A Definitive Optical Detection of a Supercluster at z = 0.91

We present the results from a multi-band optical imaging program which has definitively confirmed the existence of a supercluster at z = 0.91. Two massive clusters of galaxies, CL1604+4304 at z = 0.897 and CL1604+4321 at z = 0.924, were originally observed in the high-redshift cluster survey of Oke, Postman & Lubin (1998). They are separated by 4300 km/s in radial velocity and 17 arcminutes on the plane of the sky. Their physical and redshift proximity suggested a promising supercluster candidate. Deep BRi imaging of the region between the two clusters indicates a large population of red galaxies. This population forms a tight, red sequence in the color--magnitude diagram at (R-i) = 1.4. The characteristic color is identical to that of the spectroscopically-confirmed early-type galaxies in the two member clusters. The red galaxies are spread throughout the 5 Mpc region between CL1604+4304 and CL1604+4321. Their spatial distribution delineates the entire large scale structure with high concentrations at the cluster centers. In addition, we detect a significant overdensity of red galaxies directly between CL1604+4304 and CL1604+4321 which is the signature of a third, rich cluster associated with this system. The strong sequence of red galaxies and their spatial distribution clearly indicate that we have discovered a supercluster at z = 0.91.Comment: Accepted for publication in Astrophysical Journal Letters. 13 pages, including 5 figure

Multi-object spectroscopy of low-redshift EIS clusters IV. Reliability of matched-filter results at z~0.3-0.4

abridged) In this paper we present new redshifts for 747 galaxies in 23 ESO Imaging Survey (EIS) cluster fields. We use the "gap"-technique to search for significant overdensities in redshift space for identifying groups/clusters of galaxies. In this way we spectroscopically confirm systems in 10 of the 23 cluster candidate fields with a matched-filter estimated redshift z_MF=0.3-0.4 and with spectroscopic redshifts in the range from z=0.158 to z=0.534. We find that the systems identified in the present paper span a broad range of one-dimensional velocity dispersion (175-497 km/s) and richness (12L*<=L<=65L*). Both undersampling and contamination by substructures contribute to the uncertainty of these measurements. From the analysis of the colours of the galaxy populations we find that ~60% of the spectroscopically confirmed systems have a "significant" red sequence with a colour matching passive stellar evolution models. With this paper we complete our spectroscopic survey of the fields of 58 EIS cluster candidates with estimated redshifts z<=0.4. We have measured a total of 1954 galaxy redshifts in the range z=0.0065 to z=0.6706. Of the 58 systems we confirm 42 (~75%) with redshifts between z=0.095 and z=0.534.Comment: Accepted for publication in A&A, 19 pages, 11 figure

The CFH Optical PDCS survey (COP) I: The Data

This paper presents and gives the COP (COP: CFHT Optical PDCS; CFHT: Canada-France-Hawaii Telescope; PDCS: Palomar Distant Cluster Survey) survey data. We describe our photometric and spectroscopic observations with the MOS multi-slit spectrograph at the CFH telescope. A comparison of the photometry from the PDCS (Postman et al. 1996) catalogs and from the new images we have obtained at the CFH telescope shows that the different magnitude systems can be cross-calibrated. After identification between the PDCS catalogues and our new images, we built catalogues with redshift, coordinates and V, I and Rmagnitudes. We have classified the galaxies along the lines of sight into field and structure galaxies using a gap technique (Katgert et al. 1996). In total we have observed 18 significant structures along the 10 lines of sight.Comment: 40 pages, 13 figures, accepted in A

Filter design for the detection of compact sources based on the Neyman-Pearson detector

This paper considers the problem of compact source detection on a Gaussian background in 1D. Two aspects of this problem are considered: the design of the detector and the filtering of the data. Our detection scheme is based on local maxima and it takes into account not only the amplitude but also the curvature of the maxima. A Neyman-Pearson test is used to define the region of acceptance, that is given by a sufficient linear detector that is independent on the amplitude distribution of the sources. We study how detection can be enhanced by means of linear filters with a scaling parameter and compare some of them (the Mexican Hat wavelet, the matched and the scale-adaptive filters). We introduce a new filter, that depends on two free parameters (biparametric scale-adaptive filter). The value of these two parameters can be determined, given the a priori pdf of the amplitudes of the sources, such that the filter optimizes the performance of the detector in the sense that it gives the maximum number of real detections once fixed the number density of spurious sources. The combination of a detection scheme that includes information on the curvature and a flexible filter that incorporates two free parameters (one of them a scaling) improves significantly the number of detections in some interesting cases. In particular, for the case of weak sources embedded in white noise the improvement with respect to the standard matched filter is of the order of 40%. Finally, an estimation of the amplitude of the source is introduced and it is proven that such an estimator is unbiased and it has maximum efficiency. We perform numerical simulations to test these theoretical ideas and conclude that the results of the simulations agree with the analytical ones.Comment: 15 pages, 13 figures, version accepted for publication in MNRAS. Corrected typos in Tab.

Characterizing faint galaxies in the reionization epoch: LBT confirms two L<0.2L* sources at z=6.4 behind the CLASH/Frontier Fields cluster MACS0717.5+3745

We report the LBT/MODS1 spectroscopic confirmation of two images of faint Lyman alpha emitters at $z=6.4$ behind the Frontier Fields galaxy cluster MACSJ0717.5+3745. A wide range of lens models suggests that the two images are highly magnified, with a strong lower limit of mu>5. These are the faintest z>6 candidates spectroscopically confirmed to date. These may be also multiple images of the same z=6.4 source as supported by their similar intrinsic properties, but the lens models are inconclusive regarding this interpretation. To be cautious, we derive the physical properties of each image individually. Thanks to the high magnification, the observed near-infrared (restframe ultraviolet) part of the spectral energy distributions and Ly-alpha lines are well detected with S/N(m_1500)>~10 and S/N(Ly-alpha)~10-15. Adopting mu>5, the absolute magnitudes, M_1500, and Ly-alpha fluxes, are fainter than -18.7 and 2.8x10^(-18)erg/s/cm2, respectively. We find a very steep ultraviolet spectral slope beta=-3.0+/-0.5 (F_lambda=lambda^(beta)), implying that these are very young, dust-free and low metallicity objects, made of standard stellar populations or even extremely metal poor stars (age<~30Myr, E(B-V)=0 and metallicity 0.0-0.2 Z/Zsolar). The objects are compact (< 1 kpc^(2)), and with a stellar mass M* < 10^(8) M_solar. The very steep beta, the presence of the Ly-alpha line and the intrinsic FWHM (<300 km/s) of these newborn objects do not exclude a possible leakage of ionizing radiation. We discuss the possibility that such faint galaxies may resemble those responsible for cosmic reionization.Comment: Accepted by ApJL; 6 pages, 4 figures, 1 table, emulateapj forma