Numerous methods for finding clusters at moderate to high redshifts have been proposed in recent years, at wavelengths ranging from radio to X-rays. In this paper we describe a new method for detecting clusters in two-band optical/near-IR imaging data. The method relies upon the observation that all rich clusters, at all redshifts observed so far, appear to have a red sequence of early-type galaxies. The emerging picture is that all rich clusters contain a core population of passively evolving elliptical galaxies which are coeval and formed at high redshifts. The proposed search method exploits this strong empirical fact by using the red sequence as a direct indicator of overdensity. The fundamental advantage of this approach is that with appropriate filters, cluster elliptical galaxies at a given redshift are redder than all normal galaxies at lower redshifts. A simple color cut thus virtually eliminates all foreground contamination, even at significant redshifts. In this paper, one of a series of two, we describe the underlying assumptions and basic techniques of the method in detail, and contrast the method with those used by other authors. We provide a brief demonstration of the effectiveness of the technique using a real photometric sample with redshift data, and from this conclude that the method offers a powerful yet simple way of identify galaxy clusters. We find that the method can reliably detect structures to masses as small as groups with velocity dispersions of only ∼ 300 km s −1 , with redshifts for all detected structures estimated to an accuray of ∼10%.

We present a catalog of galaxy clusters selected via their Sunyaev-Zel'dovich (SZ) effect signature from 2500 deg 2 of South Pole Telescope (SPT) data. This work represents the complete sample of clusters detected at high significance in the 2500 deg 2 SPT-SZ survey, which was completed in 2011. A total of 677 (409) cluster candidates are identified above a signal-to-noise threshold of ξ = 4.5 (5.0). Ground-and space-based optical and near-infrared (NIR) imaging confirms overdensities of similarly colored galaxies in the direction of 516 (or 76%) of the ξ > 4.5 candidates and 387 (or 95%) of the ξ > 5 candidates; the measured purity is consistent with expectations from simulations. Of these confirmed clusters, 415 were first identified in SPT data, including 251 new discoveries reported in this work. We estimate photometric redshifts for all candidates with identified optical and/or NIR counterparts; we additionally report redshifts derived from spectroscopic observations for 141 of these systems. The mass threshold of the catalog is roughly independent of redshift above z ∼ 0.25 leading to a sample of massive clusters that extends to high redshift. The median mass of the sample is M 500c (ρ crit ) ∼ 3.5 × 10 14 M h −1 70 , the median redshift is z med = 0.55, and the highest-redshift systems are at z >1.4. The combination of large redshift extent, clean selection, and high typical mass makes this cluster sample of particular interest for cosmological analyses and studies of cluster formation and evolution.

We report the discovery of eight new Milky Way companions in~1800 deg 2 of optical imaging data collected during the first year of the Dark Energy Survey (DES). Each system is identified as a statistically significant overdensity of individual stars consistent with the expected isochrone and luminosity function of an old and metal-poor stellar population. The objects span a wide range of absolute magnitudes (M V from -2.2 to -7.4 mag), physical

We present the results of a study of weak lensing by galaxies based on 45.5 deg 2 of R C band imaging data from the Red-Sequence Cluster Survey (RCS). We define a sample of lenses with 19.5 < R C < 21, and a sample of background galaxies with 21.5 < R C < 24.We present the first weak lensing detection of the flattening of galaxy dark matter halos. We use a simple model in which the ellipticity of the halo is f times the observed ellipticity of the lens. We find a best fit value of f = 0.77 +0.18 −0.21 , suggesting that the dark matter halos are somewhat rounder than the light distribution. The fact that we detect a significant flattening implies that the halos are well aligned with the light distribution. Given the average ellipticity of the lenses, this implies a halo ellipticity of e halo = 0.33 +0.07 −0.09 , in fair agreement with results from numerical simulations of CDM. We note that this result is formally a lower limit to the flattening, since the measurements imply a larger flattening if the halos are not aligned with the light distribution. Alternative theories of gravity (without dark matter) predict an isotropic lensing signal, which is excluded with 99.5% confidence. Hence, our results provide strong support for the existence of dark matter.We also study the average mass profile around the lenses, using a maximum likelihood analysis. We consider two models for the halo mass profile: a truncated isothermal sphere (TIS) and an NFW profile. We adopt observationally motivated scaling relations between the lens luminosity and the velocity dispersion and the extent of the halo. The TIS model yields a best fit velocity dispersion of σ = 136±5±3 km/s (all errors are 68% confidence limits; the first error bar indicates the statistical uncertainty, whereas the second error bar indicates the systematic error) and a truncation radius s = 185 +30 −28 h −1 kpc for a galaxy with a fiducial luminosity of L B = 10 10 h −2 L B⊙ (under the assumption that the luminosity does not evolve with redshift). Alternatively, the best fit NFW model yields a mass M 200 = (8.4 ± 0.7 ± 0.4) × 10 11 h −1 M ⊙ and a scale radius r s = 16.2 +3.6 −2.9 h −1 kpc. This value for the scale radius is in excellent agreement with predictions from numerical simulations for a halo of this mass.