We present the confirmation and characterization of a long stream (S-stream) in the southern part of M83. This feature is revealed using deep wide-field photometric data obtained by the Hyper Suprime-Cam (HSC) mounted on the Subaru Telescope. Using individual red giant branch (RGB) stars, we successfully trace the stream over a large length of ∼ 81 kpc and a considerable width of ∼ 9 kpc. With a mean surface brightness of ⟨ μ_ V⟩∼ 31.8_-1.9^+1.3 mag arcsec^-2, it is one of the most diffuse extragalactic streams currently known. The mean photometric metallicity of the stream is ⟨[ M/H]⟩ = -1.23±0.02 dex with a standard deviation of 0.28±0.01 dex, and we estimate the stellar mass to be (8.5_-2.8^+4.2) × 10^6 M_⊙ from the luminosity of RGB stars. Compared to its well-known northern counterpart, the S-stream is slightly more metal-poor, but our large-area RGB map shows compelling evidence that these two features are related, originating from a single low-mass merger event. We identify density variations along the S-stream, which more likely reflect intrinsic density structure within the progenitor rather than the interaction with dark matter subhalos. Similarities between the morphology of the S-stream and some features in the distribution suggest that a minor merger event may have disturbed and redistributed M83's outer gas, leading to triggered star formation and the formation of the XUV disk.

We present a deep photometric study of the globular cluster NGC 5466 and its tidal stream using Subaru/Hyper Suprime-Cam (HSC) imaging with the metallicity-sensitive narrowband filter NB395. We develop an improved member-selection technique based on a k-nearest neighbor algorithm applied to the color-color-magnitude diagram (CCMD), enabling reliable candidate identification down to i_2,0 < 23.5. Photometric metallicities derived from NB395 colors agree with previous measurements, supporting the robustness of our calibration. While modest residual contamination and possible offsets - potentially driven by variations in light-element abundances - may remain beyond 10 arcmin, the metallicity distribution of high-probability inner members matches the known mean metallicity of NGC 5466, demonstrating the effectiveness of our method. The spatial distribution of NB395-selected stars clearly delineates the tidal stream. Beyond the tidal radius, the azimuthally averaged radial surface density profile follows a power law with slope α= -4.53_-0.14^+0.13. We also detect a power-law component perpendicular to the stream, suggestive of multiple apogalactic passages. A density gap is identified at a projected distance of ∼200 pc from the cluster center, consistent with eTidal N-body predictions and possibly associated with a recent pericentric passage or Galactic disk interaction. Analysis of the main-sequence mass function reveals a strong negative radial gradient in the slope within the tidal radius, whereas the slope along the outer stream is relatively flat, consistent with preferential tidal stripping of low-mass stars. These results highlight the power of HSC/NB395 photometry for identifying metal-poor populations and deriving photometric metallicities, underscoring its value for future wide-field surveys.

The recently commissioned Subaru `Ōnohi`ula Prime Focus Spectrograph (PFS) will obtain spectra from nearly 2,400 fibers that cover 1.24 square degrees. The 360 night Subaru Strategic Program for PFS is dedicating approximately one-third of its allocation (130 nights) to study the structure and evolution of galaxies in the Local Group. This Galactic Archaeological survey has three pillars. (1) We will determine whether the mass density profiles of dwarf galaxies are consistent with cusps, as expected for cold dark matter, or cores, as expected from alternative dark matter theories or baryonic feedback. We will deduce the density profiles as a function of radius from modeling of the full line-of-sight velocity and abundance distributions for six dwarf galaxies. Our total sample will consist of 18,000 member stars to beyond the nominal tidal radius of each system. (2) From measurements of the [alpha/Fe] abundance ratio, we will learn the difference in assembly history of the two most massive galaxies in the Local Group: M31 and the Milky Way. We will observe 30,000 member stars over 45 square degrees of M31's halo and outer disk. (3) We will uncover how the most fragile (outer) part of the Milky Way responded to accretion events both in the distant past (such as Gaia-Sausage Enceladus) and in more recent history (such as the Sagittarius dwarf spheroidal galaxy). To support this study, PFS will provide velocities and metallicities–from which, in combination with photometry, we will deduce ages–for tens of thousands of main-sequence stars out to a Galactocentric distance of 30 kpc.

We present our photometric method, which combines Subaru/Hyper Suprime-Cam N B 515, g -band, and i- band filters to distinguish giant stars in Local Group galaxies from Milky Way dwarf contamination. The N B 515 filter is a narrowband filter that covers the MgI+MgH features at 5150 Å and is sensitive to stellar surface gravity. Using synthetic photometry derived from large empirical stellar spectral libraries, we model the N B 515 filter’s sensitivity to stellar atmospheric parameters and chemical abundances. Our results demonstrate that the N B 515 filter effectively separates dwarfs from giants, even for the reddest and coolest M-type stars. To further enhance this separation, we develop machine learning models that improve the classification on the two-color ( g − i , N B 515 − g ) diagram. We apply these models to photometric data from the Fornax dwarf spheroidal galaxy and two fields of M31, successfully identifying red giant branch stars in these galaxies.

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 six times the 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 the Hyper Suprime-Cam on the Subaru Telescope. The algorithm employs a genetic algorithm and simulated annealing to minimize a chi(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 sigma of the 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 two detected populations in the UMi dSph have 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.

We investigate a minor merger event in M 31 that simultaneously forms the Andromeda Giant Southern Stream (AGSS), the Eastern Extent (EE), the North-Eastern Shelf (NES), and the Western Shelf (WS), offering a unified model for these substructures. By varying the scale radius and mass of the dark matter halo associated with the progenitor, around the range predicted by the $\Lambda$ cold dark matter (CDM) model, we successfully reproduce the spatial features of these substructures. Across the limited range of parameters considered in this study, our analysis shows that the spatial evolution of the NES and the WS is independent of the gravitational potential of the dark matter halo associated with the progenitor, while a shallower potential shifts the EE further north. The simulations clearly demonstrate that the progenitor with a dark matter halo mass of $9\times 10^9$ M$_\odot$ colliding with M 31 850 Myr ago could simultaneously form the observed substructures, including the AGSS, EE, NES, and WS. The simulation results indicate that the EE lies several 10 kpc closer to us than the aligned Stream Cp, which is actually a metal-poor component of Stream C, whose farther distance suggests overlapping debris from distinct collision events, while both remain closely aligned in celestial coordinates. Furthermore, we predict the existence of a positive stream along the AGSS, characterized by positive line-of-sight velocities relative to M 31, which complements an already observed negative stream exhibiting negative line-of-sight velocities. Finally, we propose that three objects—Stream B, a metal-rich component of Stream C known as Stream Cr, and the EE—are components of the Andromeda Giant Southern Arc (AGSA) connected to the AGSS. Although the existence of the positive stream and a complete picture of the AGSA have yet to be confirmed observationally, we anticipate that future spectroscopic observations and further advances in theoretical studies will verify their existence.

We investigate the chemical abundance distributions of the Fornax, Sculptor, Ursa Minor, and Draco dwarf galaxies using Subaru/Hyper Suprime-Cam (HSC) photometric data. The HSC data set, which includes broadband g and i filters and the narrowband NB515 filter, offers sensitivity to iron and magnesium abundances, as well as surface gravity, enabling the identification of giant stars and foreground dwarfs. For analysis, we selected a total of 6713 giant candidates using a random forest regressor trained on medium-resolution ( R ∼ 6000) Keck/Deep Imaging Multi-Object Spectrograph spectroscopic data. Our analysis reveals the extent of radial metallicity gradients in the galaxies. Such trends, not detectable in earlier studies, are now captured owing to the substantially enlarged sample size and areal coverage provided by the HSC data. These results are also consistent with chemical abundance patterns previously observed in the central regions through spectroscopic studies. Furthermore, we infer that Fornax underwent extended star formation, whereas Sculptor formed both metal-poor and metal-rich stars over a shorter time. Ursa Minor and Draco appear to have experienced brief, intense star formation episodes leading to nearly extinguished star formation. This study underscores the critical role of the expanded HSC data set in revealing chemical gradients that were previously inaccessible. Future work incorporating additional spectra of metal-poor stars and age-sensitive isochrone modeling will enable more accurate maps of chemical abundance distributions.

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 ∼ 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.

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.

We present the demography of the dynamics and gas mass fraction of 33 extremely metal-poor galaxies (EMPGs) with metallicities of 0.015–0.195 Z _⊙ and low stellar masses of 10 ^4 –10 ^8 M _⊙ in the local universe. We conduct deep optical integral field spectroscopy (IFS) for the low-mass EMPGs with the medium-high resolution ( R = 7500) grism of the 8 m Subaru FOCAS IFU instrument by the EMPRESS 3D survey, and investigate the H α emission of the EMPGs. Exploiting the resolution high enough for the low-mass galaxies, we derive gas dynamics with the H α lines by the fitting of three-dimensional disk models. We obtain an average maximum rotation velocity ( v _rot ) of 15 ± 3 km s ^−1 and an average intrinsic velocity dispersion ( σ _0 ) of 27 ± 10 km s ^−1 for 15 spatially resolved EMPGs out of 33 EMPGs, and find that all 15 EMPGs have v _rot / σ _0 < 1 suggesting dispersion-dominated systems. There is a clear decreasing trend of v _rot / σ _0 with the decreasing stellar mass and metallicity. We derive the gas mass fraction ( f _gas ) for all 33 EMPGs, and find no clear dependence on stellar mass and metallicity. These v _rot / σ _0 and f _gas trends should be compared with young high- z galaxies observed by the forthcoming JWST IFS programs to understand the physical origins of the EMPGs in the local universe.

National Astronomical Observatory of Japan and Hamamatsu Photonics K.K. have been developing large format and high-speed readout CMOS sensors. It is designed to be 2,560 x 10,000 pixels with 7.5 mu m and three-side buttable in order to cover a wide field of view. The CMOS sensors is designed to be back-illuminated to achieve higher filling factor than front-illuminated CMOS sensors and to improve the sensitivity by avoiding photon absorption by the poly-silicon circuit. Each pixel row is equipped with an ADC to achieve the frame rate of 10 Hz. The evaluation in the laboratory shows that the sensor has excellent performance; the quantum efficiency is 80 % at maximum at 600 nm and readout noise is 3 e(-) rms at 2 fps. We are developing a wide field camera using these CMOS sensors.

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.

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 deg2 area. 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 r below 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 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.

We analyse the M31 halo and its substructure within a projected radius of 120 kpc using a combination of Subaru/HSC NB515 and Canada France Hawaii Telescope/MegaCam g and i bands. We succeed in separating M31's halo stars from foreground contamination with $\sim$90 per cent accuracy by using the surface gravity sensitive NB515 filter. Based on the selected M31 halo stars, we discover three new substructures, which associate with the Giant Southern Stream (GSS) based on their photometric metallicity estimates. We also produce the distance and photometric metallicity estimates for the known substructures. While these quantities for the GSS are reproduced in our study, we find that the north-western stream shows a steeper distance gradient than found in an earlier study, suggesting that it is likely to have formed in an orbit closer to the Milky Way. For two streams in the eastern halo (Stream C and D), we identify distance gradients that had not been resolved. Finally, we investigate the global halo photometric metallicity distribution and surface brightness profile using the NB515-selected halo stars. We find that the surface brightness of the metal-poor and metal-rich halo populations, and the all population can be fitted to a power-law profile with an index of $\alpha =-1.65\pm 0.02$, $-2.82\pm 0.01$, and $-2.44\pm 0.01$, respectively. In contrast to the relative smoothness of the halo profile, its photometric metallicity distribution appears to be spatially non-uniform with non-monotonic trends with radius, suggesting that the halo population had insufficient time to dynamically homogenize the accreted populations.

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is one of the three candidates of ISAS/JAXA's Strategic L-class mission for the 2030s. The 1.2 m aperture, 50 K cryogenic space telescope with the wide-field camera (WFC) will provide the 1,260 square arcmin field-of-view for five photometric bands between 2 and 8 mu m. The high resolution spectrometer (HRS) will observe the 10-18 mu m with a wavelength resolution of 30,000. The GREX-PLUS WFC field-of-view is 130 times larger than that of the James Webb Space Telescope and similar to those of Euclid and Roman Space Telescope. Since these two survey missions are limited to the wavelength less than around 2 mu m, GREX-PLUS will extend the wavelength coverage beyond 2 mu m, providing versatile legacy imaging survey significantly improved from previous Spitzer imaging survey in the same wavelength range. The spectral resolution of the GREX-PLUS HRS is 10 times higher than that of the James Webb Space Telescope, opening a new window of the mid-infrared high-resolution spectroscopy from space. The main scientific themes are the galaxy formation and evolution and the planetary system formation and evolution. The GREX-PLUS WFC aims to detect the first generation of "bright" galaxies at redshift z > 15. The GREX-PLUS HRS aims to resolve the Kepler motion of water vapor molecules and identify the location of the water "snowline" in similar to 100 proto-planetary disks. Both instruments will provide unique data sets for a broad range of scientific topics including galaxy mass assembly, origin of super massive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmosphere, planetary atmosphere in the Solar system, and so on. This paper presents the status of the concept design of GREX-PLUS, including telescope system, WFC, HRS, cooling system, and spacecraft bus system.

Using the Subaru/FOCAS IFU capability, we examine the spatially resolved relationships between gas-phase metallicity, stellar mass, and star-formation rate surface densities (Sigma_* and Sigma_SFR, respectively) in extremely metal-poor galaxies (EMPGs) in the local universe. Our analysis includes 24 EMPGs, comprising 9,177 spaxels, which span a unique parameter space of local metallicity (12+log(O/H) = 6.9 to 7.9) and stellar mass surface density (Sigma_* 10^5 to 10^7 Msun/kpc^2), extending beyond the range of existing large integral-field spectroscopic surveys. Through spatially resolved emission line diagnostics based on the [NII] BPT-diagram, we verify the absence of evolved active galactic nuclei in these EMPGs. Our findings reveal that, while the resolved mass-metallicity relation exhibits significant scatter in the low-mass regime, this scatter is closely correlated with local star-formation surface density. Specifically, metallicity decreases as Sigma_SFR increases for a given Sigma_*. Notably, half of the EMPGs show a distinct metal-poor horizontal branch on the resolved mass-metallicity relation. This feature typically appears at the peak clump with the highest Sigma_* and Sigma_SFR and is surrounded by a relatively metal-enriched ambient region. These findings support a scenario in which metal-poor gas infall fuels episodic star formation in EMPGs, consistent with the kinematic properties observed in these systems. In addition, we identify four EMPGs with exceptionally low central metallicities (12+log(O/H) < 7.2), which display only a metal-poor clump without a surrounding metal-rich region. This suggests that such ultra-low metallicity EMPGs, at less than a few percent of the solar metallicity, may serve as valuable analogs for galaxies in the early stages of galaxy evolution.

We analyze the photometric data in the Wide layer of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) over similar to 1200 deg(2) to uncover new halo substructures beyond the distance, D-circle dot similar to 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 Bootes at about D-circle dot similar to 60 kpc and a new stream-like feature toward Pisces at around D-circle dot similar to 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.

We report the detection of a dwarf satellite candidate (Triangulum IV: Tri IV) of the Triangulum galaxy (M33) using the deep imaging of Subaru/Hyper Suprime-Cam (HSC). From the apparent magnitude of the horizontal branch in Tri IV, the heliocentric distance of Tri IV is estimated to be 932^+49_-43 kpc, indicating that Tri IV is located at the distance of 75^+48_-40 kpc from the M33 center. This means that Tri IV is the probable satellite of M33, because its distance from M33 is within the virial radius of M33. We also estimate its surface brightness of μ_ V = 29.72^+0.10_-0.10 mag arcsec^-2, and half-light radius of r_h = 1749^+523_-425 pc, suggesting that Tri IV is an ultra-diffuse galaxy or dynamically heated galaxy. The surface brightness of Tri IV is too low to be detected in the previous survey, so this detection suggests that much fainter satellites may be present in the outskirts of M33.

We analyze the outer regions of M33, beyond 15 kpc in projected distance from its center, using Subaru/Hyper Suprime-Cam multicolor imaging. We identify red giant branch (RGB) stars and red clump (RC) stars using the surface-gravity-sensitive NB515 filter for the RGB sample and a multicolor selection for both samples. We construct the radial surface density profiles of these RGB and RC stars and find that M33 has an extended stellar population with a shallow power-law index of α > −3, depending on the intensity of the contamination. This result represents a flatter profile than the stellar halo that was detected by the previous study focusing on the central region, suggesting that M33 may have a double-structured halo component, i.e., inner/outer halos or a very extended disk. Also, the slope of this extended component is shallower than those typically found for halos in large galaxies, implying intermediate-mass galaxies may have different formation mechanisms (e.g., tidal interaction) from large spirals. We also analyze the radial color profiles of RC/RGB stars and detect a radial gradient, consistent with the presence of an old and/or metal-poor population in the outer region of M33, thereby supporting our proposal that the stellar halo extends beyond 15 kpc. Finally, we estimate that the surface brightness of this extended component is μ _V = 35.72 ± 0.08 mag arcsec ^−2 . If our detected component is the stellar halo, this estimated value is consistent with the detection limit of previous observations.