Masami Ouchi

The End of the Reionization Epoch Probed by Lyα Emitters at z = 6.5 in the Subaru Deep Field* **

We report an extensive search for Lyα emitters (LAEs) at z = 6.5 in the Subaru Deep Field. Subsequent spectroscopy with Subaru and Keck identified eight more LAEs, giving a total of 17 spectroscopically confirmed LAEs at z = 6.5. Based on this spectroscopic sample of 17, complemented by a photometric sample of 58 LAEs, we have derived a more accurate Lyα luminosity function of LAEs at z = 6.5, which reveals an apparent deficit at the bright end of ∼0.75 mag fainter L*, compared with that observed at z = 5.7. The difference in the LAE luminosity functions between z = 5.7 and 6.5 is significant at the 3 σ level, which is reduced to 2 σ when cosmic variance is taken into account. This result may imply that the reionization of the universe has not been completed at z = 6.5. We found that the spatial distribution of LAEs at z = 6.5 was homogeneous over the field. We discuss the implications of these results for the reionization of the universe.

Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies – I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission

We present the first results of a new Keck spectroscopic survey of UV faint Lyman break galaxies in the redshift range 3 1.9 with respect to the predicted z ≃ 7 value, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the abundant supply of z and Y drops now available from deep Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Lyα fraction using optical and near-infrared multi-object spectrographs, thereby extending the study conducted in this paper to 7 ≲ z ≲ 8.

Reionization and Galaxy Evolution Probed by z = 7 Lyα Emitters*

We made a narrowband NB973 (bandwidth of 200A at 9755A) imaging of the Subaru Deep Field (SDF) and found two z=7 Lyman alpha emitter (LAE) candidates down to NB973=24.9. Carrying out deep follow-up spectroscopy, we identified one of them as a real z=6.96 LAE. This has shown that galaxy formation was in progress just 750 Myr after the Big Bang. Meanwhile, the Lyman alpha line luminosity function of LAE is known to decline from z=5.7 to 6.6 in the SDF. L* at z=6.6 is 40-60% of that at z=5.7. We also confirm that the number density of z=7 LAE is only 17% of the density at z=6.6 comparing the latest SDF LAE samples. This series of significant decreases in LAE density with increasing redshift can be the result of galaxy evolution during these epochs. However, using the UV continuum luminosity functions of LAEs, those of Lyman break galaxies, and a LAE evolution model based on the hierarchical clustering, we find that galaxy evolution alone cannot explain all the decrease in density. This extra density deficit can be interpreted as the attenuation of the Lyman alpha photons from LAEs due to a rapid evolution of neutral hydrogen fraction during the ongoing cosmic reionization at z~6.6-7.

The Discovery of Primeval Large-Scale Structures with Forming Clusters at Redshift 6

We report the discovery of primeval large-scale structures (LSSs) including two protoclusters in a forming phase at z = 5.7. We carried out extensive deep narrowband imaging in the 1 deg2 sky of the Subaru/XMM-Newton Deep Field and obtained a cosmic map of 515 Lyα emitters (LAEs) in a volume with a transverse dimension of 180 Mpc × 180 Mpc and a depth of ~40 Mpc in comoving units. This cosmic map shows filamentary LSSs, including clusters and surrounding 10-40 Mpc scale voids, similar to the present-day LSSs. Our spectroscopic follow-up observations identify overdense regions in which two dense clumps of LAEs with a sphere of 1 Mpc diameter in physical units are included. These clumps show about 130 times higher star formation rate density, mainly due to a large overdensity, ~80, of LAEs. These clumps would be clusters in a formation phase involving a burst of galaxy formation.

Lyα Emitters at z = 5.7 in the Subaru Deep Field

PASJ: Publ. Astron. Soc. Japan , 1–??, c 2008. Astronomical Society of Japan. Lyα Emitters at z = 5.7 in the Subaru Deep Field Kazuhiro Shimasaku, 1,2 Nobunari Kashikawa, 3,4 Mamoru Doi, 5,2 Chun Ly, 6 Matthew A.Malkan, 6 Yuichi Matsuda, 7 Masami Ouchi, 8,‡ Tomoki Hayashino, 9 Masanori Iye, 3,4 Kentaro Motohara, 5 Takashi Murayama, Tohru Nagao, 3,11 Kouji Ohta, 12 Sadanori Okamura, 1,2 Toshiyuki Sasaki, 13 Yasuhiro Shioya, 10 Yoshiaki Taniguchi 10 arXiv:astro-ph/0602614v1 28 Feb 2006 Department of Astronomy, School of Science, The University of Tokyo, Tokyo 113-0033 Email (KS): shimasaku@astron.s.u-tokyo.ac.jp Research Center for the Early Universe, School of Science, The University of Tokyo, Tokyo 113-0033 Optical and Infrared Astronomy Division, National Astronomical Observatory, Mitaka, Tokyo 181-8588 Department of Astronomy, School of Science, Graduate University for Advanced Studies, Mitaka, Tokyo 181-8588 Institute of Astronomy, School of Science, The University of Tokyo, Mitaka 181-0015 Department of Astronomy, University of California at Los Angeles, Los Angeles, CA 90095-1547, USA Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Research Center for Neutrino Science, Graduate School of Science, Tohoku University, Aramaki, Aoba, Sendai 980-8578 Astronomical Institute, Graduate School of Science, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan INAF / Osservatorio Astrofisico di Arcetri Largo E. Fermi 5, 50125 Firenze, Italy Kouji Ohta Department of Astronomy, Kyoto University, Kyoto 606-8502 Subaru Telescope, National Astronomical Observatory of Japan, 650 N. A’ohoku Place, Hilo, HI 96720, USA (Received 2005 October 26; accepted 2006 February 27) Abstract We present the properties of Lyα emitters (LAEs) at z = 5.7 in the Subaru Deep Field. A photometric sample of 89 LAE candidates is constructed from narrow-band (NB816) data down to N B816 = 26.0 (AB) in a continuous 725 arcmin 2 area. Spectra of 39 objects satisfying the photometric selection criteria for LAEs were obtained with Subaru and Keck II Telescopes, among which 28 were confirmed LAEs, one was a nearby galaxy, and eight were unclassified. We also obtained spectra of another 24 NB816-excess objects in the field, identifying six additional LAEs. We find that the Lyα luminosity function derived from the photometric sample is reproduced well by a Schechter function with L ⋆ = 7.9 +3.0 erg s −1 and φ ⋆ = 6.3 −2.0 × 10 −4 Mpc −3 for α = −1.5 (fixed) over the whole luminosity range of L ≃ 3 × 10 42 – 3 × 10 43 erg s −1 . We then measure rest-frame Lyα equivalent widths (EWs) for the confirmed LAEs, to find that the median among the 28 objects satisfying the photometric selection criteria is W i0 = 233 ˚ A. We infer that 30% – 40% of LAEs at z = 5.7 exceed W i0 = 240 ˚ A. These large-EW objects probably cannot be accounted for by ordinary star-forming populations with a Salpeter IMF. We also find that LAEs with fainter far-UV luminosities have larger EWs. Finally, we derive the far-UV luminosity function of LAEs down to M UV ≃ −19.6 using the photometric sample, and compare it with that of Lyman-break galaxies (LBGs). We find that as high as about 80% of LBGs at z ∼ 6 have W i0 ≥ 100 ˚ A, in sharp contrast to lower-z counterparts. Key words: galaxies: evolution — galaxies: high-redshift — galaxies: luminosity function, mass function — galaxies: photometry

MOIRCS Deep Survey. IV. Evolution of Galaxy Stellar Mass Function Back to z ~ 3

We use very deep near-infrared (NIR) imaging data obtained in MOIRCS Deep Survey (MODS) to investigate the evolution of the galaxy stellar mass function back to z ~ 3. The MODS data reach J = 24.2, H = 23.1, and K = 23.1 (5σ, Vega magnitude) over 103 arcmin2 (wide) and J = 25.1, H = 23.7, and K = 24.1 over 28 arcmin2 (deep) in the GOODS-North region. The wide and very deep NIR data allow us to measure the number density of galaxies down to low stellar mass (109-1010 M ☉) even at high redshift with high statistical accuracy. The normalization of the mass function decreases with redshift, and the integrated stellar mass density becomes ~8%-18% of the local value at z ~ 2 and ~4%-9% at z ~ 3, which are consistent with results of previous studies in general fields. Furthermore, we found that the low-mass slope becomes steeper with redshift from α ~ –1.3 at z ~ 1 to α ~ –1.6 at z ~ 3 and that the evolution of the number density of low-mass (109-1010 M ☉) galaxies is weaker than that of M* (~1011 M ☉) galaxies. This indicates that the contribution of low-mass galaxies to the total stellar mass density has been significant at high redshift. The steepening of the low-mass slope with redshift is an opposite trend expected from the stellar mass dependence of the specific star formation rate reported in previous studies. The present result suggests that the hierarchical merging process overwhelmed the effect of the stellar mass growth by star formation and was very important for the stellar mass assembly of these galaxies at 1 z 3.

Clustering of Lyman Break Galaxies at z = 4 and 5 in the Subaru Deep Field: Luminosity Dependence of the Correlation Function Slope

We explored the clustering properties of Lyman break galaxies at z = 4 and 5 with an angular two-point correlation function on the basis of the very deep and wide Subaru Deep Field data. We confirmed the previous result that the clustering strength of LBGs depends on the UV luminosity in the sense that brighter LBGs are more strongly clustered. In addition, we found an apparent dependence of the correlation function slope on UV luminosity for LBGs at both z = 4 and 5. More luminous LBGs have a steeper correlation function. The bias parameter was found to be a scale-dependent function for bright LBGs, whereas it appears to be almost scale-independent for faint LBGs. Luminous LBGs have a higher bias at smaller angular scales, which decreases as the scale increases. To compare these observational results, we constructed numerical mock LBG catalogs based on a semianalytic model of hierarchical clustering combined with high-resolution N-body simulation, carefully mimicking the observational selection effects. The luminosity functions and the overall correlation functions for LBGs at z = 4 and 5 predicted by this mock catalog were found to be almost consistent with the observation. The observed dependence of the clustering on UV luminosity was not reproduced by the model, unless subsamples of distinct halo mass were considered. That is, LBGs belonging to more massive dark halos had steeper and larger amplitude correlation functions. With this model, we found that LBG multiplicity in massive dark halos amplifies the clustering strength at small scales, which steepens the correlation function. The hierarchical clustering model could therefore be reconciled with the observed luminosity dependence of the correlation function if there is a tight correlation between UV luminosity and halo mass. Our finding that the slope of the correlation function depends on luminosity could be an indication that massive dark halos hosted multiple bright LBGs.

Stellar populations of Lyα emitters at z= 3–4 based on deep large area surveys in the Subaru-SXDS/UKIDSS-UDS Field

We investigate the stellar populations of Lyα emitters (LAEs) at z = 3.1 and 3.7 in 0.65 deg 2 of the Subaru/XMM-Newton Deep Field, based on rest-frame ultraviolet-to-optical photometry obtained from the Subaru/XMM-Newton Deep Survey, the UKIRT Infrared Deep Sky Survey/ Ultra Deep Survey (UKIDSS/UDS), and the Spitzer legacy survey of the UKIDSS/UDS. Among a total of 302 LAEs (224 for z = 3.1 and 78 for z = 3.7), only 11 are detected in the K band, i.e. brighter than K(3σ) = 24.1 mag. Eight of the 11 K-detected LAEs are spectroscopically confirmed. In our stellar population analysis, we treat K-detected objects individually, while K-undetected objects are stacked at each redshift. We find that the K-undetected objects, which should closely represent the LAE population as a whole, have low stellar masses of ~10 8 -10 8. 5 M ⊙ , modest star formation rates (SFRs) of 1-100 M ⊙ yr -1 , and modest dust extinction of E(B - V) * < 0.2. The K-detected objects are massive, M star ~ 10 9 -10 10.5 M ⊙ , and have significant dust extinction with a median of E(B — V) * ≃ 0.3. Four K-detected objects with the reddest spectral energy distributions, two of which are spectroscopically confirmed, are heavily obscured with E(B — V) * ~ 0.65, and their continua resemble those of some local ultraluminous infrared galaxies (ULIRGs). Interestingly, they have large Lyα equivalent widths ≃ 70-250 A. If these four are excluded, our sample has a weak anticorrelation between Lyα equivalent width and M star . We compare the stellar masses and the specific SFRs (sSFRs) of LAEs with those of Lyman-break galaxies (LBGs), distant red galaxies, submillimetre galaxies, and I- or K-selected galaxies with photometric redshifts of z phot ~ 3. We find that the LAE population is the least massive among all the galaxy populations in question, but with relatively high sSFRs, while near-infrared (NIR)-detected LAEs have M star and sSFR similar to LBGs. Our reddest four LAEs have very high sSFRs in spite of large M star , thus occupying a unique region in the M star versus sSFR space.

The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals

We present the Spitzer Extragalactic Representative Volume Survey (SERVS), an 18 square degrees medium-deep survey at 3.6 and 4.5 microns with the post-cryogenic Spitzer Space Telescope to ~2 microJy (AB=23.1) depth of five highly observed astronomical fields (ELAIS-N1, ELAIS-S1, Lockman Hole, Chandra Deep Field South and XMM-LSS). SERVS is designed to enable the study of galaxy evolution as a function of environment from z~5 to the present day, and is the first extragalactic survey both large enough and deep enough to put rare objects such as luminous quasars and galaxy clusters at z>1 into their cosmological context. SERVS is designed to overlap with several key surveys at optical, near- through far-infrared, submillimeter and radio wavelengths to provide an unprecedented view of the formation and evolution of massive galaxies. In this paper, we discuss the SERVS survey design, the data processing flow from image reduction and mosaicing to catalogs, as well as coverage of ancillary data from other surveys in the SERVS fields. We also highlight a variety of early science results from the survey.

MOIRCS Deep Survey. I : DRG Number Counts

We use very deep near-infrared imaging data taken with Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru Telescope to investigate the number counts of Distant Red Galaxies (DRGs). We have observed a 4x7 arcmin^2 field in the Great Observatories Origins Deep Survey North (GOODS-N), and our data reach J=24.6 and K=23.2 (5sigma, Vega magnitude). The surface density of DRGs selected by J-K>2.3 is 2.35+-0.31 arcmin^-2 at K 22 is smaller than that expected from the number counts at the brighter magnitude. The result indicates that while there are many bright galaxies at 2 22 suggest that the mass-dependent color distribution, where most of low-mass galaxies are blue while more massive galaxies tend to have redder colors, had already been established at that epoch.