1 2 A ug 2 00 9 Lyman Break Galaxies at z ∼ 5 : Rest - Frame UV Spectra . III 1

We present results of optical spectroscopic observations of candidates of Lyman Break Galaxies (LBGs) at z ∼ 5 in the region including the GOODS-N and the J0053+1234 region by using GMOS-N and GMOS-S, respectively. Among 25 candidates, five objects are identified to be at z ∼ 5 (two of them were already identified by an earlier study) and one object very close to the color-selection window turned out to be a foreground galaxy. With this spectroscopically identified sample and those from previous studies, we derived the lower limits on the number density of bright (MUV < −22.0 mag) LBGs at z ∼ 5. These lower limits are comparable to or slightly smaller than the number densities of UV luminosity functions (UVLFs) that show the smaller number density among z ∼ 5 UVLFs in literature. However, by considering that there remain many LBG candidates without spectroscopic observations, the number density of bright LBGs is expected to increase by a factor of two or more. The evidence for the deficiency of UV luminous LBGs with large Lyα equivalent widths was reinforced. We discuss possible causes for the deficiency and prefer the interpretation of dust absorption. Subject headings: galaxies: evolution — galaxies: formation — galaxies: highredshift Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Okayama 719-0232, Japan Astronomical Institute, Tohoku University, Sendai 980-77, Japan Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720 Department of Astronomy and Physics, St. Mary’s University, 923 Robie St., Halifax, Nova Scotia, B3H 3C3, Canada