A Deficit of Faint Red Galaxies in the Possible Large-Scale Structures around the RDCS J1252.9-2927 Cluster at z=1.24

(Abridged) We report a discovery of possible large-scale structures around the RDCS J1252.9-2927 cluster at z=1.24 based on photometric redshifts. We carried out multi-band wide-field imaging with Suprime-Cam on the Subaru Telescope and WFCAM on the United Kingdom Infra-Red Telescope (UKIRT). The distribution of photo-z selected galaxies reveals clumpy structures surrounding the central cluster. We compare the observed structure with an X-ray map and find that two of the four plausible clumps show significant X-ray emissions and one with a marginal detection, which strongly suggest that they are dynamically bound systems. Following the discovery of the possible large-scale structure, we carried out deeper SOFI K_s-band imaging with New Technology Telescope on the four plausible clumps. We construct the optical-to-near-infrared colour-magnitude diagrams of the galaxies in the clumps, and find that the colour-magnitude relation (CMR) of the red galaxies in the clumps is sharply truncated below K_s=22. Interestingly, the main cluster shows a clear relation down to K_s=23 (Lidman et al. 2004). We suggest that galaxies follow the 'environment-dependent down-sizing' evolution. Massive galaxies in high density environments first stop forming stars and become red. Less massive galaxies in less dense environments become red at later times. Based on a few assumptions, we predict that the brightest tip of the CMR appears at z~2.5.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Morphology with Light Profile Fitting of Confirmed Cluster Galaxies at z=0.84

We perform a morphological study of 124 spectroscopically confirmed cluster galaxies in the z=0.84 galaxy cluster RX J0152.7-1357. Our classification scheme includes color information, visual morphology, and 1-component and 2-component light profile fitting derived from Hubble Space Telescope riz imaging. We adopt a modified version of a detailed classification scheme previously used in studies of field galaxies and found to be correlated with kinematic features of those galaxies. We compare our cluster galaxy morphologies to those of field galaxies at similar redshift. We also compare galaxy morphologies in regions of the cluster with different dark-matter density as determined by weak-lensing maps. We find an early-type fraction for the cluster population as a whole of 47%, about 2.8 times higher than the field, and similar to the dynamically young cluster MS 1054 at similar redshift. We find the most drastic change in morphology distribution between the low and intermediate dark matter density regions within the cluster, with the early type fraction doubling and the peculiar fraction dropping by nearly half. The peculiar fraction drops more drastically than the spiral fraction going from the outskirts to the intermediate-density regions. This suggests that many galaxies falling into clusters at z~0.8 may evolve directly from peculiar, merging, and compact systems into early-type galaxies, without having the chance to first evolve into a regular spiral galaxy.Comment: 13 pages, 11 figures, accepted for publication in A&

An Atlas of Spectrophotometric Landolt Standard Stars

We present CCD observations of 102 Landolt standard stars obtained with the R-C spectrograph on the CTIO 1.5 m telescope. Using stellar atmosphere models we have extended the flux points to our six spectrophotometric secondary standards, in both the blue and the red, allowing us to produce flux-calibrated spectra that span a wavelength range from 3050 \AA to 1.1 \micron. Mean differences between UBVRI spectrophotometry computed using Bessell's standard passbands and Landolt's published photometry is found to be 1% or less. Observers in both hemispheres will find these spectra useful for flux-calibrating spectra and through the use of accurately constructed instrumental passbands be able to compute accurate corrections to bring instrumental magnitudes to any desired standard photometric system (S-corrections). In addition, by combining empirical and modeled spectra of the Sun, Sirius and Vega, we calculate and compare synthetic photometry to observed photometry taken from the literature for these three stars.Comment: Added referee's comments, minor corrections, replaced Table 1

Spectroscopic Confirmation of a Massive Red-Sequence-Selected Galaxy Cluster at z = 1.34 in the SpARCS-South Cluster Survey

The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is a z'-passband imaging survey, consisting of deep (z' ~ 24 AB) observations made from both hemispheres using the CFHT 3.6m and CTIO 4m telescopes. The survey was designed with the primary aim of detecting galaxy clusters at z >~ 1. In tandem with pre-existing 3.6um observations from the Spitzer Space Telescope SWIRE Legacy Survey, SpARCS detects clusters using an infrared adaptation of the two-filter red-sequence cluster technique. The total effective area of the SpARCS cluster survey is 41.9 deg^2. In this paper, we provide an overview of the 13.6 deg^2 Southern CTIO/MOSAICII observations. The 28.3 deg^2 Northern CFHT/MegaCam observations are summarized in a companion paper by Muzzin et al. (2008). In this paper, we also report spectroscopic confirmation of SpARCS J003550-431224, a very rich galaxy cluster at z = 1.335, discovered in the ELAIS-S1 field. To date, this is the highest spectroscopically confirmed redshift for a galaxy cluster discovered using the red-sequence technique. Based on nine confirmed members, SpARCS J003550-431224 has a preliminary velocity dispersion of 1050 +/- 230 km/s. With its proven capability for efficient cluster detection, SpARCS is a demonstration that we have entered an era of large, homogeneously-selected z > 1 cluster surveys.Comment: 10 pages, 6 Figures, Submitted to the Ap

Star-forming fractions and galaxy evolution with redshift in rich X-ray-selected galaxy clusters

We have compared stacked spectra of galaxies, grouped by environment and stellar mass, among 58 members of the redshift z = 1.24 galaxy cluster RDCS J1252.9-2927 (J1252.9) and 134 galaxies in the z = 0.84 cluster RX J0152.7-1357 (J0152.7). These two clusters are excellent laboratories to study how galaxies evolve from star-forming to passive at z ~ 1. We measured spectral indices and star-forming fractions for our density- and mass-based stacked spectra. The star-forming fraction among low-mass galaxies (<7 × 10^(10)M_⊙) is higher in J1252.9 than in J0152.7, at about 4σ significance. Thus star formation is being quenched between z = 1.24 and z = 0.84 for a substantial fraction of low-mass galaxies. Star-forming fractions were also found to be higher in J1252.9 in all environments, including the core. Passive galaxies in J1252.9 have systematically lower D_n4000 values than in J0152.7 in all density and mass groups, consistent with passive evolution at modestly super-solar metallicities

Discovery of ram-pressure stripped gas around an elliptical galaxy in Abell 2670

Studies of cluster galaxies are increasingly finding galaxies with spectacular one-sided tails of gas and young stars, suggestive of intense ram-pressure stripping. These so-called "jellyfish" galaxies typically have late-type morphology. In this paper, we present MUSE observations of an elliptical galaxy in Abell 2670 with long tails of material visible in the optical spectra, as well as blobs with tadpole-like morphology. The spectra in the central part of the galaxy reveals a stellar component as well as ionized gas. The stellar component does not have significant rotation, while the ionized gas defines a clear star-forming gas disk. We argue, based on deep optical images of the galaxy, that the gas was most likely acquired during a past wet merger. It is possible that the star-forming blobs are also remnants of the merger. In addition, the direction and kinematics of the one-sided ionized tails, combined with the tadpole morphology of the star-forming blobs, strongly suggests that the system is undergoing ram pressure from the intracluster medium. In summary, this paper presents the discovery of a post-merger elliptical galaxy undergoing ram pressure stripping.Comment: 6 pages, 5 figures, accepted for publication in ApJ