Search

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 ∼1140 deg2. In addition to three candidates that we have already reported, we have identified two new MW satellite candidates in the constellations of Sextans, at a heliocentric distance of D⊙≃ 126 kpc, and Virgo, at D⊙≃ 151 kpc, named Sextans II and Virgo III, respectively. Their luminosities (Sext II: MV≃ −3.9 mag; Vir III: MV≃ −2.7 mag) and half-light radii (Sext II: rh≃ 154 pc; Vir III: rh≃ 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.

We investigate a mechanism to form and keep a planar spatial distribution of satellite galaxies in the Milky Way (MW), which is called the satellite plane. It has been pointed out that the ΛCDM cosmological model hardly explains the existence of such a satellite plane, so it is regarded as one of the serious problems in the current cosmology. We here focus on a rotation of the gravitational potential of a host galaxy, i.e., a so-called figure rotation, following the previous suggestion that this effect can induce the tilt of a so-called tube orbit. Our calculation shows that a figure rotation of a triaxial potential forms a stable orbital plane perpendicular to the rotational axis of the potential. Thus, it is suggested that the MW’s dark halo is rotating with its axis being around the normal line of the satellite plane. Additionally, we find that a small velocity dispersion of satellites is required to keep the flatness of the planar structure, namely the standard derivation of their velocities perpendicular to the satellite plane needs to be smaller than their mean rotational velocity on the plane. Although not all the MW’s satellites satisfy this condition, a fraction of them, called member satellites, which are prominently on the plane, do satisfy it. We suggest that this picture explaining the observed satellite plane can be achieved by the filamentary accretion of dark matter associated with the formation of the MW and a group infall of member satellites along this cosmic filament.

The Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is Galactic archaeology with a Galactic Center survey, which aims to reveal the Milky Way’s central core structure and formation history from Gaia-level (∼25 ${\mu} $as) astrometry in the near-infrared (NIR) Hwband (1.0–1.6 ${\mu} $m). The other is an exoplanet survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic Center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information on the stars in the Galactic Center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic Center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars, and microlensing studies, including discovery of (intermediate-mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

We analyze the photometric data in the Wide layer of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) over ∼1200 deg2to uncover new halo substructures beyond the distance, D⊙∼ 30 kpc, from the Sun. For this purpose, we develop an isochrone filter for an old, metal-poor stellar system to extract the faint main-sequence stars at a range of distances. With this method, we detect not only the previously discovered substructures such as the Orphan Stream, but also a new overdensity toward Boötes at about D⊙∼ 60 kpc and a new stream-like feature toward Pisces at around D⊙∼ 60 kpc. It has been suggested that a small-scale overdensity exists in this direction of Pisces (the so-called Pisces Overdensity), but our results show that the overdensity is widely spread with a tidally elongated feature. Combining our results with the ongoing Hyper Suprime-Cam narrow-band survey and the near-future spectroscopic survey with Prime Focus Spectrograph (PFS) will allow us to place strong constraints on the origin of these halo substructures.

The structure and kinematics of the recognized stellar components of the Milky Way are explored, based on well-determined atmospheric parameters and kinematic quantities for 32360 "calibration stars" from the Sloan Digital Sky Survey (SDSS) and its first extension, SDSS-II, which included the sub-survey Sloan Extension for Galactic Understanding and Exploration (SEGUE). Full space motions for a sub-sample of 16,920 stars, exploring a local volume within 4 kpc of the Sun, are used to derive velocity ellipsoids for the inner- and outer-halo components of the Galaxy, as well as for the canonical thick-disk and proposed metal-weak thick-disk (MWTD) populations. This new sample of calibration stars represents an increase of 60% relative to the numbers used in a previous analysis. We first examine the question of whether the data require the presence of at least a two-component halo in order to account for the rotational behavior of likely halo stars in the local volume, and whether more than two components are needed. We also address the question of whether the proposed MWTD is kinematically and chemically distinct from the canonical thick disk, and point out that the Galactocentric rotational velocity inferred for the MWTD, as well as its mean metallicity, appear quite similar to the values derived previously for the Monoceros stream, suggesting a possible association between these structures. In addition, we consider the fractions of each component required to understand the nature of the observed kinematic behavior of the stellar populations of the Galaxy as a function of distance from the plane. Scale lengths and scale heights for the thick-disk and MWTD components are determined. Spatial density profiles for the inner- and outer-halo populations are inferred from a Jeans theorem analysis. The full set of calibration stars (including those outside the local volume) is used to test for the expected changes in the observed stellar metallicity distribution function with distance above the Galactic plane in situ, due to the changing contributions from the underlying stellar populations. The above issues are considered, in concert with theoretical and observational constraints from other Milky-Way-like galaxies, in light of modern cold dark matter galaxy formation models.

We present the chemical abundances of 57 metal-poor ([Fe$/$H] $< -$1) stars that are likely constituents of the outer stellar halo in the Milky Way. Almost all of the sample stars have an orbit reaching a maximum vertical distance ($Z_{\rm max}$) of $>$5 kpc above and below the Galactic plane. High-resolution ($R \sim$50000$-$55000), high signal-to-noise (S$/$N $>$100) spectra for the sample stars obtained with Subaru/HDS were used to derive the chemical abundances of Na, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, and Ba with an LTE abundance analysis code. The resulting abundance data were combined with those presented in the literature that mostly targeted at smaller $Z_{\rm max}$stars, and both data were used to investigate any systematic trends in detailed abundance patterns depending on their kinematics. It was shown that, in the metallicity range of $-$2 $<$[Fe$/$H] $< -$1, the [Mg$/$Fe] ratios for stars with $Z_{\rm max} >$5 kpc are systematically lower ($\sim$0.1 dex) than those with a smaller $Z_{\rm max}$. For this metallicity range, a modest degree of depression in the [Si$/$Fe] and the [Ca$/$Fe] ratios was also observed. This result of lower [$\alpha/$Fe] for the assumed outer halo stars is consistent with previous studies that found a signature of lower [$\alpha/$Fe] ratios for stars with extreme kinematics. The distribution of the [Mg$/$Fe] ratios for the outer halo stars partly overlaps with that for stars belonging to the Milky Way dwarf satellites in the metallicity interval of $-$2 $<$[Fe$/$H] $< -$1 and spans a range intermediate between the distributions for the inner halo stars and the stars belonging to the satellites. Our results confirm the inhomogeneous nature of the chemical abundances within the Milky Way stellar halo, depending on the kinematic properties of the constituent stars, as suggested by earlier studies. Possible implications for the formation of the Milky Way halo and its relevance to the suggested dual nature of the halo are discussed.

We present a photometric survey of the stellar halo of the nearest giant spiral galaxy, Andromeda (M31), using Suprime-Cam on the Subaru Telescope. A detailed analysis of VI color-magnitude diagrams of the resolved stellar population is used to measure properties such as line-of-sight distance, surface brightness, metallicity, and age. These are used to isolate and characterize different components of the M31 halo: (1) the giant southern stream (GSS); (2) several other substructures; and (3) the smooth halo. First, the GSS is characterized by a broad red giant branch (RGB) and a metal-rich/intermediate-age red clump (RC). The I magnitude of the well-defined tip of the RGB suggests that the distance to the observed GSS field is (m - M)0 = 24.73 +- 0.11 (883 +- 45 kpc) at a projected radius of R [?] 30 kpc from M31's center. The GSS shows a high metallicity peaked at [Fe/H][?]-0.5 with a mean (median) of -0.7 (-0.6), estimated via comparison with theoretical isochrones. Combined with the luminosity of the RC, we estimate the mean age of its stellar population to be [?]8 Gyr. The mass of its progenitor galaxy is likely in the range of 107-109 M . Second, we study M31's halo substructure along the northwest/southeast minor axis out to R [?] 100 kpc and the southwest major-axis region at R [?] 60 kpc. We confirm two substructures in the southeast halo reported by Ibata et al. and discover two overdense substructures in the northwest halo. We investigate the properties of these four substructures as well as other structures including the western shelf and find that differences in stellar populations among these systems, thereby suggesting each has a different origin. Our statistical analysis implies that the M31 halo as a whole may contain at least 16 substructures, each with a different origin, so its outer halo has experienced at least this many accretion events involving dwarf satellites with mass 107-109 M since a redshift of z [?] 1. Third, we investigate the properties of an underlying, smooth, and extended halo component out to R>100 kpc. We find that the surface density of this smooth halo can be fitted to a Hernquist model of scale radius [?]17 kpc or a power-law profile with S(R) [?] R -2.17+-0.15. In contrast to the relative smoothness of the halo density profile, its metallicity distribution appears to be spatially non-uniform with non-monotonic variations with radius, suggesting that the halo population has not had sufficient time to dynamically homogenize the accreted populations. Further implications for the formation of the M31 halo are discussed.

Chemical abundances of 26 metal-poor dwarfs and giants are determined from high-resolution and high signal-to-noise ratio spectra obtained with the Subaru/High Dispersion Spectrograph. The sample is selected so that most of the objects have outer-halo kinematics. Self-consistent atmospheric parameters were determined by an iterative procedure based on spectroscopic analysis. Abundances of 13 elements, including a-elements (Mg, Si, Ca, Ti), odd-Z light elements (Na, Sc), iron-peak elements (Cr, Mn, Fe, Ni, Zn), and neutron-capture elements (Y, Ba), are determined by two independent data reduction and local thermodynamic equillibrium analysis procedures, confirming the consistency of the stellar parameters and abundances results. We find a decreasing trend of [a/Fe] with increasing [Fe/H] for the range of -3.5< [Fe/H] <-1, as found by Stephens & Boesgaard. [Zn/Fe] values of most objects in our sample are slightly lower than the bulk of halo stars previously studied. These results are discussed as possible chemical properties of the outer halo in the Galaxy.

We investigate the dynamical effects of dark matter subhalos on the structure and evolution of a galactic disk, using the semi-analytic method that includes approximated and empirical relations as achieved in detailed numerical simulations of the cold dark matter model. We calculate the upper limit for the size of a galactic disk at a specific redshift z, based on the orbital properties of subhalos characterized by their pericentric distances from the center of a host halo. We find that this possibly largest size of a disk as determined by the smallest pericentric distances of subhalos shows the characteristic properties, which are basically in agreement with an observed galactic disk at low and high z. Namely, it is found that a massive disk can have a larger size than a less massive one, because of its stability against the destruction effect of subhalos. Also, with fixed mass, the size of a galactic disk at low z can be larger than that at high z, reflecting the orbital evolution of subhalos with respect to a host halo. These results suggest that the presence and structure of a galactic disk may be dynamically limited by the interaction with dark matter substructures, especially at high z.

We present mid-infrared imaging at 11.7 mm for the quadruple lens systems, MG0414+0534 and Q2237+030, using the cooled mid-infrared camera and spectrometer attached on the Subaru telescope. MG0414+0534 is characterized by a bright pair of lensed images (A1, A2) and their optical flux ratio A2/A1 deviates significantly from the prediction of a smooth-lens model. Q2237+030 is "the Einstein Cross" being comprised of four lensed images, which are significantly affected by microlensing in a foreground lensing galaxy. Our mid-infrared observations of these lensed images have revealed that the mid-infrared flux ratio for A2/A1 of MG0414+0534 is nearly unity (0.90 +- 0.04). We find that this flux ratio is systematically small, at 4-5s level, compared with the prediction of a best smooth-lens model (1.09) represented by a singular isothermal ellipsoid and external shear. The smooth-lens model, which also considers the additional lensing effect of the possible faint satellite, object X, still provides a large flux ratio of A2/A1=1.06, thereby suggesting the presence of more substructures that can explain our observational result. In contrast, for Q2237+030, our high signal-to-noise observation indicates that the mid-infrared flux ratios between all the four images of Q2237+030 are virtually consistent with the prediction of a smooth-lens model. Based on the size estimate of the dust torus surrounding the nuclei of these QSOs, we set limits on the mass of a substructure in these lens systems, which can cause anomalies in the flux ratios. For MG0414+0534, since the required mass of a substructure inside its Einstein radius is [?]360 M , millilensing by a cold dark matter substructure is most likely. If it is modeled as a singular isothermal sphere, the mass inside a radius of 100 pc is given as [?]1.0 x 105 M . For Q2237+030, there is no significant evidence of millilensing, so the reported anomalous flux ratios in shorter wavelengths are entirely caused due to microlensing by stars.

We first present chemodynamical simulations to investigate how stellar winds of massive stars influence early dynamical and chemical evolution of forming globular clusters (GCs). In our numerical models, GCs form in turbulent, high-density giant molecular clouds (GMCs), which are embedded in a massive dark matter halo at high redshifts. We show how high-density, compact stellar systems are formed from GMCs influenced both by physical processes associated with star formation and by tidal fields of their host halos. We also show that chemical pollution of GC-forming GMCs by stellar winds from massive stars can result in star-to-star abundance inhomogeneities among light elements (e.g., C, N, and O) of stars in GCs. The present model with a canonical initial mass function (IMF) also shows a C-N anticorrelation that stars with smaller [C/Fe] have larger [N/Fe] in a GC. Although these results imply that ``self-pollution'' of GC-forming GMCs by stellar winds from massive stars can cause abundance inhomogeneities of GCs, the present models with different parameters and canonical IMFs cannot show N-rich stars with [N/Fe] ~ 0.8 observed in some GCs (e.g., NGC 6752). We discuss this apparent failure in the context of massive star formation preceding low-mass ones within GC-forming GMCs (``bimodal star formation scenario''). We also show that, although almost all stars (~97%) show normal He abundances (Y) of ~0.24, some stars later formed in GMCs can have Y as high as ~0.3 in some models. The number fraction of He-rich stars with Y > 0.26 is, however, found to be small (~10-3) for most models. We discuss this result in the context of the possibly large Y-values observed in a few Galactic GCs.

We have investigated the dynamical interaction between a galactic disk and numerous surrounding dark subhalos as expected for a galaxy-sized halo in the cold dark matter models. Our particular interest is to what extent accretion events of subhalos into a disk are allowed in light of the observed thinness of a disk. Several models of subhalos were considered in terms of their internal density distribution, mass function, and spatial and velocity distributions. Based on a series of $N$-body simulations, we find that disk thickening, quantified by the change of its scale height, $\Delta z_{\mathrm{d}}$, depends strongly on the individual mass of an interacting subhalo, $M_\mathrm{sub}$. This is described by the relation $\Delta z_{\mathrm{d}} / R_{\mathrm{d}} \simeq 8 \sum_{j=1}^N (M_{\mathrm{sub},j} / M_{\mathrm{d}})^2$, where $R_{\mathrm{d}}$is the disk scale length, $M_{\mathrm{d}}$is the disk mass, and $N$is the total number of accretion events of subhalos inside a disk region ($\le 3 R_{\mathrm{d}}$). Using this relation, we find that an observed thin disk has never interacted with subhalos with a total mass of more than 15% of the disk mass. Also, a less-massive disk with a smaller circular velocity, $V_{\mathrm{c}}$is more affected by subhalos than a disk with a larger $V_{\mathrm{c}}$, in agreement with observations. Further implications of our results for the origin of a thick disk component are also discussed.

We investigate the extended source size effects on gravitational lensing in which a lens consists of a smooth potential and small mass clumps ("substructure lensing"). We first consider a lens model that consists of a clump modeled as a singular isothermal sphere (SIS) and a primary lens modeled as an external background shear and convergence. For this simple model, we derive analytic formulae for (de)magnification of circularly symmetric top-hat sources with three types of parity for their lensed images, namely, positive, negative, and doubly negative parities. Provided that the source size is sufficiently larger than the Einstein radius of the SIS, we find that in the positive (doubly negative) parity case, an extended source is always magnified (demagnified) in comparison with the unperturbed macrolens system, whereas in the negative parity case, the (de)magnification effect, which depends on the sign of convergence minus unity, is weaker than those in other parities. It is shown that a measurement of the distortion pattern in a multiply lensed image enables us to break the degeneracy between the lensing effects of clump mass and those of clump distance if the lensing parameters of the relevant macrolens model are determined from the position and flux of multiple images. We also show that an actual density profile of a clump can be directly measured by analyzing the "fine structure" in a multiply lensed image within the Einstein radius of the clump.

The cold dark matter (CDM) structure formation model predicts that about 5%-10% of a typical galactic halo of mass ~1012 M is in substructures with masses [?]108 M. To directly detect such substructures, we propose to observe dust continuum emission from a strongly lensed QSO host galaxy using a large submillimeter interferometer array with a high angular resolution of ~0.''01, such as the planned Atacama Large Submillimeter Array (ALMA). To assess their observational feasibility, we numerically simulate millilensing of an extended circular source by a CDM substructure modeled as a tidally truncated singular isothermal sphere (SIS) embedded in a typical QSO-galaxy lens system, B1422+231, modeled as a singular isothermal ellipsoid (SIE) with an external constant shear and a constant convergence. Assuming an angular resolution of 0.''01, we find that the angular positions of ~108 M substructures at several kpc from the center of the macrolens halo can be directly measured if the size of the dust continuum emission region and the gradient of the surface brightness at the position of the perturber are sufficiently large. From the astrometric shift on a scale of a few times 10 mas of an image perturbed by a subhalo with respect to an unperturbed macrolensed image, we can break the degeneracy between subhalo mass and distance provided that macrolensing parameters are determined from positions and fluxes of multiple images.

We present mid-infrared imaging at 11.7 mm for the quadruple lens systems PG 1115+080 and B1422+231 using the cooled mid-infrared camera and spectrometer (COMICS) attached on the Subaru Telescope. These lensed QSOs are characterized by their anomalous optical and radio flux ratios, as obtained for A1 and A2 images of PG 1115+080 and A, B, and C images of B1422+231, respectively, i.e., such flux ratios are hardly able to be reproduced by lens models with a smooth mass distribution. Our mid-infrared observations for these images have revealed that the mid-infrared flux ratio A2/A1 of PG 1115+080 is virtually consistent with smooth lens models (but inconsistent with the optical flux ratio), whereas for B1422+231, the mid-infrared flux ratios among the A, B, and C images are in good agreement with the radio flux ratios. We also identify a clear infrared bump in the spectral energy distributions of these QSOs, thereby indicating that the observed mid-infrared fluxes originate from a hot dust torus around a QSO nucleus. Based on the size estimate of the dust torus, we place limits on the mass of a substructure in these lens systems causing the anomalous optical or radio flux ratios. For PG 1115+080, the mass of a substructure inside an Einstein radius ME is [?]16 M, corresponding to either a star or a low-mass CDM subhalo having a mass of Mimg1.gif [?] 2.2 x 104 M inside a radius of 100 pc if modeled as a singular isothermal sphere (SIS). For B1422+231, we obtain ME [?] 209 M, indicating that a CDM subhalo is more likely, having a mass of Mimg1.gif [?] 7.4 x 104 M.

AbstractThe kinematic properties of tidal debris from an orbiting Galactic satellite is presented, on the assumption that its central part once contained the most massive Galactic globular cluster, ? Cen. We simulate dynamical evolution of a satellite galaxy that follows the present-day and likely past orbits of ? Cen, and analyse the kinematic nature of their tidal debris and randomly generated Galactic stars comprising spheroidal halo and flat disk components. It is found that the debris stars show a retrograde rotation at ~–100 km s–1, which may accord with a recently discovered stellar stream at a radial velocity of ~300 km s–1towards the Galactic longitude of ~270°. These stars also contribute, only in part, to a reported retrograde motion of the outer halo in the north Galactic pole direction, without significantly modifying local halo kinematics near the Sun. The prospects for future debris searches and the implications for the early evolution of the Galaxy are briefly presented.

The kinematic properties of tidal debris from an orbiting Galactic satellite is presented, on the assumption that its central part once contained the most massive Galactic globular cluster, ω Cen. We simulate dynamical evolution of a satellite galaxy that follows the present-day and likely past orbits of ω Cen, and analyse the kinematic nature of their tidal debris and randomly generated Galactic stars comprising spheroidal halo and flat disk components. It is found that the debris stars show a retrograde rotation at ~–100 km s–1, which may accord with a recently discovered stellar stream at a radial velocity of ~300 km s–1 towards the Galactic longitude of ~270°. These stars also contribute, only in part, to a reported retrograde motion of the outer halo in the north Galactic pole direction, without significantly modifying local halo kinematics near the Sun. The prospects for future debris searches and the implications for the early evolution of the Galaxy are briefly presented.

We present further observations of the extremely red object ERO J094258+4659.2, identified earlier by Iye et al. as ERO R1 in their deep images of the cluster A851. We estimate its redshift independently by eight-band photometric redshift determination and cross-correlation of a new H-band spectrum with the optical spectra of local E/S0 galaxies and conclude that it lies at z ~ 1.5. Although its colors are consistent with both an elliptical galaxy and an S0 galaxy at that redshift, its elongated shape and exponential luminosity profile suggest the presence of an evolved stellar disk component. We rule out the possibility that these properties are strongly influenced by gravitational lensing by the foreground cluster and therefore conclude that this object is more likely to be an S0-like galaxy rather than a lensed elliptical. The H-band spectrum does not show strong Ha emission, and the star formation rate therefore appears to be very modest. The presence of such a galaxy with an apparently relaxed disk of stars at this high redshift provides a new and strong constraint on theoretical models that aim to explain the formation and evolution of galaxies.

The origin of metal-rich, highly flattened, and rapidly rotating disk globular cluster systems in the Galaxy is one of long-standing issues in the context of the Galaxy formation. Our numerical simulations suggest a new, twofold scenario in which the disk globular clusters are first formed in the high-pressure, dense central region of a gas-rich dwarf galaxy, as induced during the tidal interaction with the pre-existing, young, thin disk of the Galaxy, and then dispersed into the disk region owing to the final tidal destruction of the merging dwarf. We also demonstrate that spatial distribution, total number, and metallicity distribution of the clusters formed in this minor merging depend on the mass ratio of the host to dwarf galaxy and the orbital configuration of merging. Based on these results, we discuss whether a minor merging event about 10 Gyr ago can explain both the Galactic thick disk and disk globular clusters. Several other implications for the possible relation between the properties of disk galaxies and their disk globular clusters are also discussed.

We investigate the effects of numerous dark matter subhalos in a galaxy-sized halo on the events of strong lensing to assess their presence as expected from the cold dark matter scenario. Lens galaxies are represented by a smooth ellipsoid in an external shear field and additional cold dark matter subhalos taken from Monte Carlo realizations that accord with recent N-body results. We also consider other possible perturbers, globular clusters and luminous dwarf satellites, for comparison. We then apply the models to the particular lens systems with four images, B1422+231 and PG 1115+080, for which smooth lens models are unable to simultaneously reproduce both the positions of the images and their radio flux ratios or dust-free optical flux ratios. We show that the perturbations by both globular clusters and dwarf satellites are too small to change the flux ratios, whereas cold dark matter subhalos are the most likely perturbers to reproduce the observed flux ratios in a statistically significant manner. This result suggests to us the presence of numerous subhalos in lens galaxies, which is consistent with the results of cosmological N-body simulations.

We perform numerical simulations for the formation of the Galactic stellar halo, based on the currently favored cold dark matter theory of galaxy formation. Our numerical models, taking into account both dynamical and chemical evolution processes in a consistent manner, are aimed at explaining the observed structure and kinematics of the stellar halo in the context of hierarchical galaxy formation. The main results of the present simulations are summarized as follows: (1) Basic physical processes involved in the formation of the stellar halo, composed of metal-deficient stars with [Fe/H] [?] -1.0, are described by both dissipative and dissipationless merging of subgalactic clumps and their resultant tidal disruption in the course of gravitational contraction of the Galaxy at high redshift (z > 1). (2) The simulated halo has a density profile similar to the observed power-law form of r(r) ~ r-3.5 and also has a metallicity distribution similar to the observations. The halo shows virtually no radial gradient for stellar ages and only a small gradient for metallicities. (3) The dual nature of the halo, i.e., its inner flattened and outer spherical density distribution, is reproduced, at least qualitatively, by the present model. The outer spherical halo is formed via essentially dissipationless merging of small subgalactic clumps, whereas the inner flattened one is formed via three different mechanisms, i.e., dissipative merging between larger, more massive clumps, adiabatic contraction due to the growing Galactic disk, and gaseous accretion onto the equatorial plane. (4) For the simulated metal-poor stars with [Fe/H] [?] -1.0, there is no strong correlation between metal abundances and orbital eccentricities, in good agreement with the recent observations. Moreover, the observed fraction of the low-eccentricity stars is reproduced correctly for [Fe/H] [?] -1.6 and approximately for the intermediate-abundance range of -1.6 < [Fe/H] [?] -1.0. (5) The mean rotational velocity of the simulated halo,img1.gif , is somewhat positive (prograde) at [Fe/H] < -2.2 and increases linearly with [Fe/H] at [Fe/H] > -2.2. The stars at smaller distance from the disk plane appear to show systematically largerimg1.gif . Based on these results, we discuss how early processes of dissipationless and dissipative merging of subgalactic clumps can reproduce plausibly and consistently the recent observational results on the Galactic stellar halo. We also present a possible scenario for the formation of the entire Galaxy structure, including bulge and disk components, in conjunction with halo formation.

We develop a method for recovering the global density distribution of the ancient Galactic stellar halo prior to disk formation based on the presentorbits of metal-poor stars observed in the solar neighborhood. The method relies on the adiabatic invariance of the action integrals of motion for the halo population during the slow accumulation of a disk component subsequent to earlier halo formation. The method is then applied to a sample of local stars with [Fe/H] ? -1.5, which are likely to be dominated by the halo component, taken from Beers et al.'s recently revised and supplemented catalog of metal-poor stars selected without kinematic bias. We find that even if the Galactic potential is made spherical by removing the disk component in an adiabatic manner, the halo density distribution in the inner halo region (R? 15 kpc) remains moderately flattened, with an axial ratio of about 0.8 for stars in the abundance range [Fe/H] ? -1.8 and about 0.7 for the more metal-rich interval -1.8 < [Fe/H] ? -1.5. The outer halo remains spherical for both abundance intervals. We also find that this initial flattening of the inner halo is caused by the anisotropic velocity dispersions of the halo stars. These results suggest that the two-component nature of the present-day stellar halo, characterized by a highly flattened inner halo and nearly spherical outer halo, is a consequence of bothan initially two-component density distribution of the halo (perhaps a signature of dissipative halo formation) andof the adiabatic flattening of the inner part by later disk formation. Further implications of our results for the formation of the Galaxy are also discussed, in particular in the context of the hierarchical clustering scenario of galaxy formation.

We investigated global axisymmetric (m = 0) and non-axisymmetric (m = 1) modes of magnetic fields generated by the galactic dynamo including the α2-dynamo. The α2-dynamo is responsible for the field generation in the central region of galaxies where the shear of galactic rotation is weak (e.g. M51). The highest growth rate of m = 1 modes is always smaller than that of m = 0 modes; thus m = 1 modes of the standard galactic dynamo cannot explain the dominance of the bisymmetric fields in spiral galaxies. Radial extent of each m = 1 mode is too narrow to reproduce the observed bisymmetric structure extending over a disk.

We present new limits on cosmological parameters from the statistics of gravitational lensing, based on the recently revised knowledge of the luminosity function and internal dynamics of E/S0 galaxies that are essential in lensing high-redshift QSOs. We find that the lens models using updated Schechter parameters for such galaxies, derived from the recent redshift surveys combined with morphological classification, are found to give smaller lensing probabilities than were calculated earlier. Inconsistent adoption of these parameters from a mixture of various galaxy surveys gives rise to systematic biases in the results. We also show that less compact dwarf-type galaxies that largely dominate the faint part of the Schechter form luminosity function contribute little to lensing probabilities, so that earlier lens models overestimate incidents of small separation lenses. Applications of the lens models to the existing lens surveys indicate that reproduction of both the lensing probability of optical sources and the image separations of optical and radio lenses is significantly improved in the revised lens models. The likelihood analyses allow us to conclude that a flat universe with O0=0.3$\mathstrut{^{+0.2}_{-0.1}}$ and O0+l0=1 is preferable, and a matter-dominated flat universe with l0=0 is ruled out at the 98% confidence level. These new limits are unaffected by the inclusion of uncertainties in the lens properties.

We present new calculations of gravitational lensing statistics in view of recent revisions in our knowledge of the luminosity function and internal velocity dispersion of E/S0 galaxies that work as effective lenses for background high-redshift QSOs. We show that the theoretical prediction of the lensing statistics is much smaller than previously expected. In sharp contrast to earlier statistics supporting an O0 = 1 universe, the small lensing probability reported from the Hubble Space Telescope (HST) Snapshot Lens Survey is in best agreement with a low-density, flat universe with O0 [?] 0.2 and O0 + l0 = 1. The age of this universe, combined with the HST measurement of a high value for the Hubble constant H0, can be reconciled with the age of the oldest globular clusters in the Milky Way.

We investigate the hypothesis that Lya absorption lines arise in two populations of halos--minihalos of small circular velocity (Vc [?] 55 km s-1), in which star formation and metal production are inhibited by photoionization of the UV background radiation, and large galactic halos (55 [?] Vc [?] 250 km s-1) that contain stars and metals. Based on the model of Lya-absorbing gas confined in both populations of halos, we attempt to explain the recent observations of (1) associations of visible galaxies with Lya lines at low redshifts z [?] 1 and (2) metal lines associated with a nonnegligible fraction of low H I column density Lya lines at z ~ 3. For galactic halos, we find that photoionized gas clouds confined in the pressure of ambient hot gas can produce Lya absorption for H I column densities as low as 1014 cm-2 and that the impact parameter of a sight line for such absorptions matches well with the observed radius of the gaseous envelope of a typical luminous galaxy. Using the Press-Schechter prescription for the mass function of halos, we also show that the fraction of Lya lines with associated metal lines can be understood in terms of the fraction of Lya absorbers associated with galactic halos. In particular, the reported fraction of ~0.5-0.75 at z ~ 3 is reproduced when the boundary value of Vc to separate minihalos or galactic halos is 40-60 km s-1, which is consistent with the theoretical prediction of galaxy formation under photoionization. The average metallicity of both Lya forest and damped Lya systems at z ~ 3 is explained in terms of the model of halo-formation history combined with the age-metallicity relationship of Galactic halo stars. Possible methods to test this hypothesis and alternative scenarios are also discussed.

We report on the discovery of a new Milky Way (MW) satellite in Boötes based on data from the ongoing 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 (32.3σ) overdensity of stars with 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 $r_{\rm h}=462^{+98}_{-84}\:$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 $r_{\rm h}=5.9^{+1.5}_{-1.3}\:$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, MNRAS, 479, 2853), which combined the characteristics of the satellites observed by the Sloan Digital Sky Survey and the Dark Energy Survey with the subhalos obtained in ΛCDM models, we estimate that there should be about two MW satellites at MV≤ 0 in the ∼676 deg2covered by HSC-SSP, whereas that area includes six satellites (Sextans, Leo IV, Pegasus III, Cetus III, Virgo I, and Boötes IV). 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 ΛCDM theory overpredicts the number of satellites in a MW-sized halo. This may imply that the models need more refinement for the assignment of subhalos to satellites, such as considering those found by the current deeper survey. More statistically robust constraints on this issue will be brought by further surveys of HSC-SSP over the planned ∼1400 deg2area.

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 ongoing Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) surveyed over a ∼550 deg2area. In order to select the 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 are 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 < g< 23.5 mag that the radial density distribution over a Galactocentric radius of r= 36–360 kpc can be approximated as a single power-law profile with an index of $\alpha =3.74^{+0.21}_{-0.22}$or a broken power-law profile with an index of $\alpha _{\rm in}=2.92^{+0.33}_{-0.33}$at rbelow a broken radius of $r_{\rm b}=160^{+18}_{-19}\:$kpc and a very steep slope of $\alpha _{\rm out}=15.0^{+3.7}_{-4.5}$at r> rb. 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≃ 160 kpc. 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 situinner halo as probed by RR Lyrae stars showing a relatively steep radial density profile and the ex situouter halo with a shallow profile probed by BHBs here, which is made by accretion of small stellar systems.

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 ≃ 3.3 (3.5) with (without) these fields and its global axial ratio is q≃ 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∼ 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 ≃ 5 outside a break radius of ∼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 a larger BHB sample is required from further HSC-SSP surveys to increase its statistical significance.

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 iband 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.

The Hyper Suprime-Cam (HSC) is an 870 megapixel prime focus optical imaging camera for the 8.2 m Subaru telescope. The wide-field corrector delivers sharp images of 0${^{\prime\prime}_{.}}$2 (FWHM) in the HSC-iband over the entire 1${^{\circ}_{.}}$5 diameter field of view. The collimation of the camera with respect to the optical axis of the primary mirror is done with hexapod actuators, the mechanical accuracy of which is a few microns. Analysis of the remaining wavefront error in off-focus stellar images reveals that the collimation of the optical components meets design specifications. While there is a flexure of mechanical components, it also is within the design specification. As a result, the camera achieves its seeing-limited imaging on Maunakea during most of the time; the median seeing over several years of observing is 0${^{\prime\prime}_{.}}$67 (FWHM) in the iband. The sensors use p-channel, fully depleted CCDs of 200 μm thickness (2048 × 4176 15 μm square pixels) and we employ 116 of them to pave the 50 cm diameter focal plane. The minimum interval between exposures is 34 s, including the time to read out arrays, to transfer data to the control computer, and to save them to the hard drive. HSC on Subaru uniquely features a combination of a large aperture, a wide field of view, sharp images and a high sensitivity especially at longer wavelengths, which makes the HSC one of the most powerful observing facilities in the world.

Hyper Suprime-Cam (HSC) is a wide-field imaging camera on the prime focus of the 8.2-m Subaru telescope on the summit of Mauna Kea 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 deg2in five broad bands (grizy), with a 5 σ point-source depth of r≈ 26. The Deep layer covers a total of 26 deg2in 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 deg2). 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 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) ∼300 deg2and 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 MV= −2.4 ± 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 ∼300 deg2HSC-SSP data, in rough agreement with the discovery rate so far. The further survey and completion of HSC-SSP over ∼1400 deg2will provide robust insights into the missing satellites problem.

The Subaru Prime Focus Spectrograph (PFS) is a massively multiplexed fiber-fed optical and near-infrared three-arm spectrograph (Nfiber= 2400, 380 ≤ λ ≤ 1260 nm, 1 $_{.}^{\circ}$3 diameter field of view). Here, we summarize the science cases in terms of provisional plans for a 300-night Subaru survey. We describe plans to constrain the nature of dark energy via a survey of emission line galaxies spanning a comoving volume of 9.3 h−3Gpc3in the redshift range 0.8 < z< 2.4. In each of six redshift bins, the cosmological distances will be measured to 3% precision via the baryonic acoustic oscillation scale, and redshift-space distortion measures will constrain structure growth to 6% precision. In the near-field cosmology program, radial velocities and chemical abundances of stars in the Milky Way and M 31 will be used to infer the past assembly histories of spiral galaxies and the structure of their dark matter halos. Data will be secured for 106stars in the Galactic thick-disk, halo, and tidal streams as faint as V∼ 22, including stars with V< 20 to complement the goals of the Gaia mission. A medium-resolution mode with R= 5000 to be implemented in the red arm will allow the measurement of multiple α-element abundances and more precise velocities for Galactic stars. For the galaxy evolution program, our simulations suggest the wide wavelength range of PFS will be powerful in probing the galaxy population and its clustering over a wide redshift range. We plan to conduct a color-selected survey of 1 < z< 2 galaxies and AGN over 16 deg2to J≃ 23.4, yielding a fair sample of galaxies with stellar masses above ∼1010M⊙at z≃ 2. A two-tiered survey of higher redshift Lyman break galaxies and Lyman alpha emitters will quantify the properties of early systems close to the reionization epoch.

Statistics of gravitational lensing of QSOs by intervening galaxies provides a powerful tool to set constraints on cosmological parameters. In particular, lensing statistics are sensitive indicators of a nonvanishing cosmological constant. We review the present status of the subject and recent progress of theoretical works. Taking into account the current knowledge of early-type galaxies that effectively work as lenses, we present plausible lens models for calculating lensing statistics and give a detailed discussion on the possible value of a cosmological constant to reproduce the observed frequency of lensed QSOs and their image-separation distribution.