We report the discovery of a redshift z = 6.56 galaxy lying behind the cluster Abell 370. The object HCM 6A was found in a narrowband imaging survey using a 118Å bandpass filter centered at 9152Å in the LRIS camera on the 10 m Keck II Telescope. Candidate Lyα emitters were identified by the equivalent width of the emission and the absence of lower wavelength flux in ultradeep broadband images. HCM 6A is the first galaxy to be confirmed at redshift z > 6, and has W λ (observed)=190Å, flux = 2.7 × 10 −17 erg cm −2 s −1 . Spectra obtained with LRIS confirm the emission line and the continuum break across the line, and show an asymmetric line profile with steep fall-off on the blue side. Deep Subaru nearinfrared CISCO images in J, H and K ′ which extend the sampled continuum to longer wavelengths give a consistent estimate of the continuum flux density in these line-free regions of 2.6 ± 0.7 × 10 −30 erg cm −2 s −1 Hz −1 . The line width and strength, asymmetric profile, and very deep spectral break are only consistent with the interpretation of the line as a redshifted Lyα feature. From the detailed lensing model of this cluster, we estimate a lensing amplification of 4.5 for this galaxy, which is located slightly over an arcminute from the center of the cluster, for an unlensed flux of 6.5 × 10 −18 erg cm −2 s −1 . The presence of such a galaxy suggests that the reionizing epoch is beyond z = 6.6.

We present a stellar mass-metallicity relation at z ∼ 1.4 with an unprecedentedly large sample of ∼ 340 star-forming galaxies obtained with FMOS on the Subaru Telescope. We observed K-band selected galaxies at 1.2 ≤ z ph ≤ 1.6 in the SXDS/UDS fields with M * ≥ 10 9.5 M ⊙ , and expected F(Hα) ≥ 5 × 10 −17 erg s −1 cm −2 . Among the observed ∼ 1200 targets, 343 objects show significant Hα emission lines. The gasphase metallicity is obtained from [N ii]λ6584/Hα line ratio, after excluding possible active galactic nuclei (AGNs). Due to the faintness of the [N ii]λ6584 lines, we apply the stacking analysis and derive the mass-metallicity relation at z ∼ 1.4. Our results are compared to past results at different redshifts in the literature. The mass-metallicity relation at z ∼ 1.4 is located between those at z ∼ 0.8 and z ∼ 2.2; it is found that the metallicity increases with decreasing redshift from z ∼ 3 to z ∼ 0 at fixed stellar mass. Thanks to the large size of the sample, we can study the dependence of the mass-metallicity relation on various galaxy physical properties. The average metallicity from the stacked spectra is close to the local FMR in the higher metallicity part but > ∼ 0.1 dex higher in metallicity than the FMR in the lower metallicity part. We find that galaxies with larger E(B − V ), B − R, and R − H colours tend to show higher metallicity by ∼ 0.05 dex at fixed stellar mass. We also find relatively clearer size dependence that objects with smaller half light radius tend to show higher metallicity by ∼ 0.1 dex at fixed stellar mass, especially in the low mass part.

Deep optical and near-infrared galaxy counts are utilized to estimate the extragalactic background light (EBL) coming from normal galactic light in the universe. Although the slope of number-magnitude relation of the faintest counts is flat enough for the count integration to converge, considerable fraction of EBL from galaxies could still have been missed in deep galaxy surveys because of various selection effects including the cosmological dimming of surface brightness of galaxies. Here we give an estimate of EBL from galaxy counts, in which these selection effects are quantitatively taken into account for the first time, based on reasonable models of galaxy evolution which are consistent with all available data of galaxy counts, size, and redshift distributions. We show that the EBL from galaxies is best resolved into discrete galaxies in the near-infrared bands (J, K) by using the latest data of the Subaru Deep Field; more than 80-90% of EBL from galaxies has been resolved in these bands. Our result indicates that the contribution by missing galaxies cannot account for the discrepancy between the count integration and recent tentative detections of diffuse EBL in the K-band (2.2 µm), and there may be a very diffuse component of EBL which has left no imprints in known galaxy populations.

The Subaru Deep Field (SDF) project is a program of Subaru Observatory to carry out a deep galaxy survey over a blank field as large as 34 ′ ×27 ′ . The program consists of very deep multi-band optical imaging, near infrared imaging for smaller portions of the field and follow-up optical spectroscopy. Major scientific goals of the project are to construct large samples of Lyman-break galaxies at z ≃ 4 − 5 and Lyman alpha emitters at z ≃ 5.7 and 6.6, and to make detailed studies these very high-redshift galaxy populations.In this paper, we describe the optical imaging observations and data reduction, presenting mosaicked images and object catalogs in seven bandpasses. The optical imaging was made through five broad-band filters, B, V , R, i ′ , z ′ , and two narrow-band filters, NB816 (λ c = 8150Å) and NB921 (λ c = 9196Å) with almost 10 hours long integrations for each band. The limiting magnitudes measured at 3σ on a 2 ′′ aperture are B = 28.45, V = 27.74, R = 27.80, i ′ = 27.43, z ′ = 26.62, NB816 = 26.63, and NB921 = 26.54 in the AB system. The object catalog constructed for each of the seven bands contains more than 10 5 objects. The galaxy number counts corrected for detection incompleteness and star count contribution are found to be consistent with previous results in the literature. The mosaicked images and catalogs of all the bands have been made open to the public on Oct. 1, 2004 on the SDF project website at http://soaps.naoj.org/sdf/.

We present new results of a deep optical imaging survey using a narrow band filter (NB921) centered at λ = 9196Å together with B, V , R, i ′ , and z ′ broadband 1 filters in the sky area of the Subaru Deep Field, which has been promoted as one of legacy programs of the 8.2 m Subaru Telescope. We obtained a photometric sample of 58 Lyα emitter candidates at z ≈ 6.5 -6.6 among ∼ 180 strong NB921-excess (z ′ − NB921 > 1.0) objects together with a color criterion of i ′ − z ′ > 1.3. We then obtained optical spectra of 20 objects in our NB921-excess sample, and identified at least nine Lyα emitters at z ∼ 6.5 -6.6, including the two emitters reported by Kodaira et al. (2003, PASJ, 55, L17). Since our Lyα-emitter candidates are free from strong amplification of gravitational lensing, we are able to discuss their observational properties from a statistical point of view. Based on these new results, we obtained a lower limit of the star-formation rate density of ρ SFR ≃ 5.7 × 10 −4 h 0.7 M ⊙ yr −1 Mpc −3 at z ≈ 6.6, being consistent with our previous estimate. We discuss the nature of star-formation activity in galaxies beyond z = 6.