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.