We describe updates to the redMaPPer algorithm, a photometric red-sequence cluster finder specifically designed for large photometric surveys. The updated algorithm is applied to 150 deg 2 of Science Verification (SV) data from the Dark Energy Survey (DES), and to the Sloan Digital Sky Survey (SDSS) DR8 photometric data set. The DES SV catalog is locally volume limited, and contains 786 clusters with richness λ > 20 (roughly equivalent to M 500c 10 14 h −1 70 M ) and 0.2 < z < 0.9. The DR8 catalog consists of 26311 clusters with 0.08 < z < 0.6, with a sharply increasing richness threshold as a function of redshift for z 0.35. The photometric redshift performance of both catalogs is shown to be excellent, with photometric redshift uncertainties controlled at the σ z /(1 + z) ∼ 0.01 level for z 0.7, rising to ∼ 0.02 at z ∼ 0.9 in DES SV. We make use of Chandra and XMM X-ray and South Pole Telescope Sunyaev-Zeldovich data to show that the centering performance and massrichness scatter are consistent with expectations based on prior runs of redMaPPer on SDSS data. We also show how the redMaPPer photo-z and richness estimates are relatively insensitive to imperfect star/galaxy separation and small-scale star masks.

We present a detailed overview of the cosmological surveys that we aim to carry out with Phase 1 of the Square Kilometre Array (SKA1) and the science that they will enable. We highlight three main surveys: a medium-deep continuum weak lensing and low-redshift spectroscopic HI galaxy survey over 5 000 deg2; a wide and deep continuum galaxy and HI intensity mapping (IM) survey over 20 000 deg2 from $z = 0.35$ to 3; and a deep, high-redshift HI IM survey over 100 deg2 from $z = 3$ to 6. Taken together, these surveys will achieve an array of important scientific goals: measuring the equation of state of dark energy out to $z \sim 3$ with percent-level precision measurements of the cosmic expansion rate; constraining possible deviations from General Relativity on cosmological scales by measuring the growth rate of structure through multiple independent methods; mapping the structure of the Universe on the largest accessible scales, thus constraining fundamental properties such as isotropy, homogeneity, and non-Gaussianity; and measuring the HI density and bias out to $z = 6$ . These surveys will also provide highly complementary clustering and weak lensing measurements that have independent systematic uncertainties to those of optical and near-infrared (NIR) surveys like Euclid, LSST, and WFIRST leading to a multitude of synergies that can improve constraints significantly beyond what optical or radio surveys can achieve on their own. This document, the 2018 Red Book, provides reference technical specifications, cosmological parameter forecasts, and an overview of relevant systematic effects for the three key surveys and will be regularly updated by the Cosmology Science Working Group in the run up to start of operations and the Key Science Programme of SKA1.

We use Chandra X-ray and Spitzer infrared (IR) observations to explore the active galactic nucleus (AGN) and starburst populations of XMMXCS J2215.9−1738 at z = 1.46, one of the most distant spectroscopically confirmed galaxy clusters known. The high-resolution X-ray imaging reveals that the cluster emission is contaminated by point sources that were not resolved in XMM-Newton observations of the system, and have the effect of hardening the spectrum, leading to the previously reported temperature for this system being overestimated. From a joint spectroscopic analysis of the Chandra and XMM-Newton data, the cluster is found to have temperature T = 4.1 +0.6 −0.9 keV and luminosity L X = (2.9244 erg s −1 , extrapolated to a radius of 2 Mpc. As a result of this revised analysis, the cluster is found to lie on the σ v -T relation, but the cluster remains less luminous than would be expected from self-similar evolution of the local L X -T relation. Two of the newly discovered X-ray AGNs are cluster members, while a third object, which is also a prominent 24 µm source, is found to have properties consistent with it being a high-redshift, highly obscured object in the background. We find a total of eight >5σ 24 µm sources associated with cluster members (four spectroscopically confirmed and four selected using photometric redshifts) and one additional 24 µm source with two possible optical/near-IR counterparts that may be associated with the cluster. Examining the Infrared Array Camera colors of these sources, we find that one object is likely to be an AGN. Assuming that the other 24 µm sources are powered by star formation, their IR luminosities imply star formation rates ∼100 M ⊙ yr −1 . We find that three of these sources are located at projected distances of <250 kpc from the cluster center, suggesting that a large amount of star formation may be taking place in the cluster core, in contrast to clusters at low redshift.

We present deep J and K s band photometry of 20 high redshift galaxy clusters between z = 0.8−1.5, 19 of which are observed with the MOIRCS instrument on the Subaru Telescope. By using nearinfrared light as a proxy for stellar mass we find the surprising result that the average stellar mass of Brightest Cluster Galaxies (BCGs) has remained constant at ∼ 9 × 10 11 M ⊙ since z ∼ 1.5. We investigate the effect on this result of differing star formation histories generated by three well known and independent stellar population codes and find it to be robust for reasonable, physically motivated choices of age and metallicity. By performing Monte Carlo simulations we find that the result is unaffected by any correlation between BCG mass and cluster mass in either the observed or model clusters. The large stellar masses imply that the assemblage of these galaxies took place at the same time as the initial burst of star formation. This result leads us to conclude that dry merging has had little effect on the average stellar mass of BCGs over the last 9 − 10 Gyr in stark contrast to the predictions of semi-analytic models, based on the hierarchical merging of dark matter haloes, which predict a more protracted mass build up over a Hubble time. We discuss however that there is potential for reconciliation between observation and theory if there is a significant growth of material in the intracluster light over the same period.

The current consensus is that galaxies begin as small density fluctuations in the early Universe and grow by in situ star formation and hierarchical merging 1 . Stars begin to form relatively quickly in sub-galactic sized building blocks called haloes which are subsequently assembled into galaxies. However, exactly when this assembly takes place is a matter of some debate 2, 3 . Here we report that the stellar masses of brightest cluster galaxies, which are the most luminous objects emitting stellar light, some 9 billion years ago are not significantly different from their stellar masses today. Brightest cluster galaxies are almost fully assembled 4 − 5 Gyrs after the Big Bang, having 2 grown to more than 90% of their final stellar mass by this time. Our data conflict with the most recent galaxy formation models 4, 5 based on the largest simulations of dark matter halo development 1 . These models predict protracted formation of brightest cluster galaxies over a Hubble time, with only 22% of the stellar mass assembled at the epoch probed by our sample. Our findings suggest a new picture in which brightest cluster galaxies experience an early period of rapid growth rather than prolonged hierarchical assembly.Brightest cluster galaxies (BCGs) are located at the centres of galaxy clusters. They constitute a separate population from bright elliptical galaxies 6 and both their homogeneity and extreme luminosity have motivated their use as standard candles for cosmology 7-9 .Our investigation focuses on BCGs in the most distant X-ray emitting galaxy clusters at redshifts z = 1.2 −1.5, where (1 + z) is the expansion factor of the Universe relative to the present. It has been shown that X-ray cluster selection is currently the optimum strategy for an unbiased investigation of BCG evolution 10 . Properties of our BCGs and their host clusters are listed in Table 1. All five clusters were discovered serendipitously in X-rays and they are the most distant clusters discovered in their respective X-ray surveys 11-15 .The cluster J2215 was discovered as part of the XMM Cluster Survey (XCS 16,17 ) and has the highest redshift of any spectroscopically confirmed cluster 12,18 . 3The stellar mass of a BCG depends upon the hierarchical build up of its host dark matter halo and its stellar evolution history, along with the baryonic physics of the galaxy.We base our study of BCGs on photometry in the infrared wavebands J (1.26 µm) and K s (2.14 µm). Infrared imaging is essential at these large redshifts to compensate for the redshifting of the early-type galaxy spectra. Also, these wavebands are less sensitive than optical light to the presence of young stars and are a more accurate tracer of the underlying old stellar population and, hence, of the stellar mass of the systems. Fig. 1 shows an infrared image of the cluster J2235 from our sample (see also Supplementary Fig. 1).We start by examining the ages of the stars themselves in these galaxies using the run of J − K s colour evolution with redshift as shown in Fig. 2. For BCGs at ...

The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters using all publicly available data in the XMM–Newton Science Archive. Its main aims are to measure cosmological parameters and trace the evolution of X‐ray scaling relations. In this paper we present the first data release from the XMM Cluster Survey (XCS‐DR1). This consists of 503  optically confirmed, serendipitously detected, X‐ray clusters. Of these clusters, 256  are new to the literature and 357  are new X‐ray discoveries. We present 463  clusters with a redshift estimate (0.06 < z < 1.46), including 261  clusters with spectroscopic redshifts. The remainder have photometric redshifts. In addition, we have measured X‐ray temperatures (TX) for 401  clusters (0.4 < TX < 14.7 keV). We highlight seven interesting subsamples of XCS‐DR1 clusters: (i) 10  clusters at high redshift (z > 1.0, including a new spectroscopically confirmed cluster at z= 1.01); (ii) 66  clusters with high TX (>5 keV); (iii) 130  clusters/groups with low TX (<2 keV); (iv) 27  clusters with measured TX values in the Sloan Digital Sky Survey (SDSS) ‘Stripe 82’ co‐add region; (v) 77  clusters with measured TX values in the Dark Energy Survey region; (vi) 40  clusters detected with sufficient counts to permit mass measurements (under the assumption of hydrostatic equilibrium); (vii) 104  clusters that can be used for applications such as the derivation of cosmological parameters and the measurement of cluster scaling relations. The X‐ray analysis methodology used to construct and analyse the XCS‐DR1 cluster sample has been presented in a companion paper, Lloyd‐Davies et al.

The Square Kilometre Array (SKA) is a planned large radio interferometer designed to operate over a wide range of frequencies, and with an order of magnitude greater sensitivity and survey speed than any current radio telescope. The SKA will address many important topics in astronomy, ranging from planet formation to distant galaxies. However, in this work, we consider the perspective of the SKA as a facility for studying physics. We review four areas in which the SKA is expected to make major contributions to our understanding of fundamental physics: cosmic dawn and reionisation; gravity and gravitational radiation; cosmology and dark energy; and dark matter and astroparticle physics. These discussions demonstrate that the SKA will be a spectacular physics machine, which will provide many new breakthroughs and novel insights on matter, energy, and spacetime.

We present a study of the morphological fractions and color-magnitude relation (CMR) in the most distant X-ray selected galaxy cluster currently known, XMMXCS J2215.9−1738 at z = 1.46, using a combination of optical imaging data obtained with the Hubble Space Telescope Advanced Camera for Surveys, and infrared data from the Multi-Object Infrared Camera and Spectrograph, mounted on the 8.2 m Subaru telescope. We find that the morphological mix of the cluster galaxy population is similar to clusters at z ∼ 1. Within the central 0.5 Mpc, approximately ∼62% of the galaxies identified as likely cluster members are ellipticals or S0s; and ∼38% are spirals or irregulars. Therefore, early-type galaxies were already entrenched as the dominant galaxy population in at least some clusters approximately ∼ 4.5 Gyr after the big bang. We measure the CMRs for the early-type galaxies, finding that the slope in the z 850-J relation is consistent with that measured in the Coma cluster, some ∼9 Gyr earlier, although the uncertainty is large. In contrast, the measured intrinsic scatter about the CMR is more than three times the value measured in Coma, after conversion to rest-frame U−V. From comparison with stellar population synthesis models, the intrinsic scatter measurements imply mean luminosity-weighted ages for the earlytype galaxies in J2215.9−1738 of ≈3 Gyr, corresponding to the major epoch of star formation coming to an end at z f ≈ 3-5. We find that the cluster exhibits evidence of the "downsizing" phenomenon: the fraction of faint cluster members on the red sequence expressed using the Dwarf-to-Giant Ratio (DGR) is 0.32 ± 0.18 within a radius of 0.5R 200. This is consistent with extrapolation of the redshift evolution of the DGR seen in cluster samples at z < 1. In contrast to observations of some other z > 1 clusters, we find a lack of very bright galaxies within the cluster.