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 report the source size distribution, as measured by ALMA millimetric continuum imaging, of a sample of 13 AzTEC-selected submillimeter galaxies (SMGs) at z 3 phot ~-6. Their infrared luminosities and star formation rates (SFRs) are L IR ~2-6 10 12 ´L and ∼200-600 M  yr −1 , respectively. The sizes of these SMGs range from 0″. 10 to 0″. 38, with a median of 0″. 20 0. 05to that seen in local merger-driven (U)LIRGs rather than in extended disk galaxies at low and high redshifts. The discovery of compact starbursts in z 3  SMGs strongly supports a massive galaxy formation scenario wherein z 3 ~-6 SMGs evolve into the compact stellar components of z 2 ~cQGs. These cQGs are then thought to evolve into the most massive ellipticals in the local universe, mostly via dry mergers. Our results thus suggest that z 3  SMGs are the likely progenitors of massive local ellipticals, via cQGs, meaning that we can now trace the evolutionary path of the most massive galaxies over a period encompassing ∼90% of the age of the universe.

We compare a large sample of galaxies between 0.5 < z < 2.6 with theoretical predictions for how the optical diagnostic line ratios in galaxy ensembles change as a function of cosmic time. We show that star forming galaxies at high redshift (z > 1.5) are consistent with a model in which the ISM conditions are more extreme at high redshift than seen in the global spectra of local galaxies. We speculate that global spectra of our high redshift galaxies may be dominated by H II regions similar to the extreme clumpy, dense star-forming complexes in the Antennae and M82. The transition to local-type conditions occurs between 0.8 < z < 1.5. We conclude that classification schemes developed for local samples should not be applied at high redshift (z ≥ 1.5). We use our theoretical models to derive a new redshift-dependent classification line that utilizes the standard optical diagnostic line ratios [O III]/Hβ and [N II]/Hα. Our new line can be used to separate star-forming galaxies from AGN between z = 0 to z ∼ 3.5. We anticipate that our redshift-dependent optical classification line will be useful for future large surveys with near-infrared multi-object spectrographs. We apply our classification line to a sample of gravitationally lensed galaxies at z ∼ 2.5. Although limited by small numbers, we show that our classification line is consistent with the position of AGN that have been independently confirmed via other methods.

We report two secure (z = 3.775, 4.012) and one tentative (z ≈ 3.767) spectroscopic confirmations of massive and quiescent galaxies close to their quenching epoch through K-band observations with Keck/MOSFIRE and VLT/X-Shooter. The stellar continuum emission, the absence of strong nebular emission lines and the lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of log(M /M ) ∼ 11 and ongoing star formation rates placing these galaxies 1 − 2 dex below the main sequence at their redshifts. The adopted parametrization of the star formation history suggests that these sources experienced a strong ( SFR ∼ 1200 − 3500 M yr −1 ) and short (∼ 50 Myr) burst of star formation, peaking ∼ 150 − 500 Myr before the time of observation, all properties reminiscent of the characteristics of sub-millimeter galaxies (SMGs) at z > 4. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest sub-mm surveys detecting not only the most extreme 2 Valentino et al.starbursts, but also more normal galaxies. We support these findings by further exploring the Illustris-TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at z ∼ 3 − 4 and SMGs at z ∼ 4 − 5, with number densities and properties in broad agreement with the observations at z ∼ 3, but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past and, similarly, not all bright SMGs quench by z ∼ 3, both fractions depending on the threshold assumed to define the SMGs themselves. This cautions against the blind application of the assumption of a univocal connection between the two populations at high redshift.

We present the results of Spectral Energy Distribution (SED) fitting analysis for Lyman Break Galaxies (LBGs) at z ∼ 5 in the GOODS-N and its flanking fields (the GOODS-FF). With the publicly available IRAC images in the GOODS-N and IRAC data in the GOODS-FF, we constructed the rest-frame UV to optical SEDs for a large sample (∼ 100) of UV-selected galaxies at z ∼ 5. Comparing the observed SEDs with model SEDs generated with a population synthesis code, we derived a best-fit set of parameters (stellar mass, age, color excess, and star formation rate) for each of sample LBGs. The derived stellar masses range from 10 8 to 10 11 M ⊙ with a median value of 4.1 × 10 9 M ⊙ . The comparison with z = 2 − 3 LBGs shows that the stellar masses of z ∼ 5 LBGs are systematically smaller by a factor of 3 − 4 than those of z = 2 − 3 LBGs in a similar rest-frame UV luminosity range. The star formation ages are relatively younger than those of the z = 2 − 3 LBGs. We also compared the results for our sample with other studies for the z = 5 − 6 galaxies. Although there seem to be similarities and differences in the properties, we could not conclude its significance. We also derived a stellar mass function of our sample by correcting for incompletenesses. Although the number densities in the massive end are comparable to the theoretical predictions from semi-analytic models involving AGN feedback, the number densities in the low-mass part are smaller than the model predictions. By integrating the stellar mass function down to 10 8 M ⊙ , the stellar mass density at z ∼ 5 is calculated to be (0.7 − 2.4) × 10 7 M ⊙ Mpc −3 . The stellar mass density at z ∼ 5 is dominated by massive part of the stellar mass function. Compared with other observational studies and the model predictions, the mass density of our sample is consistent with general trend of the increase of the stellar mass density with time.

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 the faint end of number counts at 1.3 mm (238 GHz) obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Band 6 observations were carried out targeting 20 starforming galaxies at z ∼ 1.4 in the Subaru/XMM-Newton Deep Survey field. In the observations, we serendipitously detect 15 sources (≥3.8σ, S 1.3 mm = 0.15-0.61 mJy) other than the targeted sources. We create number counts by using these 'sub-mJy sources', which probe the faintest flux range among surveys at millimeter wavelengths. The number counts are consistent with (flux-scaled) number counts at 850 µm and 870 µm obtained with gravitational lensing clusters. The ALMA number counts agree well with model predictions, which suggest that these sub-mJy populations are more like 'normal' star-forming galaxies than 'classical' SMGs with intense star-forming activity. In this flux range, ∼80% of the extragalactic background light at 1.3 mm is resolved into individual sources.

We measure the redshift-space correlation function from a spectroscopic sample of 2783 emission line galaxies from the FastSound survey. The survey, which uses the Subaru Telescope and covers the redshift ranges of 1.19 < z < 1.55, is the first cosmological study at such high redshifts. We detect clear anisotropy due to redshift-space distortions (RSD) both in the correlation function as a function of separations parallel and perpendicular to the line of sight and its quadrupole moment. RSD has been extensively used to test general relativity on cosmological scales at z < 1. Adopting a ΛCDM cosmology with the fixed expansion history and no velocity dispersion σ v = 0, and using the RSD measurements on scales above 8 h −1 Mpc, we obtain the first constraint on the growth rate at the redshift, f (z)σ 8 (z) = 0.482 ± 0.116 at z ∼ 1.4 after marginalizing over the galaxy bias parameter b(z)σ 8 (z). This corresponds to 4.2σ detection of RSD. Our constraint is consistent with the prediction of general relativity f σ 8 ∼ 0.392 within the 1 − σ confidence level. When we allow σ v to vary and marginalize it over, the growth rate constraint becomes f σ 8 = 0.494−0.120 . We also demonstrate that by combining with the lowz constraints on f σ 8 , high-z galaxy surveys like the FastSound can be useful to distinguish modified gravity models without relying on CMB anisotropy experiments.