Cluster Member Galaxies Research Articles

CLASH-VLT: a full dynamical reconstruction of the mass profile of Abell S1063 from 1 kpc out to the virial radius

Context. The shape of the mass density profiles of cosmological halos informs us of the nature of dark matter (DM) and DM-baryons interactions. Previous estimates of the inner slope of the mass density profiles of clusters of galaxies are in opposition to predictions derived from numerical simulations of cold dark matter (CDM). Aims. We determine the inner slope of the DM density profile of a massive cluster of galaxies, Abell S1063 (RXC J2248.7−4431) at z = 0.35, with a dynamical analysis based on an extensive spectroscopic campaign carried out with the VIMOS and MUSE spectrographs at the ESO VLT. This new data set provides an unprecedented sample of 1234 spectroscopic members, 104 of which are located in the cluster core (R ≲ 200 kpc), extracted from the MUSE integral field spectroscopy. The latter also allows the stellar velocity dispersion profile of the brightest cluster galaxy (BCG) to be measured out to 40 kpc. Methods. We used an upgraded version of the MAMPOSSt technique to perform a joint maximum likelihood fit to the velocity dispersion profile of the BCG and to the velocity distribution of cluster member galaxies over a radial range from 1 kpc to the virial radius (r200 ≈ 2.7 Mpc). Results. We find a value of γDM = 0.99 ± 0.04 for the inner logarithmic slope of the DM density profile after marginalizing over all the other parameters of the mass and velocity anisotropy models. Moreover, the newly determined dynamical mass profile is found to be in excellent agreement with the mass density profiles obtained from the independent X-ray hydrostatic analysis based on deep Chandra data, as well as the strong and weak lensing analyses. Conclusions. Our value of the inner logarithmic slope of the DM density profile γDM is in very good agreement with predictions from cosmological CDM simulations. We will extend our analysis to more clusters in future works. If confirmed on a larger cluster sample, our result makes this DM model more appealing than alternative models.

Calibration of systematics in constraining modified gravity models with galaxy cluster mass profiles

Joint lensing and dynamical mass profile determinations of galaxy clusters are an excellent tool to constrain modification of gravity at cosmological scales. However, search for tiny departures from General Relativity (GR) calls for an accurate control of the systematics affecting the method. In this analysis we concentrate on the systematics in the reconstruction of mass profiles from the dynamics of cluster member galaxies, while assuming that lensing provides unbiased mass profile reconstructions. In particular, in the case study of linear f(R) gravity, we aim at verifying whether in realistic simulations of cluster formation a spurious detection of departure from GR can be detected due to violation of the main assumptions (e.g. dynamical equilibrium and spherical symmetry) on which the method is based. We aim at identifying and calibrating the impact of those systematics by analyzing a set of Dark Matter halos taken from ΛCDM N-body cosmological simulations performed with the GADGET-3 code. We evaluate how the constraints on the additional degree of freedom mfR suffer the lack of dynamical relaxation and departures from spherical symmetry. If no selection criteria are applied, ∼ 60% of clusters in a ΛCDM Universe (where GR is assumed) produce a spurious detection of modified gravity. {We find that the probability of finding cluster in agreement with GR predictions PGR mainly depends on the properties of the halo's projected phase-space and on shape orientation of the cluster along the line-of-sight projection.} We further define two observational criteria which correlate with PGR and which can be used to select, among a generic population of galaxy clusters in the local Universe, those objects that are more suitable for the application of the proposed method. In particular, we find that according to these criteria the percentage of spurious detection can be lowered down to ∼ 20% in the best case. We discuss how PGR changes when a simplified treatment of the chameleon screening mechanism is considered. Our results are relevant in view of the availability of precise measurements of lensing mass profiles from imaging data and dynamics mass profiles from spectroscopic data that will be available with the next generation surveys.

Stellar-mass Measurements in A133 with Magellan/IMACS

Abstract We present the analysis of deep optical imaging of the galaxy cluster A133 with the IMACS instrument on Magellan. Our multi-band photometry enables stellar-mass measurements in the cluster member galaxies down to a mass limit of (≈0.1 of the Large Magellanic Cloud stellar mass). We observe a clear difference in the spatial distribution of large and dwarf galaxies within the cluster. Modeling these galaxy populations separately, we can confidently track the distribution of stellar mass locked in the galaxies to the cluster’s virial radius. The extended envelope of the cluster’s brightest galaxy can be tracked to ∼200 kpc. The central galaxy contributes ∼1/3 of the total cluster stellar mass within the radius .

GMRT observations of IC 711 – the longest head-tail radio galaxy known

ABSTRACTWe present low frequency, GMRT observations at 240, 610, and 1300 MHz of IC 711, a narrow angle tail (NAT) radio galaxy. The total angular extent of the radio emission, ∼22 arcmin, corresponds to a projected linear size of ∼900 kpc, making it the longest among the known head-tail radio galaxies. The objectives of the GMRT observations were to investigate the radio morphology, especially of the long tail structure, at low frequencies. The radio structure, especially initial ∼10 arcmin of tail being a long straight feature, does not seem to be consistent with a simple circular motion around the cluster centre, as previously suggested in the literature. Two sharp bends after the straight section of the tail cast doubt on the prevailing idea in the literature that the long narrow tails represent trails left behind by the fast moving parent optical galaxy with respect to the cluster medium, as the optical galaxy could not have undergone such sharp bends in its path, under any conceivable gravitational influence of some individual galaxy or of the overall cluster gravitational potential. In fact, the tail does not seem to have been influenced by the gravitational field of any of the cluster-member galaxies. The radio spectrum of the head, coinciding with the optical galaxy, is flat ($\alpha \stackrel{\lt }{_{\sim }}0.4$ for S∝ ν−α), but steadily steepens along the radio tail, with the end part of the tail showing the steepest spectrum ($\alpha \, {\sim}$ 4–5) ever seen in any diffuse radio emission region.

Dark Energy Survey Year 1 results: validation of weak lensing cluster member contamination estimates from P(z) decomposition

ABSTRACT Weak lensing source galaxy catalogues used in estimating the masses of galaxy clusters can be heavily contaminated by cluster members, prohibiting accurate mass calibration. In this study, we test the performance of an estimator for the extent of cluster member contamination based on decomposing the photometric redshift P(z) of source galaxies into contaminating and background components. We perform a full scale mock analysis on a simulated sky survey approximately mirroring the observational properties of the Dark Energy Survey Year One observations (DES Y1), and find excellent agreement between the true number profile of contaminating cluster member galaxies in the simulation and the estimated one. We further apply the method to estimate the cluster member contamination for the DES Y1 redMaPPer cluster mass calibration analysis, and compare the results to an alternative approach based on the angular correlation of weak lensing source galaxies. We find indications that the correlation based estimates are biased by the selection of the weak lensing sources in the cluster vicinity, which does not strongly impact the P(z) decomposition method. Collectively, these benchmarks demonstrate the strength of the P(z) decomposition method in alleviating membership contamination and enabling highly accurate cluster weak lensing studies without broad exclusion of source galaxies, thereby improving the total constraining power of cluster mass calibration via weak lensing.

Identification of clusters of galaxies and physical properties of clusters and their member galaxies

Clusters of galaxies are the largest gravitationally bound systems in the Universe and are located at the knots of cosmic web. They provide powerful constraints on galaxy evolution, the formation of large scale structure of the Universe and many astrophysical processes. This paper summarizes the results we obtained in the last decade on identification of galaxy clusters, studies of the properties of clusters and cluster member galaxies. We developed a method to identify galaxy clusters by using photometric redshifts of galaxies. The method was applied to the survey data of Sloan Digital Sky Survey (SDSS), and we identified 39668 clusters from the sixth data release of SDSS, and later identified 158103 clusters from the twelfth data release of SDSS, which doubles the number of known clusters and extends the redshift range of clusters from 0.3 to 0.8. Recently, we identified 47600 clusters from all sky data of Two Micron All-Sky Survey (2MASS), Wide-field Infrared Survey Explorer (WISE) and SuperCOSMOS, which significantly enlarge the number of clusters outside of the SDSS sky region. More high-redshift clusters were also recently identified from SDSS×WISE and the deep survey data, which significantly enlarge the number of cluster at high redshift. By inspecting the SDSS color images, we found many clusters showing giant arcs, acting as the lensed system for background galaxies. Many following-up studies have been done to understand the properties of galaxy clusters, including their spatial distribution in the Universe, the dynamical state and diffuse radio emission. The two point correlation function of 79091 clusters was calculated to detect the Baryon Acoustic Oscillations (BAO) signal with 3.7σ confidence, which is the first significant detection of the BAO signal by using galaxy clusters as tracers. Based on optical photometric data, we developed a method to quantify the dynamical state of clusters and calculated the relaxation parameter for 2092 rich clusters. We found that the cluster dynamical state is significantly correlated with the absolute magnitude of Brightest Cluster Galaxy (BCG) and also depends on the magnitude difference between the first BCG and the second BCG. The diffuse radio emission of galaxy clsuters detected in merging clusters is related to dynamical state, so that the radio power of diffuse radio emission, cluster mass proxy and dynamical parameters form a fundamental plane. The properties of cluster member galaxies have been investigated in several aspects. By using 2092 clusters with dynamical states, we calculated the composite luminosity function of member galaxies. We found that more relaxed clusters have fewer bright member galaxies, but host a brighter BCG. By cross-matching the largest optical cluster sample with radio data obtained by the Very Large Array (VLA) of National Radio Astronomy Observatory (NRAO) through the NRAO VLA Sky Survey (NVSS) and Faint Images of the Radio Sky at Twenty-centimeters (FIRST), we got the largest complete sample of radio loud BCG. We obtained the radio luminosity function of BCGs, and found that it is related to the optical luminosity of BCG and dynamical state of clusters. BCGs with larger optical luminosity have higher fraction of radio loud. BCGs in more relaxed clusters have higher radio power. It is gareat era now as more deeper sky surveys on many bands are going on. We expect many breakthroughs in the research areas related to galaxy clusters.

Use Density-Based Spatial Clustering of Applications with Noise (DBSCAN) Algorithm to Identify Galaxy Cluster Members

Galaxies are important structures for studying the universe, and clusters are the physical environment of galaxies. Their study is of great significance for understanding the evolution of galaxies and the distribution of matter. Classification of galaxies into clusters is an urgent subject. How do we classify some observed galaxy data points as clusters? How to ensure the correctness of classification? Based on the results of CoDECS numerical simulation and combining DBSCAN algorithm, this paper attempts to classify the data and compare and explain the results of the three methods. Then, based on the data of Abell 383 cluster, further comparison and analysis of the three methods were made. This research can be a basis on measuring new stars.

Future constraints on the gravitational slip with the mass profiles of galaxy clusters

The gravitational slip parameter is an important discriminator between large classes of gravity theories at cosmological and astrophysical scales. In this work we use a combination of simulated information of galaxy cluster mass profiles, inferred by Strong+Weak lensing analyses and by the study of the dynamics of the cluster member galaxies, to reconstruct the gravitational slip parameter $\eta$ and predict the accuracy with which it can be constrained with current and future galaxy cluster surveys. Performing a full-likelihood statistical analysis, we show that galaxy cluster observations can constrain $\eta$ down to the percent level already with a few tens of clusters. We discuss the significance of possible systematics, and show that the cluster masses and numbers of galaxy members used to reconstruct the dynamics mass profile have a mild effect on the predicted constraints.

RELICS: Strong Lensing Analysis of MACS J0417.5–1154 and Predictions for Observing the Magnified High-redshift Universe with JWST

Abstract Strong gravitational lensing by clusters of galaxies probes the mass distribution at the core of each cluster and magnifies the universe behind it. MACS J0417.5−1154 at z = 0.443 is one of the most massive clusters known based on weak lensing, X-ray, and Sunyaev–Zel’dovich analyses. Here we compute a strong lens model of MACS J0417 based on Hubble Space Telescope imaging observations collected, in part, by the Reionization Lensing Cluster Survey (RELICS), and recently reported spectroscopic redshifts from the MUSE instrument on the Very Large Telescope (VLT). We measure an Einstein radius of at z = 9 and a mass projected within 200 kpc of M . Using this model, we measure a ratio between the mass attributed to cluster-member galaxy halos and the main cluster halo of order 1:100. We assess the probability to detect magnified high-redshift galaxies in the field of this cluster, both for comparison with RELICS HST results and as a prediction for the James Webb Space Telescope (JWST) Guaranteed Time Observations upcoming for this cluster. Our lensing analysis indicates that this cluster has similar lensing strength to other clusters in the RELICS program. Our lensing analysis predicts a detection of at least a few z ∼ 6–8 galaxies behind this cluster, at odds with a recent analysis that yielded no such candidates in this field. Reliable strong lensing models are crucial for accurately predicting the intrinsic properties of lensed galaxies. As part of the RELICS program, our strong lensing model produced with the Lenstool parametric method is publicly available through the Mikulski Archive for Space Telescopes.

Spectroscopic Confirmation of Five Galaxy Clusters at z > 1.25 in the 2500 deg2 SPT-SZ Survey

Abstract We present spectroscopic confirmation of five galaxy clusters at 1.25 < z < 1.5, discovered in the 2500 deg2 South Pole Telescope Sunyaev–Zel’dovich (SZ) survey. These clusters, taken from a mass-limited sample with a nearly redshift-independent selection function, have multiwavelength follow-up imaging data from the X-ray to near-IR and currently form the most homogeneous massive high-redshift cluster sample known. We identify 44 member galaxies, along with 25 field galaxies, among the five clusters, and describe the full set of observations and data products from Magellan/LDSS3 multiobject spectroscopy of these cluster fields. We briefly describe the analysis pipeline and present ensemble analyses of cluster member galaxies that demonstrate the reliability of the measured redshifts. We report z = 1.259, 1.288, 1.316, 1.401, and 1.474 for the five clusters from a combination of absorption-line (Ca ii H&K doublet—λλ3968, 3934) and emission-line ([O ii] λλ3727, 3729) spectral features. Moreover, the calculated velocity dispersions yield dynamical cluster masses in good agreement with the SZ masses for these clusters. We discuss the velocity and spatial distributions of passive and [O ii]-emitting galaxies in these clusters, showing that they are consistent with velocity segregation and biases observed in lower redshift South Pole Telescope clusters. We identify modest [O ii] emission and pronounced CN and Hδ absorption in a stacked spectrum of 28 passive galaxies with Ca ii H&K-derived redshifts. This work increases the number of spectroscopically confirmed SZ-selected galaxy clusters at z > 1.25 from three to eight, further demonstrating the efficacy of SZ selection for the highest redshift massive clusters and enabling detailed study of these systems.

Introducingromulusc: a cosmological simulation of a galaxy cluster with an unprecedented resolution

We present the first results from RomulusC, the highest resolution cosmological hydrodynamic simulation of a galaxy cluster run to date. RomulusC, a zoom-in simulation of a halo with $z=0$ mass $10^{14}$ M$_{\odot}$, is run with the same sub-grid physics and resolution as Romulus25 (Tremmel et al. 2017). With unprecedented mass and spatial resolution, RomulusC represents a unique opportunity to study the evolution of galaxies in dense environments down to dwarf masses. We demonstrate that RomulusC results in an intracluster medium (ICM) consistent with observations. The star formation history and stellar mass of the brightest cluster galaxy (BCG) is consistent with observations and abundance matching results, indicating that our sub-grid models, optimized only to reproduce observations of field dwarf and Milky Way mass galaxies, are able to produce reasonable galaxy masses and star formation histories in much higher mass systems. Feedback from supermassive black holes (SMBHs) regulates star formation by driving large-scale, collimated outflows that coexist with a low entropy core. We find that non-BCG cluster member galaxies are substantially quenched compared to the field down to dwarf galaxy masses and, at low masses, quenching is seen to have no dependence on mass or distance from the cluster center. This enhanced quenched population extends beyond $R_{200}$ and is in place at high redshift. Similarly, we predict that SMBH activity is significantly suppressed within clusters outside of the BCG, but show how the effect could be lost when only focusing on the brightest AGN in the most massive galaxies.

The XXL Survey

Context. An X-ray survey with the XMM-Newton telescope, XMM-XXL, has identified hundreds of galaxy groups and clusters in two 25 deg2 fields. Combining spectroscopic and X-ray observations in one field, we determine how the kinetic energy of galaxies scales with hot gas temperature and also, by imposing prior constraints on the relative energies of galaxies and dark matter, infer a power-law scaling of total mass with temperature. Aims. Our goals are: i) to determine parameters of the scaling between galaxy velocity dispersion and X-ray temperature, T300 kpc, for the halos hosting XXL-selected clusters, and; ii) to infer the log-mean scaling of total halo mass with temperature, ⟨lnM200 | T300 kpc, z⟩. Methods. We applied an ensemble velocity likelihood to a sample of >1500 spectroscopic redshifts within 132 spectroscopically confirmed clusters with redshifts z < 0.6 to model, ⟨lnσgal | T300 kpc, z⟩, where σgal is the velocity dispersion of XXL cluster member galaxies and T300 kpc is a 300 kpc aperture temperature. To infer total halo mass we used a precise virial relation for massive halos calibrated by N-body simulations along with a single degree of freedom summarising galaxy velocity bias with respect to dark matter. Results. For the XXL-N cluster sample, we find σgal ∝ T300 kpc0.63±0.05, a slope significantly steeper than the self-similar expectation of 0.5. Assuming scale-independent galaxy velocity bias, we infer a mean logarithmic mass at a given X-ray temperature and redshift, 〈ln(E(z)M200/1014 M⊙)|T300 kpc, z〉 = πT + αT ln (T300 kpc/Tp) + βT ln (E(z)/E(zp)) using pivot values kTp = 2.2 keV and zp = 0.25, with normalization πT = 0.45 ± 0.24 and slope αT = 1.89 ± 0.15. We obtain only weak constraints on redshift evolution, βT = −1.29 ± 1.14. Conclusions. The ratio of specific energies in hot gas and galaxies is scale dependent. Ensemble spectroscopic analysis is a viable method to infer mean scaling relations, particularly for the numerous low mass systems with small numbers of spectroscopic members per system. Galaxy velocity bias is the dominant systematic uncertainty in dynamical mass estimates.

New insights on the dissociative merging galaxy cluster Abell 2034

We present here new insights about the merging galaxy cluster Abell 2034 ($\bar{z}=0.114$) based on a combined weak lensing and dynamical analysis. From our deep Subaru $BR_Cz'$ images plus Gemini-GMOS/N low-resolution spectra accompanied by available redshift data, we have obtained the individual masses of the colliding subclusters as well as estimated a timeline of the process. The collision event happened $0.56^{+0.15}_{-0.22}$ Gyr ago between the subclusters A2034S ($M_{200}^S=2.35_{-0.99}^{+0.84}\times 10^{14}$ M$_{\odot}$) and A2034N ($M_{200}^N=1.08_{-0.71}^{+0.51}\times 10^{14}$ M$_{\odot}$) with the gas content of both subclusters displaced in relation to their galaxy and dark matter distributions, in a scenario similar to that found in the Bullet Cluster. Following our data and modelling the collision event is, most likely, taking place not so far from the plane of the sky, with an angle of $27^\circ\pm14^\circ$ in relation to that. In spite of the intrinsic degeneracy inherent to the system (whether it has been observed incoming or outgoing), the comparison of our calculated time since the closest approximation with the estimated age of the observed X-ray shock front and the increment experienced by the velocity dispersion of the galaxy cluster members points toward an outgoing movement. Besides, we found a strong evidence for the presence of a third structure which we called A2034W.

Multi-object spectroscopy of CL1821+643: a dynamically relaxed cluster with a giant radio halo?

We present the study of the dynamical status of the galaxy cluster CL1821+643, a rare and intriguing cool-core cluster hosting a giant radio halo. We base our analysis on new spectroscopic data for 129 galaxies acquired at the Italian Telescopio Nazionale Galileo. We also use spectroscopic data available from the literature and photometric data from the Sloan Digital Sky Survey. We select 120 cluster member galaxies and compute the cluster redshift <z> ~ 0.296 and the global line-of-sight velocity dispersion $\sigma_{\rm V}$ ~ 1100 km/s. The results of our analysis are consistent with CL1821+643 being a massive dynamically relaxed cluster dominated by a big and luminous elliptical at the centre of the cluster potential well. None of the tests employed to study the cluster galaxies kinematics in the 1D (velocity information), 2D (spatial information), and 3D (combined velocity and spatial information) domains is able to detect significant substructures. While this picture is in agreement with previous results based on X-ray data and on the existence of the central cool core, we do not find any evidence of a merging process responsible for the radio halo discovered in this cluster. Thus, this radio halo remains an open problem that raises doubts about our understanding of diffuse radio sources in clusters.

The shape of galaxy dark matter haloes in massive galaxy clusters: insights from strong gravitational lensing

We assess how much unused strong lensing information is available in the deep \emph{Hubble Space Telescope} imaging and VLT/MUSE spectroscopy of the \emph{Frontier Field} clusters. As a pilot study, we analyse galaxy cluster MACS\,J0416.1-2403 ($z$$=$$0.397$, $M(R<200\,{\rm kpc})$$=$$1.6$$\times$$10^{14}\msun$), which has 141 multiple images with spectroscopic redshifts. We find that many additional parameters in a cluster mass model can be constrained, and that adding even small amounts of extra freedom to a model can dramatically improve its figures of merit. We use this information to constrain the distribution of dark matter around cluster member galaxies, simultaneously with the cluster's large-scale mass distribution. We find tentative evidence that some galaxies' dark matter has surprisingly similar ellipticity to their stars (unlike in the field, where it is more spherical), but that its orientation is often misaligned. When non-coincident dark matter and baryonic halos are allowed, the model improves by 35\%. This technique may provide a new way to investigate the processes and timescales on which dark matter is stripped from galaxies as they fall into a massive cluster. Our preliminary conclusions will be made more robust by analysing the remaining five \emph{Frontier Field} clusters.

Evolution of Late-type Galaxies in a Cluster Environment: Effects of High-speed Multiple Encounters with Early-type Galaxies

Abstract Late-type galaxies falling into a cluster would evolve being influenced by the interactions with both the cluster and the nearby cluster member galaxies. Most numerical studies, however, tend to focus on the effects of the former with little work done on those of the latter. We thus perform a numerical study on the evolution of a late-type galaxy interacting with neighboring early-type galaxies at high speed using hydrodynamic simulations. Based on the information obtained from the Coma cluster, we set up the simulations for the case where a Milky Way–like late-type galaxy experiences six consecutive collisions with twice as massive early-type galaxies having hot gas in their halos at the closest approach distances of 15–65 h −1 kpc at the relative velocities of 1500–1600 km s−1. Our simulations show that the evolution of the late-type galaxy can be significantly affected by the accumulated effects of the high-speed multiple collisions with the early-type galaxies, such as on cold gas content and star formation activity of the late-type galaxy, particularly through the hydrodynamic interactions between cold disk and hot gas halos. We find that the late-type galaxy can lose most of its cold gas after the six collisions and have more star formation activity during the collisions. By comparing our simulation results with those of galaxy–cluster interactions, we claim that the role of the galaxy–galaxy interactions on the evolution of late-type galaxies in clusters could be comparable with that of the galaxy–cluster interactions, depending on the dynamical history.

First results on the cluster galaxy population from the Subaru Hyper Suprime-Cam survey. II. Faint end color–magnitude diagrams and radial profiles of red and blue galaxies at 0.1 &amp;lt; z &amp;lt; 1.1

Abstract We present a statistical study of the redshift evolution of the cluster galaxy population over a wide redshift range from 0.1 to 1.1, using ∼1900 optically-selected CAMIRA clusters from ∼232 deg2 of the Hyper Suprime-Cam (HSC) Wide S16A data. Our stacking technique with a statistical background subtraction reveals color–magnitude diagrams of red-sequence and blue cluster galaxies down to faint magnitudes of mz ∼ 24. We find that the linear relation of red-sequence galaxies in the color–magnitude diagram extends down to the faintest magnitudes we explore with a small intrinsic scatter σint(g − r) &amp;lt; 0.1. The scatter does not evolve significantly with redshift. The stacked color–magnitude diagrams are used to define red and blue galaxies in clusters in order to study their radial number density profiles without resorting to photometric redshifts of individual galaxies. We find that red galaxies are significantly more concentrated toward cluster centers and blue galaxies dominate the outskirts of clusters. We explore the fraction of red galaxies in clusters as a function of redshift, and find that the red fraction decreases with increasing distances from cluster centers. The red fraction exhibits a moderate decrease with increasing redshift. The radial number density profiles of cluster member galaxies are also used to infer the location of the steepest slope in the three-dimensional galaxy density profiles. For a fixed threshold in richness, we find little redshift evolution in this location.

Radio Selection of the Most Distant Galaxy Clusters

Abstract We show that the most distant X-ray-detected cluster known to date, Cl J1001 at , hosts a strong overdensity of radio sources. Six of them are individually detected (within ) in deep resolution VLA 3 GHz imaging, with . Of the six, an active galactic nucleus (AGN) likely affects the radio emission in two galaxies, while star formation is the dominant source powering the remaining four. We searched for cluster candidates over the full COSMOS 2 deg2 field using radio-detected 3 GHz sources and looking for peaks in density maps. Cl J1001 is the strongest overdensity by far with , with a simple preselection. A cruder photometric rejection of radio foregrounds leaves Cl J1001 as the second strongest overdensity, while even using all radio sources Cl J1001 remains among the four strongest projected overdensities. We conclude that there are great prospects for future deep and wide-area radio surveys to discover large samples of the first generation of forming galaxy clusters. In these remarkable structures, widespread star formation and AGN activity of massive galaxy cluster members, residing within the inner cluster core, will ultimately lead to radio continuum as one of the most effective means for their identification, with detection rates expected in the ballpark of 0.1–1 per square degree at . Samples of hundreds such high-redshift clusters could potentially constrain cosmological parameters and test cluster and galaxy formation models.

Galaxy overdensities around 3C radio galaxies and quasars at 1 &lt; z &lt; 2.5 revealed by Spitzer 3.6/4.5 µm and Pan‐STARRS

Luminous radio sources are thought to lie in galaxy clusters or proto‐clusters. The complete sample of 64 high‐redshift 3C sources at 1 &lt; z &lt; 2.5 has been mapped with the Spitzer Space Telescope. The IRAC 3.6 and 4.5 µm 5σ detection limit of 4 µJy (22.4 AB mag) allows us to search for the brightest candidate cluster member galaxies associated with the 3C sources. To remove the contamination of foreground stars and galaxies along the lines of sight toward the 3C sources, we apply color cuts: the removed sources satisfy either the IRAC1/2 cut [3.6]–[4.5] &lt; −0.3 or the Pan‐STARRS/IRAC cut i − [4.5] &lt; 0.5 if detected by Pan‐STARRS. For both selection methods, about half of the 3C radio sources show significant overdensities (&gt; 3σ) within 30″ (∼250 kpc) projected distance from the radio source compared to the surrounding galaxy densities measured in the 50–120″ annulus. The Pan‐STARRS/IRAC cut reveals higher average overdensities than the IRAC1/2 cut and also a decline of overdensities at z &gt; 1.4. To infer the nature of the cluster members, we rerun the analysis using a stronger IRAC1/2 cut [3.6]–[4.5] &lt; −0.1, which removes 1 &lt; z &lt; 1.4 passive ellipticals but not star‐forming galaxies. For the strong cut, the overdensities, on average, completely disappear at 1 &lt; z &lt; 1.4. We therefore suggest that the 4.5 µm detected cluster member galaxies are mainly passive ellipticals.

An optically-selected cluster catalog at redshift 0.1 &amp;lt; z &amp;lt; 1.1 from the Hyper Suprime-Cam Subaru Strategic Program S16A data

Abstract We present an optically-selected cluster catalog from the Hyper Suprime-Cam (HSC) Subaru Strategic Program. The HSC images are sufficiently deep to detect cluster member galaxies down to M* ∼ 1010.2 M⊙ even at z ∼ 1, allowing a reliable cluster detection at such high redshifts. We apply the CAMIRA algorithm to the HSC Wide S16A dataset covering ∼232 deg2 to construct a catalog of 1921 clusters at redshift 0.1 &amp;lt; z &amp;lt; 1.1 and richness ${\skew7\hat{N}}_{\rm mem}&amp;gt;15$ that roughly corresponds to M200m ≳ 1014 h−1 M⊙. We confirm good cluster photometric redshift performance, with the bias and the scatter in Δz/(1 + z) being better than 0.005 and 0.01, respectively, over most of the redshift range. We compare our cluster catalog with large X-ray cluster catalogs from the XXL and XMM-LSS (the XMM Large Scale Structure) surveys and find good correlation between richness and X-ray properties.We also study the mis-centering effect from the distribution of offsets between optical and X-ray cluster centers. We confirm the high (&amp;gt;0.9) completeness and purity for high-mass clusters by analyzing mock galaxy catalogs.