11 Msun within 0.5 R_200 shows no detection of CO emission line, giving the upper limit of molecular gas mass and molecular gas fraction to be <1E+10 Msun and <10%, respectively. The phase-space diagram indicates that the gas-rich galaxies have entered the cluster more recently than the gas-poor star-forming galaxies and passive galaxies located in the virialized region of this cluster, however, the massive galaxies in the cluster center must quench the star formation activity while consuming most of the gas reservoirs.

High Resolution Observations of the SZ effect

Mroczkowski, Tony

At sub-arcminute resolutions, the Sunyaev-Zeldovich effect (SZE) provides several powerful tools for probing the formation of high-redshift galaxy clusters, complementary to X-ray observations. First, the thermal SZE measures the line of sight integral of electron pressure, serving to provide calorimetry of the intra-cluster medium. Second, the kinetic SZE measures line of sight peculiar velocity with respect to our most universal reference frame, the CMB. And finally, both effects have redshift independent surface brightness, making them suitable for pushing to higher and higher redshifts. I will present recent SZE observations with MUSTANG-2, NIKA2, and ALMA, which now attain resolutions approaching those of X-ray observations, and will discuss prospects for a next generation ~40-meter-class telescope in the southern hemisphere called AtLAST, the Atacama Large Aperture Submm/mm Telescope (atlast-telescope.org).

Herschel-Planck Clusters - Protoclusters in the far-IR

Clements, David

Cross identification of Planck compact sources with Herschel data from the HerMES and H-ATLAS large area surveys has identified a class of objects that appear to be z~2 clusters or protoclusters of galaxies, many of whose members are rapidly forming stars in what appear to be near-simultaneous starbursts. We describe the selection of these sources, discuss their properties as revealed by Herschel and followup observations, compare them to other high redshift cluster/protocluster selections and to theoretical models. These clusters/protoclusters can solve a number of outstanding questions regarding galaxy cluster evolution, but raise other questions about the origin of these objects in the context of currently favoured models of galaxy formation and cosmology. Prospects for further advancing our understanding of these systems are also discussed. 

Confirming Herschel candidate proto-clusters from CO ALMA/VLA observations

Gómez-Guijarro, Carlos

Wide-field surveys with the Herschel Space Observatory are excellent tools for the identification of rare dusty star-forming galaxies (DSFGs), with the best examples being gravitationally lensed DSFGs and high-redshift z>4 DSFGs. Now, our team has used ALMA 870um continuum imaging to discover that wide-field Herschel surveys can also be used to uncover another class of rare galaxies: blends of multiple DSFGs with pairwise separations that are much smaller than what is found from ALMA follow-up of other single-dish surveys. Theoretical simulations overall fail to reproduce this population of multi-component and small-separation DSFGs, but they do suggest that a portion of the population may be due to line-of-sight projection effects and are not physically related. Investigation of this possibility requires spatially-resolved spectroscopic data. Using ALMA and VLA we have conducted a redshift search for a sample of 3 Herschel candidate proto-clusters successfully detecting CO(3-2) and CO(1-0) and thus being able to confirm that the multiple DSFGs are physically associated.

Subaru/HSC Identifications of 42 Protocluster Candidates at z~6-7: Implications for Cosmic Reionization

Higuchi, Ryo

We report fourteen and twenty-eight protocluster candidates at z = 5.7 and 6.6 over 14 and 19 deg2areas, respectively, selected from 2,230 Lya emitters (LAEs) photometrically identified with Subaru/Hyper Suprime-Cam (HSC) deep images. Six out of the 42 protocluster candidates include 1-12 spectroscopically confirmed LAEs at redshifts up to z=6.574. By the comparisons with the cosmological Lya radiative transfer (RT) model reproducing LAEs with the reionization effects, we find that more than a half of these protocluster candidates are progenitors of the present-day clusters with a mass of > 10^14 M_sun. We also investigate the correlation between LAE overdensity and Lya rest-frame equivalent width (EW), because the cosmological Lya RT model suggests that a slope of EW-overdensity relation is steepened towards the epoch of cosmic reionization (EoR), due to the existence of the ionized bubbles around galaxy overdensities easing the escape of Lya emission from the partly neutral intergalactic medium. The available HSC data suggest that the slope of the EW-overdensity correlation does not evolve from the post-reionization epoch z = 5.7 to the EoR z = 6.6 beyond the moderately large statistical errors. 

Cold gas in protoclusters : star formation efficiency and gas kinematics of star forming galaxies in z=2.5 protocluster

Lee, Minju

 We report on cold gas observations toward star-forming galaxies associated with one of the protoclusters found at z=2.5, of two CO lines (CO(3–2) and CO(4–3)), and dust continuum at 1.1 mm using ALMA, aiming for understanding galaxy evolution during the early epoch of cluster formation. From the CO(3-2) line and dust continuum detection, we measure the global cold gas content of seven massive (>4x1010 Msun) star-forming galaxies. The measured cold gas mass is on average similar to that of star forming galaxies in general fields, with the gas mass fraction fgas ~ 0.5, but a positive correlation between stellar mass and star formation efficiency is found within the stellar mass range of log Mstar=[10.64, 11.30], suggesting a potential role for environment at z=2.5. Higher angular resolution images in CO(4-3) are obtained at 0.4 arcsec, toward the subsamples of the CO(3-2)-detected galaxies for further investigation. This allows us to model the gas kinematics on two disk-like galaxies with S/N>~8, located in a high local density region. The best-fit models on CO(4-3) data cubes suggest that the overall kinematic parameters (e.g., rotation velocity, velocity dispersion) are similar to those found in the comparable field disk-like galaxies, so are the (baryonic) Tully-Fisher relation and the specific angular momentum, though there is a tension in the model. Potential mergers are also found in the protocluster. We discuss the physical interpretation of the discovery of both disk-like galaxies and mergers in the protocluster, from the theoretical standpoint. In conclusion, we find no significant difference in cold gas properties between protocluster and field galaxies at z~2.5 on average, though there are tentative trends that need to be confirmed with a finer beam. This is one of the first studies ever made toward protoclusters using cold gas, which still lacks statistical significance, and thus, future observations are necessary to investigate more galaxies.

A systematic search for protoclusters at z~4 based on the >100deg^2 area

Toshikawa, Jun

We conduct a systematic search for galaxy protoclusters at z~3.8 based on the Hyper Suprime-Cam Subaru strategic program (HSC-SSP). In the Wide layer of the HSC-SSP, we investigate the large-scale projected sky distribution of g-dropout galaxies over an area of 121deg2, and identify 216 large-scale overdense regions (>4s overdensity significance) that are likely protocluster candidates. The unprecedented size of our protocluster candidate catalog allows us to perform, for the first time, an angular clustering analysis of the systematic sample of protocluster candidates. We find a correlation length of 35.0h-1Mpc. The relation between correlation length and number density of z~3.8 protocluster candidates is consistent with the prediction of the CDM model, and the correlation length is similar to that of rich clusters in the local universe. This result suggests that our protocluster candidates are tracing similar spatial structures to those expected from the progenitors of rich clusters. Following this protocluster search, we are now performing follow-up observations to investigate the physical properties of protoclusters. Optical spectroscopy is carried out for four protocluster candidates by Keck/DEIMOS and Gemini/GMOS. The fraction of Lya emitters among g-dropout galaxies in overdense regions is found to be significantly smaller than that in field. This implies that protocluster galaxies are dustier or there is a large amount of neutral hydrogen gas in protocluster regions. In addition, five protocluster candidates are observed by the submm imaging of JCMT/SCUBA2 to search dusty star-burst galaxies. We will present the systematic search for protoclusters and the initial results of these follow-up observations. In years to come, our protocluster search will be extended to the entire HSC-SSP Wide sky coverage of ~1400deg2 to probe cluster formation over a wide redshift range of z~2-6.

Stellar populations of high-z protoclusters from the CARLA survey

Amodeo, Stefania

Studying the evolution of galaxy properties in clusters is important to test theories of galaxy formation and the role of the environment on galaxy evolution. We will present results on the stellar populations of galaxies in confirmed dense structures at z>1.4. At this epoch, clusters are still in the process of forming and most of the assembly is ongoing,  while the cosmic star formation rate density peaks in the history of the Universe (Madau & Dickinson 2014). The Spitzer/IRAC survey CARLA (Clusters Around Radio-Loud AGN) has targeted high-z radio-loud AGNs,  which are known to reside in dense environments (Wylezalek et al. 2013), and selected the 20 richest fields to be followed-up with HST, from which 16 structures in the range 1.4 < z < 2.8 were spectroscopically confirmed (Noirot et al. 2018). Optical (i-band) imaging for eight of these targets have also been obtained with the Gemini and William Hershel telescopes (Cooke et al. 2015). This is the largest sample of protoclusters at z>1.5 at present. We will discuss the colour-magnitude relation of the spectroscopically confirmed galaxies and of the galaxies selected by their Spitzer colours ([3.6] - [4.5])AB > - 0.1.  We will compare our data to different stellar populations models and discuss the presence or absence of a red-sequence, the fraction of star-forming and quiescent galaxies from rest-frame UVJ colors, as well as the dependence of the galaxy properties on the proximity to the AGN. This study, done for the first time on an homogeneous statistical sample of spectroscopically confirmed clusters at high redshift, is ideal to investigate the quenching mechanisms in dense environments.

Mergers, Shocks, Radio Relics, and the Dynamical State of Clusters of Galaxies

Sarazin, Craig

Recent XMM-Newton, Chandra, and NuSTAR observations of merging clusters of galaxies will be presented.  Cluster mergers drive shocks into the intracluster gas, and these shocks can (re)accelerate relativistic electrons, producing the observed radio relics.  XMM observations of the merger shock in Abell 3667, which is associated with the NW radio relic, will be presented.  The complex merger in Abell 2061 will also be discussed based on Chandra and XMM-Newton X-ray observations, and comparison to the radio relics and halos will be made.  The origin of the radio relic in the poor cluster Abell S743 will be probed with the recent XMM, Chandra, and NuSTAR observations. The moderate redshift cluster ACT-CL J0256.5+0006 has a very low power radio halo, and may be an early stage merger.  X-ray and SZ observations of high redshift clusters, including IDCS J1426.5+3508, will be described.  The results on the thermal and non thermal physics of merger shocks will be given. 

First results on the cluster galaxy population from the Subaru Hyper Suprime-Cam survey: Brightest cluster galaxies, stellar mass distribution, and active galaxies

Lin, Yen-Ting

The unprecedented depth and area surveyed by the Subaru Strategic Program with the Hyper Suprime-Cam (HSC-SSP) have enabled us to construct and publish the largest distant cluster sample out to z~1 to date.  In this study of cluster galaxy evolution from z=1 to z=0.3, we investigate the stellar mass assembly history of brightest cluster galaxies (BCGs), evolution of stellar mass and luminosity distributions, stellar mass surface density profile, as well as the population of radio galaxies.  Our analysis is the first high redshift application of the top N richest cluster selection, which is shown to allow us to trace the cluster galaxy evolution faithfully.  Over the 230 deg2 area of the HSC-SSP footprint in 2017, selecting the top 100 clusters in each of the 4 redshift bins allows us to observe the buildup of galaxy population in descendants of clusters whose z~1 mass is about 2x1014Msun.  Our stellar mass is derived from a machine-learning algorithm, which is found to be unbiased and accurate with respect to the COSMOS data.  We find very mild stellar mass growth in BCGs (about 35% between z=1 and 0.3), and no evidence for evolution in both the total stellar mass--cluster mass correlation and the shape of the stellar mass surface density profile.  We also present the first measurement of the radio luminosity distribution in clusters out to z~1, and show hints of changes in the dominant accretion mode powering the cluster radio galaxies at z~0.8.

On the turbulent life of disk galaxies in distant clusters

Böhm, Asmus

I will present first results from VLT-KMOS integral field spectroscopy of galaxies in two evolving clusters at redshift z=1.5. Our targets were selected from narrow-band imaging of the HyperSuprimeCam Survey and are following the star formation main sequence at their redshift. The KMOS data cubes of 26 cluster galaxies allowed to extract extended Halpha rotation velocity fields. We found a high fraction (>50%) of kinematically disturbed objects. Only roughly one third of the galaxies with derived kinematics are regular rotators, in the sense that they establish a rotation velocity-stellar mass (Tully-Fisher) relation. Including a comparison to our own studies at lower redshifts, I will discuss the connection between galaxy evolution and environment during cluster build-up 9 Gyr ago.

The evolution of the luminosity function of cluster galaxies in the Cluster-EAGLE simulation

Negri, Andrea

The galaxy luminosity function (LF) is a fundamental tool for exploring galaxy evolution over cosmic time. In the last decade observations have been able to probe the evolution of the galaxy LF with redshift, in particular showing a variation of its low-mass end with redshift. On the numerical side, there is currently no extended study of the evolution of the LF of galaxies in clusters. We employ the data of the Cluster-EAGLE project, a set of cosmological, hydrodynamical zoom simulations of 30 galaxy clusters, to study the evolution of the galaxy LF in clusters as function of redshift. We compile a catalogue of simulated galaxies’ luminosities in the SDSS bands using the E-MILESspectra database, and taking into account dust attenuation. Stacked luminosity functions show a good agreement with observations, presenting little evolution with redshift of the faint-end slope. The environment effects are strong for intermediate-luminosity galaxies. In the clusters infall region (r > 3 r200), star-forming galaxies dominate in number the knee of the LF, whereas, inside the clusters (r < r200), the fraction of passive galaxies is larger. The relative fraction of passive and star-forming galaxies within r200 evolves with redshift, with the star-forming fraction being larger above z~0.5. On the other hand, the fraction of low-luminosity, red dwarfs dominates the faint end up to z=2.

HST Grism/WFC3 observations of XLSSC122, a mature cluster z~2.

Canning, Rebecca

We present HST grism spectroscopy of XLSSC 122 confirming its redshift of z=1.98. This redshift is in good agreement with that derived from spectral fitting of the redshifted X-ray Fe 6.7keV line. The cluster displays a clear red sequence as well as significant AGN activity and SF in the confirmed cluster members. Here we present our cycle 21 HST observations, redshift determination, emission line analysis and analysis of the F105W-F140W cluster colour magnitude diagram.

Cluster star formation probed with scaling relations

Perez Martinez, Jose Manuel

To put strong constraints on the environmental impact on galaxies we investigate quantitatively the kinematic status of galaxies in several clusters at z~1.5, z~0.9 and z~0.5 using KMOS, FORS2, VIMOS and OSIRIS-GTC data. Our high-z KMOS observations provide spatially resolved star formation and H-alpha gas kinematics of galaxies enabling us to examine their rotational support and star formation status before they become the passive population we see in clusters at later stages. At the same time, the good spatial resolution of our data at lower redshift allows a thorough analysis of subtle kinematical irregularities that trace accretion and stripping events, tidal and harassment processes, as well as mergers, and their impact on different scaling relations.We use different representations of the Tully-Fisher relation (TFR) to study galaxy evolution between the starting point of the hierarchical assembly of clusters, and the epoch when half of the cluster mass has been already accreted. While the B-band TFR is sensitive to recent episodes of star formation, the stellar mass TFR tracks the overall evolution of the underlying stellar population. The combination of these two schemes allows us to identify galaxies that might be suffering cluster-specific interactions as outliers in the B-band TFR, while these very same galaxies display no significant offsets in the stellar-mass TFR. I will discuss the connection between the kinematic status of cluster galaxies and their recent star formation history during these two important epochs.

Formation, Evolution, and Galaxy Population Properties of Proto-Clusters: Insights from the Hydrodynamic Cosmological Simulation Magneticum

Remus, Rhea-Silvia

Using the power of the state-of-the-art hydrodynamical cosmological simulation suit Magneticum Pathfinder, I study the formation of proto clusters at redshifts 4-5, and the evolution of such structures into present-day massive cluster systems. I will present star-formation and gas properties of the galaxy cluster members in comparison to state-of-the-art observations, as well as kinematical properties of such very early galaxies, and I will demonstrate what we can learn from the combined approach of simulations and observations about the build-up of structures and the laws of star formation in the early phases of our Universe.

Optical rest-frame spectroscopy to study the filaments and galaxy clusters outskirts at z~1

Afonso, Ana

The VIMOS Spectroscopic Survey of a Supercluster in the COSMOS field (VIS³COS) aims to accurately map in 3D a superstructure at redshift between 0.8 and 0.9, which contains 3 massive X-rays confirmed clusters (Finoguenov et al., 2007) and shows a striking filamentary structure in the HiZELS Ha survey at z=0.84 (Sobral et al., 2011). The ~500 spectroscopic confirmed members probe a wide range of densities and environments (from fields to the clusters outskirts and rich groups). The unprecedented nature of this survey (Paulino-Afonso et al., A&A submitted) allows a detailed study of the evolution of these objects across cosmic time as well as link observations and theories in and around cluster galaxies by e.g. characterizing the star formation activity and the ionized gas properties of galaxies in a large density range. I will present the observed depression of star formation and local changes on electron densities with the environment (depending on galaxy stellar mass) at z~1, where most changes occur in cluster outskirts and filament-like regions. At this epoch, the intense life in the cluster galaxies suburbs seems to provide essential clues on the physical mechanisms that can affect the shape of galaxy clusters and their understanding.

ALMA deep survey in a z=3.1 proto-cluster field

Umehata, Hideki

Galaxies and nuclei in the dense environment at high redshift provide a good laboratory to investigate the accelerated, most extreme evolution of galaxies at a given epoch. A 50 Mpc-scale filamentary three-dimensional structure (or large-scale ‘cosmic web’) traced by Lyman-alpha emitters (LAEs) has been found in the SSA22 field at z=3.1, which is one of the best target in this regard. We mapped a contiguous 7 arcmin^2 and 20 arcmin^2 regions at the node in ALMA band 3 and band 6, respectively, to uncover molecular gas reservoirs and dusty starburst cores at the proto-cluster core. We obtained 35 robustly detected 1.1mm sources (here after submillimeter galaxies, SMGs) with a signal-to-noise ratio (SNR) >5. More than half of the SMGs have been spectroscopically confirmed to be a genuine proto-cluster member. IR and CO luminosity functions are 2-3 orders of magnitudes higher than blank fields. Six SMGs at the core also host a X-ray luminous active galactic nuclei (AGN). Our results suggest that the vigorous star formation activity and the growth of super massive black holes (SMBHs) occurred simultaneously in the densest regions at z~3 supported by abundant gas fueling, which is likely to correspond to the most active historical phase of the massive galaxy population found in the core of the clusters in the present universe.

Tracing Protoclusters with Submillimeter Galaxies

Menéndez-Delmestre, Karín

Radio galaxies have been used quite successfully as protocluster tracers. However, because they are only visible for a short period in the evolution of a massive galaxy, a significant fraction of forming clusters remains unexplored. Clustering analysis indicate that submm-selected galaxies (SMGs) reside in very massive halos, suggesting that they trace high-density environments. Conversely, SMGs have been identified as tracers of structures with more modest masses caught in highly active periods. This suggests that SMGs may be tracers of a wider range of environments beyond the progenitors of today’s rich clusters, opening a window for a more complete exploration of protoclusters. With narrow-band Ly-alpha imaging and multi-object spectroscopy using Palomar/Keck/Magellan/Gemini telescopes we probe for galaxy overdensities in SMG environments at z~1-5. With >100 spectroscopically-confirmed Ly-alpha emitters, we quantify the overdensities in these regions. We are also studying galaxy properties according to the maturity of SMG-traced protoclusters to explore the way galaxy and local environment relate to each other within the broader picture of a cosmologically-evolving large scale structure

Studying the star formation activity in two massive proto-clusters at z>2

Zavala, Jorge

Taking census of the protocluster's star formation activity -and to compare it to a sample of mass-matched galaxies in normal environments- is crucial to understand how environment impacts the growth of galaxies. We focus on two particularly unique protoclusters: the z=2.10 and z=2.47 structures in the COSMOS field. These structures contain several starbursting dusty star-forming galaxies and extends up to half a degree in the sky, in line with expectation of massive clusters in formation. Using ALMA continuum observations, we find evidence of an accelerated evolution in the most massive galaxies, while normal, less massive sources are in agreement with those galaxies in the field. 

Galaxy Evolution in 3D

Kewley, Lisa

Tracing matter and chemical elements in the Universe is critical for understanding the formation of the first galaxies, the formation and growth of supermassive black holes, and ultimately the evolution of galaxies like our Milky Way. Throughout the history of the universe, large-scale gas flows have moulded the arms of spiral galaxies, formed the bulges of the most massive galaxies in the universe, fed supermassive black holes in the centers of galaxies, fueled generation upon generation of new stars, and enriched the intergalactic medium with metals. The physics and impact of these processes can now be traced through new efficient, wide-field 3D integral field spectrographs.  With multi-object integral field spectroscopy, we can now probe these transformative galaxy processes statistically for thousands of galaxies. I will describe how our understanding of galaxy evolution has been changed through these surveys to help us understand the relationship between gas inflows, galactic-scale outflows, star-formation, chemical enrichment, and active galactic nuclei in galaxies.  I will link these studies with new insights into the early universe traced through gravitational lensing and infrared adaptive optics.  I will finish by discussing how this field will be transformed in the JWST and ELT era.(This is the abstract for my invited plenary talk scheduled for 17:15 to 18:15, on Thursday 23rd of August)

A Talk of Ice and Fire: Cold Gas Growth, Survival, and Entrainment in Hot Environments

Gronke, Max

A variety of astrophysical systems are multiphase, and we commonly observe the cold gas moving at high velocities, e.g., in outflows around star-forming galaxies. The usual picture is that this gas has been accelerated by ram pressure forces due to the hot gas. However, reproducing this ubiquitous observation in hydrodynamical simulations has proven to be challenging - simply because the destruction time (equal to the Kevin-Helmholtz timescale) is shorter than the time it takes to entrain the cold gas. In my talk, I will propose a solution to this puzzle, and discuss it in light of different astrophysical phenomena.

XXX IAU General Assembly | ACV - Austria Center Vienna  | Bruno-Kreisky-Platz 1  | 1220 Vienna