180 research outputs found

    A Systematic Survey of Protoclusters at z36z\sim3\mathrm{-}6 in the CFHTLS Deep Fields

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    We present the discovery of three protoclusters at z34z\sim3\mathrm{-}4 with spectroscopic confirmation in the Canada-France-Hawaii Telescope (CFHT) Legacy Survey Deep Fields. In these fields, we investigate the large-scale projected sky distribution of z36z\sim3\mathrm{-}6 Lyman break galaxies and identify 21 protocluster candidates from regions that are overdense at more than 4σ4\sigma overdensity significance. Based on cosmological simulations, it is expected that more than 76%76\% of these candidates will evolve into a galaxy cluster of at least a halo mass of 1014M10^{14}\,\mathrm{M_\odot} at z=0z=0. We perform follow-up spectroscopy for eight of the candidates using Subaru/FOCAS, KeckII/DEIMOS, and Gemini-N/GMOS. In total we target 462 dropout candidates and obtain 138 spectroscopic redshifts. We confirm three real protoclusters at z=34z=3\mathrm{-}4 with more than five members spectroscopically identified, and find one to be an incidental overdense region by mere chance alignment. The other four candidate regions at z56z\sim5\mathrm{-}6 require more spectroscopic follow-up in order to be conclusive. A z=3.67z=3.67 protocluster, which has eleven spectroscopically confirmed members, shows a remarkable core-like structure composed of a central small region (Mpc}) and an outskirts region (1.0physicalMpc\sim1.0\,\mathrm{physical\>Mpc}). The Lyα\alpha equivalent widths of members of the protocluster are significantly smaller than those of field galaxies at the same redshift while there is no difference in the UV luminosity distributions. These results imply that some environmental effects start operating as early as at z4z\sim4 along with the growth of the protocluster structure.Comment: 25 pages, 6 tables, 25 figures, accepted for publication in Ap

    A New Milky Way Satellite Discovered In The Subaru/Hyper Suprime-Cam Survey

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    We report the discovery of a new ultra-faint dwarf satellite companion of the Milky Way based on the early survey data from the Hyper Suprime-Cam Subaru Strategic Program. This new satellite, Virgo I, which is located in the constellation of Virgo, has been identified as a statistically significant (5.5 sigma) spatial overdensity of star-like objects with a well-defined main sequence and red giant branch in their color-magnitude diagram. The significance of this overdensity increases to 10.8 sigma when the relevant isochrone filter is adopted for the search. Based on the distribution of the stars around the likely main sequence turn-off at r ~ 24 mag, the distance to Virgo I is estimated as 87 kpc, and its most likely absolute magnitude calculated from a Monte Carlo analysis is M_V = -0.8 +/- 0.9 mag. This stellar system has an extended spatial distribution with a half-light radius of 38 +12/-11 pc, which clearly distinguishes it from a globular cluster with comparable luminosity. Thus, Virgo I is one of the faintest dwarf satellites known and is located beyond the reach of the Sloan Digital Sky Survey. This demonstrates the power of this survey program to identify very faint dwarf satellites. This discovery of VirgoI is based only on about 100 square degrees of data, thus a large number of faint dwarf satellites are likely to exist in the outer halo of the Milky Way.Comment: typos are corrected, 6 pages, 4 figures, accepted for publication in Ap

    Minor Contribution of Quasars to Ionizing Photon Budget at z~6: Update on Quasar Luminosity Function at the Faint-end with Subaru/Suprime-Cam

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    We constrain the quasar contribution to cosmic reionization based on our deep optical survey of z~6 quasars down to z_R=24.15 using Subaru/Suprime-Cam in three UKIDSS-DXS fields covering 6.5 deg^2. In Kashikawa et al. (2015), we select 17 quasar candidates and report our initial discovery of two low-luminosity quasars (M_1450~ -23) from seven targets, one of which might be a Lyman alpha emitting galaxy. From an additional optical spectroscopy, none of the four candidates out of the remaining ten turn out to be genuine quasars. Moreover, the deeper optical photometry provided by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) shows that, unlike the two already-known quasars, the i-z and z-y colors of the last six candidates are consistent with M- or L-type brown dwarfs. Therefore, the quasar luminosity function (QLF) in the previous paper is confirmed. Compiling QLF measurements from the literature over a wide magnitude range, including an extremely faint AGN candidate from Parsa et al. (2017}, to fit them with a double power-law, we find that the best-fit faint-end slope is alpha=-2.04^+0.33_-0.18 (-1.98^+0.48_-0.21) and characteristic magnitude is M_1450^*=-25.8^+1.1_-1.9 (-25.7^+1.0_-1.8) in the case of two (one) quasar detection. Our result suggests that, if the QLF is integrated down to M_1450=-18, quasars produce ~1-12% of the ionizing photons required to ionize the whole universe at z~6 with 2sigma confidence level, assuming that the escape fraction is f_esc=1 and the IGM clumpy factor is C=3. Even when the systematic uncertainties are taken into account, our result supports the scenario that quasars are the minor contributors of reionization.Comment: 8 pages, 3 figures, ApJL accepte

    Comoving Space Density and Obscured Fraction of High-Redshift Active Galactic Nuclei in the Subaru/{\it XMM-Newton} Deep Survey

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    We study the comoving space density of X-ray-selected luminous active galactic nuclei (AGNs) and the obscured AGN fraction at high redshifts (3<z<53 < z < 5) in the Subaru/{\it XMM-Newton} Deep Survey (SXDS) field. From an X-ray source catalog with high completeness of optical identification thanks to deep optical images, we select a sample of 30 AGNs at z>3z > 3 with intrinsic (de-absorbed and rest-frame 2--10 keV) luminosities of LX=104445L_{\rm X} = 10^{44-45} erg s1^{-1} detected in the 0.5--2 keV band, consisting of 20 and 10 objects with spectroscopic and photometric redshifts, respectively. Utilizing the 1/Vmax1/V_{\rm max} method, we confirm that the comoving space density of luminous AGNs decreases with redshift above z>3z > 3. When combined with the {\it Chandra}-COSMOS result of Civano et al.\ (2011), the density decline of AGNs with LX=104445L_{\rm X} = 10^{44-45} erg s1^{-1} is well represented by a power law of (1+z)6.2±0.9(1 + z)^{-6.2 \pm 0.9}. We also determine the fraction of X-ray obscured AGNs with NH>1022N_{\rm H} > 10^{22} cm2^{-2} in the Compton-thin population to be 0.540.19+0.17^{+0.17}_{-0.19}, by carefully taking into account observational biases including the effects of photon statistics for each source. This result is consistent with an independent determination of the type-2 AGN fraction based on optical properties, for which the fraction is found to be 0.59±\pm0.09. Comparing our result with that obtained in the local Universe, we conclude that the obscured fraction of luminous AGNs increases significantly from z=0z=0 to z>3z>3 by a factor of 2.5±\pm1.1.Comment: 12 pages, 12 figures, 1 table. Accepted for publication in Ap
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