Hyper Suprime-Cam (HSC) is a wide-field imaging camera on the prime focus of the 8.2m Subaru telescope on the summit of Maunakea in Hawaii. A team of scientists from Japan, Taiwan and Princeton University is using HSC to carry out a 300-night multi-band imaging survey of the high-latitude sky. The survey includes three layers: the Wide layer will cover 1400 deg 2 in five broad bands (grizy), with a 5 σ point-source depth of r ≈ 26. The Deep layer covers a total of 26 deg 2 in four fields, going roughly a magnitude fainter, while the UltraDeep layer goes almost a magnitude fainter still in two pointings of HSC (a total of 3.5 deg 2). Here we describe the instrument, the science goals of the survey, and the survey strategy and data processing. This paper serves as an introduction to a special issue of the Publications of the Astronomical Society of Japan, which includes a large number of technical and scientific papers describing results from the early phases of this survey.

We present the deepest study to date of the Lyα luminosity function in a blank field using blind integral field spectroscopy from MUSE. We constructed a sample of 604 Lyα emitters (LAEs) across the redshift range 2.91 < z < 6.64 using automatic detection software in the Hubble Ultra Deep Field. The deep data cubes allowed us to calculate accurate total Lyα fluxes capturing low surface-brightness extended Lyα emission now known to be a generic property of high-redshift star-forming galaxies. We simulated realistic extended LAEs to fully characterise the selection function of our samples, and performed flux-recovery experiments to test and correct for bias in our determination of total Lyα fluxes. We find that an accurate completeness correction accounting for extended emission reveals a very steep faint-end slope of the luminosity function, α, down to luminosities of log 10 L erg s −1 < 41.5, applying both the 1/V max and maximum likelihood estimators. Splitting the sample into three broad redshift bins, we see the faint-end slope increasing from −2.03 +1.42 −0.07 at z ≈ 3.44 to −2.86 +0.76 −∞ at z ≈ 5.48, however no strong evolution is seen between the 68% confidence regions in L *-α parameter space. Using the Lyα line flux as a proxy for star formation activity, and integrating the observed luminosity functions, we find that LAEs' contribution to the cosmic star formation rate density rises with redshift until it is comparable to that from continuum-selected samples by z ≈ 6. This implies that LAEs may contribute more to the star-formation activity of the early Universe than previously thought, as any additional inter-glactic medium (IGM) correction would act to further boost the Lyα luminosities. Finally, assuming fiducial values for the escape of Lyα and LyC radiation, and the clumpiness of the IGM, we integrated the maximum likelihood luminosity function at 5.00 < z < 6.64 and find we require only a small extrapolation beyond the data (< 1 dex in luminosity) for LAEs alone to maintain an ionised IGM at z ≈ 6.

We present diffuse Lyα halos (LAHs) identified in the composite Subaru narrowband images of 100 − 3600 Lyα emitters (LAEs) at z = 2.2, 3.1, 3.7, 5.7, and 6.6. First, we carefully examine potential artifacts mimicking LAHs that include a large-scale point-spread function (PSF) made by instrumental and atmospheric effects. Based on our critical test with composite images of non-LAE samples whose narrowbandmagnitude and source-size distributions are the same as our LAE samples, we confirm that no artifacts can produce a diffuse extended feature similar to our LAHs. After this test, we measure the scale lengths of exponential profile for the LAHs estimated from our z = 2.2 − 6.6 LAE samples of L Lyα 2 × 10 42 erg s −1 . We obtain the scale lengths of ≃ 5 − 10 kpc at z = 2.2 − 5.7, and find no evolution of scale lengths in this redshift range beyond our measurement uncertainties. Combining this result and the previously-known UV-continuum size evolution, we infer that the ratio of LAH to UVcontinuum sizes is nearly constant at z = 2.2 − 5.7. The scale length of our z = 6.6 LAH is larger than 5 − 10 kpc just beyond the error bar, which is a hint that the scale lengths of LAHs would increase from z = 5.7 to 6.6. If this increase is confirmed by future large surveys with significant improvements of statistical and systematical errors, this scale length change at z 6 would be a signature of increasing fraction of neutral hydrogen scattering Lyα photons, due to cosmic reionization.

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layered imaging survey aimed at addressing some of the most important outstanding questions in astronomy today, including the nature of dark matter and dark energy. The survey has been awarded 300 nights of observing time at the Subaru Telescope, and it started in 2014 March. This paper presents the first public data release of HSC-SSP. This release includes data taken in the first 1.7 yr of observations (61.5 nights), and each of the Wide, Deep, and UltraDeep layers covers about 108, 26, and 4 square degrees down to depths of i ∼ 26.4, ∼26.5, and ∼27.0 mag, respectively (5 σ for point sources). All the layers are observed in five broad bands (grizy), and the Deep and UltraDeep layers are observed in narrow bands as well. We achieve an impressive image quality of 0${^{\prime\prime}_{.}}$6 in the i band in the Wide layer. We show that we achieve 1%–2% point spread function (PSF) photometry (root mean square) both internally and externally (against Pan-STARRS1), and ∼10 mas and 40 mas internal and external astrometric accuracy, respectively. Both the calibrated images and catalogs are made available to the community through dedicated user interfaces and database servers. In addition to the pipeline products, we also provide value-added products such as photometric redshifts and a collection of public spectroscopic redshifts. Detailed descriptions of all the data can be found online. The data release website is https://hsc-release.mtk.nao.ac.jp.

We study the UV luminosity functions (LFs) at z ∼ 4, 5, 6, and 7 based on the deep largearea optical images taken by the Hyper Suprime-Cam (HSC) Subaru strategic program (SSP). On the 100 deg 2 sky of the HSC SSP data available to date, we make enormous samples consisting of a total of 579,565 dropout candidates at z ∼ 4 − 7 by the standard color selection technique, 358 out of which are spectroscopically confirmed by our follow-up spectroscopy and other studies. We obtain UV LFs at z ∼ 4 − 7 that span a very wide UV luminosity range of ∼ 0.002 -100 L * UV (−26 < M UV < −14 mag) by combining LFs from our program and the ultra-deep Hubble Space Telescope legacy surveys. We derive three parameters of the best-fit Schechter function, φ * , M * UV , and α, of the UV LFs in the magnitude range where the AGN contribution is negligible, and find that α and φ * decrease from z ∼ 4 to 7 with no significant evolution of M * UV . Because our HSC SSP data bridge the LFs of galaxies and AGNs with great statistical accuracy, we carefully investigate the bright end of the galaxy UV LFs that are estimated by the subtraction of the AGN contribution either aided with spectroscopy or the best-fit AGN UV LFs. We find that the bright end of the galaxy UV LFs cannot be explained by the Schechter function fits at > 2σ significance, and require either double power-law functions or modified Schechter functions that consider a magnification bias due to gravitational lensing.

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.

We present statistical properties of diffuse Lyα halos (LAHs) around high-z starforming galaxies with large Subaru samples of Lyα emitters (LAEs) at z = 2.2. We make subsamples defined by the physical quantities of LAEs' central Lyα luminosities, UV magnitudes, Lyα equivalent widths, and UV slopes, and investigate LAHs' radial surface brightness (SB) profiles and scale lengths r n as a function of these physical quantities. We find that there exist prominent LAHs around LAEs with faint Lyα luminosities, bright UV luminosities, and small Lyα equivalent widths in cumulative radial Lyα SB profiles. We confirm this trend with the anti-correlation between r n and Lyα luminosities (equivalent widths) based on the Spearman's rank correlation coefficient that is ρ = −0.9 (−0.7) corresponding to the 96% (93%) confidence level, although the correlation between r n and UV magnitudes is not clearly found in the rank correlation coefficient. Our results suggest that LAEs with properties similar to typical Lyman-break galaxies (with faint Lyα luminosities and small equivalent widths) possess more prominent LAHs. We investigate scenarios for the major physical origins of LAHs with our results, and find that the cold stream scenario is not preferred, due to the relatively small equivalent widths up to 77Å in LAHs that include LAEs' central components. There remain two possible scenarios of Lyα scattering in circumgalactic medium and satellite galaxies that cannot be tested with our observational data.

We present the results of structure analyses for a large sample of 426 Lyα emitters (LAEs) at z ∼ 2.2 that are observed with HST/ACS and WFC3-IR by deep extra-galactic legacy surveys. We confirm that the merger fraction and the average ellipticity of LAE's stellar component are 10 − 30% and 0.4 − 0.6, respectively, that are comparable with previous study results. We successfully identify that some LAEs have a spatial offset between Lyα and stellar-continuum emission peaks, δ Lyα , by ∼ 2.5−4 kpc beyond our statistical errors. To uncover the physical origin of strong Lyα emission found in LAEs, we investigate Lyα equivalent width (EW) dependences of these three structural parameters, merger fraction, δ Lyα , and ellipticity of stellar distribution in the range of EW (Lyα) = 20 − 250Å. Contrary to expectations, we find that merger fraction does not significantly increase with Lyα EW. We reveal an anti-correlation between δ Lyα and EW (Lyα) by Kolmogorov-Smirnov (KS) test. There is a trend that the LAEs with a large Lyα EW have a small ellipticity. This is consistent with the recent theoretical claims that Lyα photons can more easily escape from face-on disks having a small ellipticity, due to less inter-stellar gas along the line of sight, although our KS test indicates that this trend is not statistically significant. Our results of Lyα-EW dependence generally support the idea that an H i column density is a key quantity determining Lyα emissivity.