arXiv subjects

Nobuo Arimoto

Publications and source records attributed to Nobuo Arimoto.

At least 19 recordsLinked to original sources

arXiv2025

The Extended Stellar Distribution in the Outskirts of the Ursa Minor Dwarf Spheroidal Galaxy

We discover an extended distribution of main-sequence (MS) stars along the minor axis of the Ursa Minor (UMi) dwarf spheroidal galaxy (dSph). This study is enabled by deep, wide Subaru/Hyper Suprime-Cam data, reaching photometric uncertainties below 0.1 mag at $g,i \sim 26$ mag. Color-magnitude diagrams along the major and minor axes reveal a clear excess of MS stars beyond the nominal tidal radius along the minor axis. To characterize this structure, we derive radial number density profiles in seven azimuthal directions and fit them with an exponential+power-law function to assess the symmetry of the extended component. The power-law slopes tend to be shallower toward the minor axis, though the symmetry remains inconclusive within 1$σ$ uncertainties. This may indicate that the extended component is preferentially distributed along the minor axis, and could be different from the previously suggested tidal features along the major axis. Comparing with simulations, we find that the fraction of stars beyond five effective radii is consistent with expectations from an intermediate mass ratio merger scenario with a stellar mass ratio around 6:1. While these findings provide new insights into the structural complexity and dynamical history of the UMi dSph, alternative mechanisms such as stellar or supernova feedback have also been proposed for extended stellar halos in dwarfs and cannot be ruled out.

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arXiv2025

The Star Formation and Chemical Evolution Histories of Ursa Minor Dwarf Spheroidal Galaxy

We derive the star formation history (SFH) and chemical evolution history (CEH) of the Ursa Minor (UMi) dwarf spheroidal galaxy (dSph). We detect two distinct stellar populations that exist over 6 times half-light radius from its center. The results are obtained by applying a newly developed algorithm to the deep and wide-field photometric dataset taken with Hyper Suprime-Cam on the Subaru Telescope. The algorithm employs the genetic algorithm and the simulated annealing to minimize a $χ^{2}$ value between the observed color-magnitude diagram (CMD) and synthetic CMD generated from the stellar isochrones. The age and metallicity resolutions are set to $0.5$ Gyr and $0.1$ dex, respectively. The accuracy assessment with mock galaxies shows that it returns the peaks of metallicity distributions and star formation period within 1 $σ$ of input value in the case of a single population. In tests with two populations, two distinct metallicity peaks are identified without an offset from the input values, indicating the robustness of this algorithm. The detected two populations in the UMi dSph have the metallicity peaks of [Fe/H] = $-2.2$ and $-2.5$; the metal-rich population started its star formation about 1 Gyr later than the metal-poor one. The SFH of both metal-rich and metal-poor populations varies with distance from the center of the UMi dSph, without any age-gradients. These results suggest that the UMi dSph underwent a complex formation process, contrary to the simple formation history of dwarf galaxies previously thought.

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arXiv2024

Final results for mapping the Milky Way's stellar halo with blue horizontal-branch stars selected from the Subaru Hyper Supreme-Cam Survey

We select blue-horizontal branch stars (BHBs) from the internal data release of the Hyper Suprime-Cam Subaru Strategic Program to reveal the global structure of the Milky Way (MW) stellar halo. The data are distributed over $\sim 1,100$~deg$^2$ area in the range of $18.5 r_{\rm b}$; the statistical difference between these fitting profiles is small. Both profiles are found to show prolate shapes having axial ratios of $q=1.47^{+0.30}_{-0.33}$ and $1.56^{+0.34}_{-0.23}$, respectively. We also find a signature of the so-called "splashback radius" for the candidate BHBs, which can reach as large as $r \sim 575$~kpc, although it is still inconclusive owing to rather large distance errors in this faintest end of the sample. Our results suggest that the MW stellar halo consists of the two overlapping components: the {\it in situ} inner halo showing a relatively steep radial density profile and the {\it ex situ} outer halo with a shallower profile, being characteristic of a component formed from accretion of small stellar systems.

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arXiv2024

Final Results of Search for New Milky Way Satellites in the Hyper Suprime-Cam Subaru Strategic Program Survey: Discovery of Two More Candidates

We present the final results of our search for new Milky Way (MW) satellites using the data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) survey over $\sim 1,140$ deg$^2$. In addition to three candidates that we already reported, we have identified two new MW satellite candidates in the constellation of Sextans at a heliocentric distance of $D_{\odot} \simeq 126$kpc, and Virgo at $D_{\odot} \simeq 151$kpc, named Sextans II and Virgo III, respectively. Their luminosities (Sext II:$M_V\simeq-3.9$mag; Vir III:$M_V\simeq-2.7$mag) and half-light radii (Sext II:$r_h\simeq154$ pc; Vir III:$r_h\simeq 44$ pc) place them in the region of size-luminosity space of ultra-faint dwarf galaxies (UFDs). Including four previously known satellites, there are a total of nine satellites in the HSC-SSP footprint. This discovery rate of UFDs is much higher than that predicted from the recent models for the expected population of MW satellites in the framework of cold dark matter models, thereby suggesting that we encounter a too many satellites problem. Possible solutions to settle this tension are also discussed.

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arXiv2024

Uncovering the Ghostly Remains of an Extremely Diffuse Satellite in the Remote Halo of NGC 253

We present the discovery of NGC253-SNFC-dw1, a new satellite galaxy in the remote stellar halo of the Sculptor Group spiral, NGC 253. The system was revealed using deep resolved star photometry obtained as part of the Subaru Near-Field Cosmology Survey that uses the Hyper Suprime-Cam on the Subaru Telescope. Although rather luminous ($\rm{M_{V}} = -11.7 \pm 0.2$) and massive ($M_* \sim 1.25\times 10^7~\rm{M}_{\odot}$), the system is one of the most diffuse satellites yet known, with a half-light radius of $\rm{R_{h}} = 3.37 \pm 0.36$ kpc and an average surface brightness of $\sim 30.1$ mag arcmin$^{-2}$ within the $\rm{R_{h}}$. The colour-magnitude diagram shows a dominant old ($\sim 10$ Gyr) and metal-poor ($\rm{[M/H]}=-1.5 \pm 0.1$ dex) stellar population, as well as several candidate thermally-pulsing asymptotic giant branch stars. The distribution of red giant branch stars is asymmetrical and displays two elongated tidal extensions pointing towards NGC 253, suggestive of a highly disrupted system being observed at apocenter. NGC253-SNFC-dw1 has a size comparable to that of the puzzling Local Group dwarfs Andromeda XIX and Antlia 2 but is two magnitudes brighter. While unambiguous evidence of tidal disruption in these systems has not yet been demonstrated, the morphology of NGC253-SNFC-dw1 clearly shows that this is a natural path to produce such diffuse and extended galaxies. The surprising discovery of this system in a previously well-searched region of the sky emphasizes the importance of surface brightness limiting depth in satellite searches.

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arXiv2023

The Progenitor of the Peculiar Galaxy NGC3077

We present a study of the structural properties and metallicity distribution of the nearby peculiar galaxy NGC3077. Using data from our survey of the M81 Group with the Hyper Suprime-Cam on the Subaru Telescope, we construct deep color-magnitude diagrams that are used to probe the old red giant branch population of NGC3077. We map these stars out to and beyond the nominal tidal radius, which allows us to derive the structural properties and stellar content of the peripheral regions. We show that NGC3077 has an extended stellar halo and pronounced ``S-shaped" tidal tails that diverge from the radial profile of the inner region. The average metallicity of the old population in NGC3077 is estimated from individual RGBs to be $\rm{[M/H]}=-0.98 \pm 0.26$, which decreases with the distance from the galaxy center as $\rm{[M/H]}=-0.17$ dex $\rm{R_{h}}^{-1}$. The metallicity of the S-shaped structure is similar to that of the regions lying at $r\sim4\times\rm{R_{h}} (\sim 30$~kpc), indicating that the stellar constituents of the tidal tails have come from the outer envelope of NGC3077. These results suggest that this peculiar galaxy was probably a rather normal dwarf elliptical galaxy before the tidal interaction with M81 and M82. We also examine the evidence in our dataset for the six recently-reported ultra-faint dwarf candidates around NGC3077. We recover a spatial overdensity of sources coinciding with only one of these.

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arXiv2022

A Tale of a Tail: A Tidally-Disrupting Ultra-Diffuse Galaxy in the M81 Group

We present the discovery of a giant tidal tail of stars associated with F8D1, the closest known example of an ultra-diffuse galaxy (UDG). F8D1 sits in a region of the sky heavily contaminated by Galactic cirrus and has been poorly studied since its discovery two decades ago. The tidal feature was revealed in a deep map of resolved red giant branch stars constructed using data from our Subaru Hyper Suprime-Cam survey of the M81 Group. It has an average surface brightness of $μ_g \sim 32$ mag arcsec$^{-2}$ and can be traced for over a degree on the sky (60 kpc at the distance of F8D1) with our current imagery. We revisit the main body properties of F8D1 using deep multiband imagery acquired with MegaCam on CFHT and measure effective radii of $1.7-1.9$ kpc, central surface brightnesses of $24.7-25.7$ mag and a stellar mass of $\sim7 \times 10^7 M_{\odot}$. Assuming a symmetric feature on the other side of the galaxy, we calculate that $30-36$% of F8D1's present-day luminosity is contained in the tail. We argue that the most likely origin of F8D1's disruption is a recent close passage to M81, which would have stripped its gas and quenched its star formation. As the only UDG that has so far been studied to such faint surface brightness depths, the unveiling of F8D1's tidal disruption is important. It leaves open the possibility that many other UDGs could be the result of similar processes, with the most telling signatures of this lurking below current detection limits.

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arXiv2020

Narrowband Ca Photometry for Dwarf Spheroidal Galaxies. I. Chemostructural Study on Draco, Sextans, and Canes Venatici I

A few dozen dwarf satellite galaxies of the Milky Way have been discovered, which are often viewed as the remaining building blocks of our Galaxy. The follow-up spectroscopy showed that dwarf galaxies have a sizeable spread in their metallicities. Several scenarios were suggested to explain the metallicity spread, which can be tested by the structural patterns of stellar subpopulations with distinct metallicities. However, such chemical plus structural examination, to which we refer to as "chemostructural study" is hindered by the lack of stars with spectroscopic metallicity. Here we propose the Ca$-$$by$ photometry as an alternative way to secure metallicities for a 2$-$3 orders of magnitude larger stellar sample than the spectroscopic sample and thus enable us to perform a chemostructural study on dwarf galaxies. In particular, we use the $hk$ index [$\equiv($Ca$-b)-(b-y)$], whose validity as a photometric metallicity indicator (and crass insensitivity to age) for red-giant-branch stars was upheld via Galactic globular clusters, and observe three dwarf spheroidal galaxies$-$Draco, Sextans, and Canes Venatici I (CVnI)$-$with Subaru/Suprime-Cam. We find that in all the galaxies the metal-rich stellar populations are more centrally concentrated than the metal-poor counterparts, suggesting that the central regions of the galaxies underwent extended star formation. Such a negative radial metallicity gradient for Sextans and CVnI opposes to the traditional spectroscopic results. We also find that their metallicity distribution functions (MDFs) can be characterized by a unimodal, skewed Gaussian shape with a metal-rich peak and a metal-poor tail. We discuss their features in the chemostructure and MDF in terms of dwarf galaxy formation theories.

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arXiv2020

Globular Clusters in Coma Cluster Ultra Diffuse Galaxies (UDGs): Evidence for Two Types of UDG?

Ultra diffuse galaxies (UDGs) reveal extreme properties. Here we compile the largest study to date of 85 globular cluster (GC) systems around UDGs in the Coma cluster, using new deep ground-based imaging of the known UDGs and existing imaging from the Hubble Space Telescope of their GC systems. We find that the richness of GC systems in UDGs generally exceeds that found in normal dwarf galaxies of the same stellar mass. These GC-rich UDGs imply halos more massive than expected from the standard stellar mass-halo mass relation. The presence of such overly massive halos presents a significant challenge to the latest simulations of UDGs in cluster environments. In some exceptional cases, the mass in the GC system is a significant fraction of the stellar content of the host galaxy. We find that rich GC systems tend to be hosted in UDGs of lower luminosity, smaller size and fainter surface brightness. Similar trends are seen for normal dwarf galaxies in the Coma cluster. A toy model is presented in which the GC-rich UDGs are assumed to be `failed' galaxies within massive halos that have largely old, metal-poor, alpha-element enhanced stellar populations. On the other hand, GC-poor UDGs are more akin to normal, low surface brightness dwarfs that occupy less massive dark matter halos. Additional data on the stellar populations of UDGs with GC systems will help to further refine and test this simplistic model.

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arXiv2019

Stellar population and structural properties of dwarf galaxies and young stellar systems in the M81 group

We use Hyper Suprime-Cam on the Subaru Telescope to investigate the structural and photometric properties of early-type dwarf galaxies and young stellar systems at the center of the M81 Group. We have mapped resolved stars to $\sim2$ magnitudes below the tip of the red giant branch over almost 6.5 square degrees, corresponding to a projected area of $160\times160 \rm{kpc}$ at the distance of M81. The resulting stellar catalogue enables a homogeneous analysis of the member galaxies with unprecedented sensitivity to low surface brightness emission. The radial profiles of the dwarf galaxies are well-described by Sersic and King profiles, and show no obvious signatures of tidal disruption. The measured radii for most of these systems are larger than the existing literature values and we find the total luminosity of IKN ($\rm{M_{V,0}}=-14.29$) to be almost 3 magnitudes brighter than previously-thought. We identify new dwarf satellite candidates, d1006+69 and d1009+68, which we estimate to lie at a distance of $4.3\pm0.2$ Mpc and $3.5\pm0.5$ Mpc. With $\rm{M_{V,0}}=-8.91\pm0.40$ and $\rm{[M/H]}=-1.83\pm0.28$, d1006+69 is one of the faintest and most metal-poor dwarf satellites currently-known in the M81 Group. The luminosity functions of young stellar systems in the outlying tidal HI debris imply continuous star formation in the recent past and the existence of populations as young as 30 Myr old. We find no evidence for old RGB stars coincident with the young MS/cHeB stars which define these objects, supporting the idea that they are genuinely new stellar systems resulting from triggered star formation in gaseous tidal debris.

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arXiv2019

Boötes IV: A New Milky Way Satellite Discovered in the Subaru Hyper Suprime-Cam Survey and Implications for the Missing Satellite Problem

We report on the discovery of a new Milky Way (MW) satellite in Boötes based on data from the on-going Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This satellite, named Boötes IV, is the third ultra-faint dwarf that we have discovered in the HSC-SSP. We have identified a statistically significant (32.3$σ$) overdensity of stars having characteristics of a metal-poor, old stellar population. The distance to this stellar system is $D_{\odot}=209^{+20}_{-18}$ kpc with a $V$-band absolute magnitude of $M_V=-4.53^{+0.23}_{-0.21}$ mag. Boötes IV has a half-light radius of $r_h=462^{+98}_{-84}$ pc and an ellipticity of $0.64^{+0.05}_{-0.05}$, which clearly suggests that this is a dwarf satellite galaxy. We also found another overdensity that appears to be a faint globular cluster with $M_V=-0.20^{+0.59}_{-0.83}$ mag and $r_h=5.9^{+1.5}_{-1.3}$ pc located at $D_{\odot}=46^{+4}_{-4}$ kpc. Adopting the recent prediction for the total population of satellites in a MW-sized halo by Newton et al. (2018), which combined the characteristics of the observed satellites by SDSS and DES with the subhalos obtained in $Λ$CDM models, we estimate that there should be about two MW satellites at $M_V\le0$ in the $\sim676$ deg$^2$ covered by HSC-SSP, whereas that area includes six satellites. Thus, the observed number of satellites is larger than the theoretical prediction. On the face of it, we have a problem of too many satellites, instead of the well-known missing satellites problem whereby the $Λ$CDM theory overpredicts the number of satellites in a MW-sized halo. This may imply that the models need more refinements for the assignment of subhalos to satellites such as considering those found by the current deeper survey. [abridged]

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arXiv2019

The stellar halo of the Milky Way traced by blue horizontal-branch stars in the Subaru Hyper Suprime-Cam Survey

We report on the global structure of the Milky Way (MW) stellar halo up to its outer boundary based on the analysis of blue-horizontal branch stars (BHBs). These halo tracers are extracted from the $(g,r,i,z)$ band multi-photometry in the internal data release of the on-going Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) surveyed over $\sim550$~deg$^2$ area. In order to select most likely BHBs by removing blue straggler stars (BSs) and other contamination in a statistically significant manner, we have developed and applied an extensive Bayesian method, instead of the simple color cuts adopted in our previous work, where each of the template BHBs and non-BHBs obtained from the available catalogs is represented as a mixture of multiple Gaussian distributions in the color-color diagrams. We found from the candidate BHBs in the range of 18.5 r_{\rm b}$. The latter profile with a prolate shape having an axial ratio of $q=1.72^{+0.44}_{-0.28}$ is most likely and this halo may hold a rather sharp boundary at r=160kpc. The slopes of the halo density profiles are compared with those from the suite of hydrodynamical simulations for the formation of stellar halos. This comparison suggests that the MW stellar halo may consist of the two overlapping components: the in situ. inner halo as probed by RR Lyrae stars showing a relatively steep radial density profile and the ex situ. outer halo with a shallow profile probed by BHBs here, which is made by accretion of small stellar systems.

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arXiv2019

Deciphering an evolutionary sequence of merger stages in infrared-luminous starburst galaxies at z ~ 0.7

Based on optical/near-IR Magellan FIRE spectra of 25 starburst galaxies at 0.5 < z < 0.9, Calabrò et al.(2018) showed that their attenuation properties can be explained by a single-parameter sequence of total obscurations ranging from A(V)=2 to A(V)=30 towards the starburst core centers in a mixed stars and dust configuration. We investigate here the origin of this sequence for the same sample. We show that total attenuations anti-correlate with the starburst sizes in radio (3 GHz) with a significance larger than 5sigma and a scatter of 0.26 dex. More obscured and compact starbursts also show enhanced N2 (=[NII]/Halpha) ratios and larger line velocity widths that we attribute to an increasing shock contribution toward later merger phases, driven by deeper gravitational potential wells at the coalescence. Additionally, the attenuation is also linked to the equivalent width (EW) of hydrogen recombination lines, which is sensitive to the luminosity weighted age of the relatively unobscured stellar populations. Overall, the correlations among A(V), radio size, line width, N2 and EW of Balmer/Paschen lines converge towards suggesting an evolutionary sequence of merger stages: all of these quantities are likely to be good time-tracers of the merger phenomenon, and their large spanned range appears to be characteristic of the different merger phases. Half of our sample at higher obscurations have radio sizes approximately 3 times smaller than early type galaxies at the same redshift, suggesting that, in analogy with local Ultraluminous Infrared galaxies (ULIRGs), these cores cannot be directly forming elliptical galaxies. Finally, we detect mid-IR AGN torus for half of our sample and additional X-ray emission for 6 starbursts; intriguingly, the latter have systematically more compact sizes, suggestive of emerging AGNs towards later merger stages, possibly precursors of a later QSO phase.

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arXiv2018

The FMOS-COSMOS survey of star-forming galaxies at $z\sim1.6$ VI: Redshift and emission-line catalog and basic properties of star-forming galaxies

We present a new data release from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS survey, which contains the measurements of spectroscopic redshift and flux of rest-frame optical emission lines (H$α$, [NII], [SII], H$β$, [OIII]) for 1931 galaxies out of a total of 5484 objects observed over the 1.7 deg$^2$ COSMOS field. We obtained $H$-band and $J$-band medium-resolution ($R\sim3000$) spectra with FMOS mounted on the Subaru telescope, which offers an in-fiber line flux sensitivity limit of $\sim 1 \times 10^{-17}~\mathrm{erg~s^{-1}~cm^{-2}}$ for an on-source exposure time of five hours. The full sample contains the main population of star-forming galaxies at $z\sim1.6$ over the stellar mass range $10^{9.5}\lesssim M_\ast/M_\odot \lesssim 10^{11.5}$, as well as other subsamples of infrared-luminous galaxies detected by Spitzer and Herschel at the same and lower ($z\sim0.9$) redshifts and X-ray emitting galaxies detected by Chandra. This paper presents an overview of our spectral analyses, a description of the sample characteristics, and a summary of the basic properties of emission-line galaxies. We use the larger sample to re-define the stellar mass--star formation rate relation based on the dust-corrected H$α$ luminosity, and find that the individual galaxies are better fit with a parametrization including a bending feature at $M_\ast\approx10^{10.2}~M_\odot$, and that the intrinsic scatter increases with $M_\ast$ from 0.19 to $0.37$ dex. We also confirm with higher confidence that the massive ($M_\ast\gtrsim10^{10.5}~M_\odot$) galaxies are chemically mature as much as local galaxies with the same stellar masses, and that the massive galaxies have lower [SII]/H$α$ ratios for their [OIII]/H$β$, as compared to local galaxies, which is indicative of enhancement in ionization parameter.

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arXiv2018

Near-infrared emission lines in starburst galaxies at 0.5 < z < 0.9 : Discovery of a merger sequence of extreme obscurations

We obtained optical/near-IR rest-frame Magellan FIRE spectra (including Pa$β$ and Pa$γ$) of 25 starburst galaxies at 0.5<z<0.9, with average star formation rates (SFR) x7 above the Main Sequence (MS). We find that Paschen-to-Balmer line ratios saturate around a constant value corresponding to $A_{\rm V}\sim$2-3 mag, while line to IR luminosity ratios suggest a large range of more extreme obscurations and appear to be uncorrelated to the former. This behavior is not consistent with standard attenuation laws derived for local and distant galaxies, while being remarkably consistent with observations of starburst cores in which young stars and dust are homogeneously mixed. This model implies $A_{\rm V}=$2-30 mag attenuation to the center of starburst cores, with a median of ~9 mag (a factor of 4000). X-ray hardness ratios for 6 AGNs in our sample and column densities derived from observed dust masses and radio sizes independently confirm this level of attenuation. In these conditions observed optical/near-IR emission comes from surface regions, while inner starburst cores are invisible. We thus attribute the high [NII]/H$α$ ratios to widespread shocks from accretion, turbulence and dynamic disturbances rather than to AGNs. The large range of optical depths demonstrates that substantial diversity is present within the starburst population, possibly connected to different merger phases or progenitor properties. The majority of our targets are, in fact, morphologically classified as mergers. We argue that the extreme obscuration provides in itself smoking gun evidence of their merger origin, and a powerful tool for identifying mergers at even higher redshifts.

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arXiv2018

Structure of the Milky Way stellar halo out to its outer boundary with blue horizontal-branch stars

We present the structure of the Milky Way stellar halo beyond Galactocentric distances of $r = 50$ kpc traced by blue horizontal-branch (BHB) stars, which are extracted from the survey data in the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We select BHB candidates based on $(g,r,i,z)$ photometry, where the $z$-band is on the Paschen series and the colors that involve the $z$-band are sensitive to surface gravity. About 450 BHB candidates are identified between $r = 50$ kpc and 300 kpc, most of which are beyond the reach of previous large surveys including the Sloan Digital Sky Survey. We find that the global structure of the stellar halo in this range has substructures, which are especially remarkable in the GAMA15H and XMM-LSS fields in the HSC-SSP. We find that the stellar halo can be fitted to a single power-law density profile with an index of $α\simeq 3.3$ ($3.5$) with (without) these fields and its global axial ratio is $q \simeq 2.2$ ($1.3$). Thus, the stellar halo may be significantly disturbed and be made in a prolate form by halo substructures, perhaps associated with the Sagittarius stream in its extension beyond $r \sim 100$ kpc. For a broken power-law model allowing different power-law indices inside/outside a break radius, we obtain a steep power-law slope of $α\sim 5$ outside a break radius of $\simeq 100$ kpc ($200$ kpc) for the case with (without) GAMA15H and XMM-LSS. This radius of $200$ kpc might be as close as a halo boundary if there is any, although larger BHB sample is required from further HSC-SSP survey to increase its statistical significance.

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arXiv2017

First Data Release of the Hyper Suprime-Cam Subaru Strategic Program

The Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) is a three-layered imaging survey aimed at addressing some of the most 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 March 2014. This paper presents the first public data release of HSC-SSP. This release includes data taken in the first 1.7 years 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 (5sigma 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 0.6 arcsec in the i-band in the Wide layer. We show that we achieve 1-2 per cent PSF photometry (rms) 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/.

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arXiv2017

Searches for New Milky Way Satellites from the First Two Years of Data of the Subaru/Hyper Suprime-Cam Survey: Discovery of Cetus~III

We present the results from a search for new Milky Way (MW) satellites from the first two years of data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) $\sim 300$~deg$^2$ and report the discovery of a highly compelling ultra-faint dwarf galaxy candidate in Cetus. This is the second ultra-faint dwarf we have discovered after Virgo~I reported in our previous paper. This satellite, Cetus~III, has been identified as a statistically significant (10.7$σ$) spatial overdensity of star-like objects, which are selected from a relevant isochrone filter designed for a metal-poor and old stellar population. This stellar system is located at a heliocentric distance of 251$^{+24}_{-11}$~kpc with a most likely absolute magnitude of $M_V = -2.4 \pm 0.6$~mag estimated from a Monte Carlo analysis. Cetus~III is extended with a half-light radius of $r_h = 90^{+42}_{-17}$~pc, suggesting that this is a faint dwarf satellite in the MW located beyond the detection limit of the Sloan Digital Sky Survey. Further spectroscopic studies are needed to assess the nature of this stellar system. We also revisit and update the parameters for Virgo~I finding $M_V = -0.33^{+0.75}_{-0.87}$~mag and $r_h = 47^{+19}_{-13}$~pc. Using simulations of $Λ$-dominated cold dark matter models, we predict that we should find one or two new MW satellites from $\sim 300$~deg$^2$ HSC-SSP data, in rough agreement with the discovery rate so far. The further survey and completion of HSC-SSP over $\sim 1,400$~deg$^2$ will provide robust insights into the missing satellites problem.

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