Extensive air showers induced from high-energy cosmic rays provide a window into understanding the most energetic phenomena in the universe. We present a new method for observing these showers using the silicon imaging detector Subaru Hyper Suprime-Cam (HSC). This method has the advantage of being able to measure individual secondary particles. When paired with a surface detector array, silicon imaging detectors like Subaru HSC will be useful for studying the properties of extensive air showers in detail. The following report outlines the first results of observing extensive air showers with Subaru HSC. The potential for reconstructing the incident direction of primary cosmic rays is demonstrated and possible interdisciplinary applications are discussed.

We obtained mid-infrared 3.6 and 4.5μm imaging of a z = 6.96 Lyα emitter (LAE), IOK-1, discovered in the Subaru Deep Field, using Spitzer Space Telescope Infrared Array Camera observations. After removal of a nearby bright source, we found that IOK-1 is not significantly detected in any of these infrared bands to m3:6μm ∼ 24.00 and m4:5μm ∼ 23.54 at 3φ. Fitting population synthesis models to the spectral energy distribution consisting of the upper limit fluxes of the optical-to-infrared non-detection images and fluxes in detection images, we constrained the stellar mass, M*, of IOK-1. This LAE could have either a mass as low as M* . ≤2-9 × 108 M for young age (≤10 Myr) and low dust reddening (AV ∼ 0) or a mass as large as M* . 1-4 × 1010 M for either old age (>100 Myr) or high dust reddening (AV ∼ 1.5). This would be within the range of masses of z ∼ 3-6.6 LAEs studied to date, ∼106-10 10M. Hence, IOK-1 is not a particularly unique galaxy with extremely high mass or low mass, but is similar to one of the LAEs seen at later epochs. © 2010. Astronomical Society of Japan.

SDF line-emitting galaxies in four narrowband filters at low and intermediate redshifts are presented. Broadband colors, follow-up optical spectroscopy, and multiple NB filters are used to distinguish Hα, [O II], and [O III] emitters at z = 0.07-1.47 to construct their LFs. These LFs are derived down to faint magnitudes, allowing for an accurate determination of the faint-end slope. With a large (N ∼ 200-900) sample for each redshift interval, a Schechter profile is fitted to each LF. Prior to dust extinction corrections, the [O III] and [O II] LFs agree reasonably well with those of Hippelein et al. The z = 0.08 Ha LF, which reaches 2 orders of magnitude fainter than Gallego et al., is steeper by 25%. This indicates that there are more low-luminosity star-forming galaxies for z < 0.1. The faint-end slope α and Φ*, show a strong redshift evolution, while L * shows little evolution. The evolution in α indicates that low-luminosity galaxies have a stronger evolution compared to brighter ones. Integrated SFR densities are derived via Hα, [O III], and [O III] for 0.07 < z < 1.47. A steep increase in the SFR density, as a function of redshift, is seen for 0.4 < z < 0.9. For z > 1, the SFR densities are similar. The latter is consistent with previous UVand [O II] measurements. Below z < 0.4, the SFR densities are consistent with several Ha, [O II], and UV measurements, but others are a factor of 2 higher. For example, the z = 0.066-0.092 LF agrees with Jones & Bland-Hawthorn, but at z = 0.24 and 0.40, their number densities are twice as high. This discrepancy can be explained by cosmic variance. © 2007. The American Astronomical Society. All rights reserved.

We report the discovery of a protocluster at z∼6 containing at least eight cluster member galaxies with spectroscopic confirmations in the wide-field image of the Subaru Deep Field (SDF). The overdensity of the protocluster is significant at the 6σ level, based on the surface number density of i′-dropout galaxies. The overdense region covers 6′ × 6′ (14 Mpc × 14 Mpc in comoving units at z = 6) and includes 30 i′-dropout galaxies. Follow-up spectroscopy revealed that 15 of these are real z 6 galaxies (5.7 < z < 6.3). Of these 15, 8 are clustering in a narrow redshift range (Δz < 0.05 centered at z = 6.01), corresponding to a seven-fold increase in number density over the average in redshift space. We found no significant difference in the observed properties, such as Lyα luminosities and UV continuum magnitudes, between the eight protocluster members and the seven non-members. The velocity dispersion of the eight protocluster members is 647 ± 124 km s-1, which is about three times higher than that predicted by the standard cold dark matter model. This discrepancy could be attributed to the distinguishing three-dimensional distribution of the eight protocluster members. We discuss two possible explanations for this discrepancy: either the protocluster is already mature, with old galaxies at the center, or it is still immature and composed of three subgroups merging to become a larger cluster. In either case, this concentration of z = 6.01 galaxies in the SDF may be one of the first sites of formation of a galaxy cluster in the universe. © © 2012. The American Astronomical Society. All rights reserved.

We present observations of SDF-05M05, an unusual optical transient discovered in the Subaru Deep Field (SDF). The duration of the transient is > 800 days in the observer frame, and the maximum brightness during observation reached approximately 23mag in the i′ and z′ bands. The faint host galaxy is clearly identified in all five optical bands of the deep SDF images. The photometric redshift of the host yields z 0.6 and the corresponding absolute magnitude at maximum is - 20. This implies that this event shone with an absolute magnitude brighter than -19mag for approximately 300days in the rest frame, which is significantly longer than a typical supernova and ultraluminous supernova. The total radiated energy during our observation was 1 × 1051 erg. The light curves and color evolution are marginally consistent with some luminous IIn supernovae. We suggest that the transient may be a unique and peculiar supernova at intermediate redshift. © 2012. The American Astronomical Society. All rights reserved..

We present our survey for optically faint variable objects using multiepoch (8-10 epochs over 2-4 years) i′-band imaging data obtained with Subaru Suprime-Cam over 0.918 deg2 in the Subaru/XMM-Newton Deep Field (SXDF). We found 1040 optically variable objects by image subtraction for all the combinations of images at different epochs. This is the first statistical sample of variable objects at depths achieved with 8-10 m class telescopes or the Hubble Space Telescope. The detection limit for variable components is i′vari ∼ 25.5 mag. These variable objects were classified into variable stars, supernovae (SNe), and active galactic nuclei (AGNs), based on the optical morphologies, magnitudes, colors, and optical-mid-infrared colors of the host objects, spatial offsets of variable components from the host objects, and light curves. Detection completeness was examined by simulating light curves for periodic and irregular variability. We detected optical variability for 36% ± 2% (51% ± 3% for a bright sample with i′ < 24.4 mag) of X-ray sources in the field. Number densities of variable objects as functions of time intervals Δt and variable component magnitudes i′vari are obtained. Number densities of variable stars, SNe, and AGNs are 120, 489, and 579 objects deg-2, respectively. Bimodal distributions of variable stars in the color-magnitude diagrams indicate that the variable star sample consists of bright (V ∼ 22 mag) blue variable stars of the halo population and faint (V ∼ 23.5 mag) red variable stars of the disk population. There are a few candidates of RR Lyrae providing a possible number density of ∼10-2 kpc-3 at a distance of >150 kpc from the Galactic center. © 2008. The American Astronomical Society. All rights reserved.

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. © 2006. The American Astronomical Society. All rights reserved.

Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera-electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray-emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.