Showing papers in "Publications of the Astronomical Society of Japan in 2019"

Cosmology from cosmic shear power spectra with Subaru Hyper Suprime-Cam first-year data

Abstract: We measure cosmic weak lensing shear power spectra with the Subaru Hyper Suprime-Cam (HSC) survey first-year shear catalog covering 137 deg^2 of the sky. Thanks to the high effective galaxy number density of ∼17 arcmin^−2, even after conservative cuts such as a magnitude cut of i < 24.5 and photometric redshift cut of 0.3 ≤ z ≤ 1.5, we obtain a high-significance measurement of the cosmic shear power spectra in four tomographic redshift bins, achieving a total signal-to-noise ratio of 16 in the multipole range 300 ≤ l ≤ 1900. We carefully account for various uncertainties in our analysis including the intrinsic alignment of galaxies, scatters and biases in photometric redshifts, residual uncertainties in the shear measurement, and modeling of the matter power spectrum. The accuracy of our power spectrum measurement method as well as our analytic model of the covariance matrix are tested against realistic mock shear catalogs. For a flat Λ cold dark matter model, we find |$S\,_{8}\equiv \sigma _8(\Omega _{\rm m}/0.3)^\alpha =0.800^{+0.029}_{-0.028}$| for α = 0.45 (⁠|$S\,_8=0.780^{+0.030}_{-0.033}$| for α = 0.5) from our HSC tomographic cosmic shear analysis alone. In comparison with Planck cosmic microwave background constraints, our results prefer slightly lower values of S_8, although metrics such as the Bayesian evidence ratio test do not show significant evidence for discordance between these results. We study the effect of possible additional systematic errors that are unaccounted for in our fiducial cosmic shear analysis, and find that they can shift the best-fit values of S_8 by up to ∼0.6 σ in both directions. The full HSC survey data will contain several times more area, and will lead to significantly improved cosmological constraints.

Big Three Dragons: A z = 7.15 Lyman-break galaxy detected in [O iii] 88 μm, [C ii] 158 μm, and dust continuum with ALMA

Abstract: We present new ALMA observations and physical properties of a Lyman break galaxy at z = 7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, M-UV approximate to -22.4, and has been spectroscopically identified in Ly alpha with a small rest-frame equivalent width of approximate to 4 angstrom. A previous Hubble Space TElescope (HST) image has shown that the target is composed of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [Oiii] 88 mu m, [Cii] 158 mu m, and their underlying dust continuum emission. In the whole system of B14-65666, the [Oiii] and [Cii] lines have consistent redshifts of 7.1520 +/- 0.0003, and the [Oiii] luminosity, (34.4 +/- 4.1)x 10(8)L(circle dot), is about three times higher than the [Cii] luminosity, (11.0 +/- 1.4) x 10(8)L(circle dot). With our two continuum flux densities, the dust temperature is constrained to be T-d approximate to 50-60K under the assumption of a dust emissivity index of beta(d) = 2.0-1.5, leading to a large total infrared luminosity of L-TIR approximate to 1 x 10(12)L(circle dot). Owing to our high spatial resolution data, we show that the [Oiii] and [Cii] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by approximate to 200kms(-1). We also find these two clumps have comparable UV, infrared, [Oiii], and [Cii] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major merger. The merger interpretation is also supported by the large specific star formation rate (defined as the star formation rate per unit stellar mass), sSFR Gyr(-1), inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [Oiii]-to-[Cii] luminosity ratio.

Achievements of Hinode in the first eleven years

Abstract: This is an open access article distributed under the terms of the Creative Commons CC BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

AKARI/IRC near-infrared asteroid spectroscopic survey: AcuA-spec

Abstract: Knowledge of water in the solar system is important for understanding of a wide range of evolutionary processes and the thermal history of the solar system. To explore the existence of water in the solar system, it is indispensable to investigate hydrated minerals and/or water ice on asteroids. These water-related materials show absorption features in the 3-$\micron$ band (wavelengths from 2.7 to 3.1 $\micron$). We conducted a spectroscopic survey of asteroids in the 3-$\micron$ band using the Infrared Camera (IRC) on board the Japanese infrared satellite AKARI. In the warm mission period of AKARI, 147 pointed observations were performed for 66 asteroids in the grism mode for wavelengths from 2.5 to 5 $\micron$. According to these observations, most C-complex asteroids have clear absorption features ($> 10\%$ with respect to the continuum) related to hydrated minerals at a peak wavelength of approximately 2.75 $\micron$, while S-complex asteroids have no significant feature in this wavelength range. The present data are released to the public as the Asteroid Catalog using AKARI Spectroscopic Observations (AcuA-spec).

Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). VIII. A less biased view of the early co-evolution of black holes and host galaxies

Abstract: We present ALMA [C II] line and far-infrared (FIR) continuum observations of three z > 6 low-luminosity quasars (M-1450 > -25 mag) discovered by our Subaru Hyper Suprime-Cam (HSC) survey. The [C II] line was detected in all three targets with luminosities of (2.4-9.5) x 10(8) L-circle dot, about one order ofmagnitude smaller than optically luminous (M-1450 less than or similar to -25 mag) quasars. The FIR continuum luminosities range from <9 x 10(10) L-circle dot (3 sigma limit) to similar to 2 x 10(12) L-circle dot, indicating a wide range in star formation rates in these galaxies. Most of the HSC quasars studied thus far show [C II]/FIR luminosity ratios similar to local star-forming galaxies. Using the [C II]-based dynamical mass (M-dyn) as a surrogate for bulge stellar mass (M-bulge), we find that a significant fraction of low-luminosity quasars are located on or even below the local M-BH-M-bulge relation, particularly at the massive end of the galaxy mass distribution. In contrast, previous studies of optically luminous quasars have found that black holes are overmassive relative to the local relation. Given the low luminosities of our targets, we are exploring the nature of the early co-evolution of supermassive black holes and their hosts in a less biased way. Almost all of the quasars presented in this work are growing their black hole mass at a much higher pace at z similar to 6 than the parallel growth model, in which supermassive black holes and their hosts grow simultaneously to match the local M-BH-M-bulge relation at all redshifts. As the low-luminosity quasars appear to realize the local co-evolutionary relation even at z similar to 6, they should have experienced vigorous starbursts prior to the currently observed quasar phase to catch up with the relation.

The mass–richness relation of optically selected clusters from weak gravitational lensing and abundance with Subaru HSC first-year data

Abstract: Constraining the relation between the richness $N$ and the halo mass $M$ over a wide redshift range for optically-selected clusters is a key ingredient for cluster-related science in optical surveys, including the Subaru Hyper Suprime-Cam (HSC) survey. We measure stacked weak lensing profiles around 1747 HSC CAMIRA clusters over a redshift range of $0.1\leq z_{\rm cl}\leq 1.0$ with $N\geq 15$ using the HSC first-year shear catalog covering $\sim$$140$ ${\rm deg^2}$. The exquisite depth and image quality of the HSC survey allow us to measure lensing signals around the high-redshift clusters at $0.7\leq z_{\rm cl}\leq 1.0$ with a signal-to-noise ratio of 19 in the comoving radius range $0.5\lesssim R\lesssim 15 h^{-1}{\rm Mpc}$. We constrain richness-mass relations $P(\ln N|M,z)$ of the HSC CAMIRA clusters assuming a log-normal distribution without informative priors on model parameters, by jointly fitting to the lensing profiles and abundance measurements under both Planck and WMAP cosmological models. We show that our model gives acceptable $p$-values when we add redshift dependent terms which are proportional to $\ln (1+z)$ and $[\ln (1+z)]^{2}$ into the mean and scatter relations of $P(\ln N|M,z)$. Such terms presumably originate from the variation of photometric redshift errors as a function of the redshift. We show that the constraints on the mean relation $\langle M|N \rangle$ are consistent between the Planck and WMAP models, whereas the scatter values $\sigma_{\ln M|N}$ for the Planck model are systematically larger than those for the WMAP model. We also show that the scatter values for the Planck model increase toward lower richness values, whereas those for the WMAP model are consistent with constant values as a function of richness. This result highlights the importance of the scatter in the mass-richness relation for cluster cosmology.

Long-term simulations of multi-dimensional core-collapse supernovae: Implications for neutron star kicks

Abstract: Core-collapse supernovae (CCSNe) are the final stage of massive stars, marking the birth of neutron stars (NSs). The aspherical mass ejection drives a natal kick of the forming NS. In this work we study the properties of the NS kick based on our long-term hydrodynamics CCSN simulations. We perform two-dimensional (2D) simulations for ten progenitors from a 10.8 to 20 $\, M_{\odot}$ star covering a wide range of the progenitor's compactness parameter, and two three-dimensional (3D) simulations for an 11.2 $\, M_{\odot}$ star. Our 2D models present a variety of explosion energies between $\sim 1.3 \times 10^{50}$ erg and $\sim 1.2 \times 10^{51}$ erg, and NS kick velocities between $\sim 100$ km s$^{-1}$ and $\sim 1500$ km s$^{-1}$. For the 2D exploding models, we find that the kick velocities tend to become higher with the progenitor's compactness. This is because the high progenitor compactness results in high neutrino luminosity from the proto-neutron star (PNS), leading to more energetic explosions. Since high-compactness progenitors produce massive PNSs, we point out that the NS masses and the kick velocities can be correlated, which is moderately supported by observation. Comparing 2D and 3D models of the 11.2 $\, M_{\odot}$ star, the diagnostic explosion energy in 3D is, as previously identified, higher than that in 2D, whereas the 3D model results in a smaller asymmetry in the ejecta distribution and a smaller kick velocity than in 2D. Our results confirm the importance of self-consistent CCSN modeling covering a long-term postbounce evolution in 3D for a quantitative prediction of the NS kicks.

Environmental impacts on molecular gas in protocluster galaxies at z ∼ 2

Abstract: We present the results from ALMA CO(3-2) observations of 66 Halpha-selected galaxies in three protoclusters around radio galaxies, PKS1138-262 (z=2.16) and USS1558-003 (z=2.53), and 4C23.56 (z=2.49). The pointing areas have an overdensity of ~100 compared to a mean surface number density of galaxies in field environments. We detect CO emission line in 16 star-forming galaxies, including previously published six galaxies, to measure the molecular gas mass. In the stellar mass range of 10.5 11.0 are comparable in mass to the scaling relation, or smaller. Our results suggest that the environmental effects on gas properties are mass-dependent: in high-density environments, gas accretion through cosmic filaments is accelerated in less massive galaxies while this is suppressed in the most massive system.

Cluster formation in the W 40 and Serpens South complex triggered by the expanding H ii region

Abstract: We present the results of mapping observations covering a large area of 1 square degree around W 40 and Serpens South carried out in the 12CO (J = 1–0), 13CO (J = 1–0), C18O (J = 1–0), CCS (JN = 87–76), and N2H+ (J = 1–0) emission lines with the 45 m Nobeyama Radio Telescope. W 40 is a blistered H ii region, and Serpens South is an infrared dark cloud accompanied by a young cluster. The relation between these two regions, which are separated by ∼20′ on the sky, has not been recognizable so far. We found the C18O emission is distributed smoothly throughout the W 40 and Serpens South regions, and that the two regions seem to be physically connected. We divided the C18O emission into four groups in terms of the spatial distributions around the H ii region which we call 5, 6, 7, and 8 km s−1 components according to their typical LSR velocity, and propose a three-dimensional model of the W 40 and Serpens South complex. We found two elliptical structures in the position–velocity diagrams, which can be explained as part of two expanding shells. One of the shells is small inner shell just around the H ii region, and the other is a large outer shell corresponding to the boundary of the H ii region. Dense gas associated with the young cluster of Serpens South is likely to be located at the surface of the outer shell, indicating that the natal clump of the young cluster is interacting with the outer shell being compressed by the expansion of the shell. We suggest that the expansion of the shell induced the formation of the young cluster.

An unbiased spectral line survey observation toward the low-mass star-forming region L1527

Abstract: An unbiased spectral line survey toward a solar-type Class 0/I protostar, IRAS04368+2557, in L1527 has been carried out in the 3 mm band with the Nobeyama 45 m telescope. L1527 is known as a warm carbon-chain chemistry (WCCC) source, which harbors abundant unsaturated organic species such as C$_n$H ($n = 3,\ 4,\ 5,\ldots$) in a warm and dense region near the protostar. The observation covers the frequency range from 80 to 116 GHz. A supplementary observation has also been conducted in the 70 GHz band to observe fundamental transitions of deuterated species. In total, 69 molecular species are identified, among which 27 species are carbon-chain species and their isomers, including their minor isotopologues. This spectral line survey provides us with a good template of the chemical composition of the WCCC source.

A molecular line survey toward the nearby galaxies NGC 1068, NGC 253, and IC 342 at 3 mm with the Nobeyama 45 m radio telescope: The data

Abstract: We present observational data of a molecular line survey toward the nearby galaxies NGC 1068, NGC 253, and IC 342 at the wavelengths of 3 mm ($\sim$85--116 GHz) obtained with the Nobeyama 45 m radio telescope. In IC 342 the line survey with high spectral resolution at the 3 mm region was reported for the first time. NGC 1068 is a nearby gas-rich galaxy with X-rays from an active galactic nucleus (AGN), and NGC 253 and IC 342 are nearby gas-rich galaxies with prototypical starbursts. These galaxies are useful to study the impacts of X-rays and ultraviolet radiation on molecular abundances. The survey was carried out with the resulting rms noise level of a few mK ($T\rm{_A^*}$). As a result we could obtain almost complete data of these galaxies at the 3 mm region: We detected 19--23 molecular species depending on the galaxies including several new detections (e.g., cyclic-C$_3$H$_2$ in IC 342). We found that the intensities of HCN, CN, and HC$_3$N relative to $^{13}$CO are significantly strong in NGC 1068 compared to those in NGC 253 and IC 342. On the other hand, CH$_3$CCH was not detected in NGC 1068. We obtained these results with the narrow beam (15$''$.2--19$''$.1) of the 45 m telescope among the single-dish telescopes, and in particular selectively observed the molecular gas close to the circumnuclear disk (CND) in NGC 1068. Our line intensities in NGC 1068 were compared to those obtained with the IRAM 30 m radio telescope already reported. As a result, the intensity ratio of each line was found to have information on the spatial distribution. Our observations obtained the line intensities and stringent constraints on the upper limit for the three galaxies with such narrow beam, and consequently, the data will be a basis for further observations with high spatial resolution.

Boötes. IV. A new Milky Way satellite discovered in the Subaru Hyper Suprime-Cam Survey and implications for the missing satellite problem

Abstract: 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 deg2 covered 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 deg2 area.

Magnetohydrodynamic simulation code CANS+: Assessments and applications

Abstract: We present a new magnetohydrodynamic (MHD) simulation package with the aim of providing accurate numerical solutions to astrophysical phenomena where discontinuities, shock waves, and turbulence are inherently important. The code implements the Harten–Lax–van Leer–discontinuitues (HLLD) approximate Riemann solver, the fifth-order-monotonicity-preserving interpolation (MP5) scheme, and the hyperbolic divergence cleaning method for a magnetic field. This choice of schemes has significantly improved numerical accuracy and stability, and saved computational costs in multidimensional problems. Numerical tests of one- and two-dimensional problems show the advantages of using the high-order scheme by comparing with results from a standard second-order total variation diminishing monotonic upwind scheme for conservation laws (MUSCL) scheme. The present code enables us to explore the long-term evolution of a three-dimensional accretion disk around a black hole, in which compressible MHD turbulence causes continuous mass accretion via nonlinear growth of the magneto-rotational instability (MRI). Numerical tests with various computational cell sizes exhibits a convergent picture of the early nonlinear growth of the MRI in a global model, and indicates that the MP5 scheme has more than twice the resolution of the MUSCL scheme in practical applications.

Interaction between the Northern Coalsack in the Cygnus OB 7 cloud complex and multiple supernova remnants including HB 21

Abstract: We report on possible interaction between multiple supernova remnants (SNRs) and the Northern Coalsack (NCS), which is a massive clump (∼1 × 103 M⊙) in the Cygnus OB 7 cloud complex and is forming a massive Class 0 object. We performed molecular observations of the 12CO(J = 1–0), 13CO(J = 1–0), and C18O(J = 1–0) emission lines using the 45 m telescope at the Nobeyama Radio Observatory, and we found that there are four main-velocity components at VLSR ≃ −20, −6, −4, and 10 km s−1. The −6 and −4 km s−1 components correspond to the systemic velocities of NCS and the Cyg OB 7 complex, respectively, and the other velocity components originate from distinct smaller clouds. Interestingly, there are apparent correlations and anticorrelations among the spatial distributions of the four components, suggesting that they are physically interacting with one another. On a larger scale, we find that a group of small clouds belonging to the −20 and 10 km s−1 components are located along two different arcs around some SNRs including HB 21, which has been suggested to be interacting with the Cyg OB 7 cloud complex, and we also find that NCS is located right at the interface of the arcs. The small clouds are likely to be the gas swept up by the stellar wind of the massive stars that created the SNRs. We suggest that the small clouds aligned along the two arcs recently encountered NCS, and the massive star formation in NCS was triggered by the strong interaction of the small clouds.

Cloud-cloud collision in the DR 21 cloud as a trigger of massive star formation

Abstract: We report on a possible cloud-cloud collision in the DR 21 region, which we found through molecular observations with the Nobeyama 45-m telescope. We mapped an area of 8'x12' around the region with twenty molecular lines including the 12CO(J=1-0) and 13CO(J=1-0) emission lines, and sixteen of them were significantly detected. Based on the 12CO and 13CO data, we found five distinct velocity components in the observed region, and we call molecular gas associated with these components -42, -22, -3, 9, and 17 km/s clouds taking after their typical radial velocities. The -3 km/s cloud is the main filamentary cloud (31,000 Mo) associated with young massive stars such as DR21 and DR21(OH), and the 9 km/s cloud is a smaller cloud (3,400 Mo) which may be an extension of the W75 region in the north. The other clouds are much smaller. We found a clear anticorrelation in the distributions of the -3 and 9 km/s clouds, and detected faint 12CO emission having intermediate velocities bridging the two clouds at their intersection. These facts strongly indicate that the two clouds are colliding against each other. In addition, we found that DR21 and DR21(OH) are located in the periphery of the densest part of the 9 km/s cloud, which is consistent with results of recent numerical simulations of cloud-cloud collisions. We therefore suggest that the -3 and 9 km/s clouds are colliding, and that the collision induced the massive star formation in the DR21 cloud. The interaction of the -3 and 9 km/s clouds was previously suggested by Dickel et al. (1978), and our results strongly support their hypothesis of the interaction.

Magnetic field structure in Serpens South

Abstract: We made near-infrared polarimetric observations toward Serpens South. This region contains three dense filaments that are roughly parallel to one another. Using the histogram of relative orientations, the three filaments are found to be roughly perpendicular to the global magnetic field. The morphology of the plane-of-sky (POS) magnetic field and molecular gas suggests that the magnetic field plays an important role in the filament formation and evolution. Applying the Davis–Chandrasekhar–Fermi method, the POS magnetic field strengths are estimated to be 10–80$\, \mu$G. The evaluated mass-to-flux ratios indicate that the center filament is magnetically supercritical, while the others are approximately magnetically critical. We speculate that the filaments are formed by fragmentation of a sheet-like cloud that was created through the gravitational contraction of a magnetized, turbulent cloud.

Stellar wind accretion and accretion disk formation: Applications to neutron star high-mass X-ray binaries

Abstract: Recent X-ray observations have revealed the complexity and diversity of high-mass X-ray binaries (HMXBs). This diversity challenges a classical understanding of the accretion process onto the compact objects. In this study, we reinforce the conventional concept of the nature of wind-fed accretion onto a neutron star considering the geometrical effect of radiatively accelerated wind, and re-evaluate the transported angular momentum by using a simple wind model. Our results suggest that even in an OB-type HMXB fed by stellar wind, a large amount of angular momentum could be transported to form an accretion disk due to the wind-inhomogeneity, if the binary separation is tight enough and/or stellar wind is slow. We apply our model into actual systems such as LMC X-4 and OAO 1657-415, and discuss the possibility of disk formations in these systems.

A Chandra and ALMA study of X-ray-irradiated gas in the central ∼100 pc of the Circinus galaxy

Abstract: We report a study of X-ray-irradiated gas in the central ~100 pc of the Circinus galaxy, hosting a Compton-thick active galactic nucleus (AGN), at 10-pc resolution using Chandra and ALMA. Based on ~200 ksec Chandra/ACIS-S data, we created an image of the Fe Kalpha line at 6.4 keV, tracing X-ray-irradiated dense gas. The ALMA data in Bands 6 (~270 GHz) and 7 (~350 GHz) cover five molecular lines: CO(J=3--2), HCN(J=3--2), HCN(J=4--3), HCO^+(J=3--2), and HCO^+(J=4--3). The detailed spatial distribution of dense molecular gas was revealed, and compared to the iron line image. The molecular gas emission appeared faint in regions with bright iron emission. Motivated by this, we quantitatively discuss the possibility that the molecular gas is efficiently dissociated by AGN X-ray irradiation (i.e., creating an X-ray-dominated region). Based on a non-local thermodynamic equilibrium model, we constrained the molecular gas densities and found that they are as low as interpreted by X-ray dissociation. Furthermore, judging from inactive star formation (SF) reported in the literature, we suggest that the X-ray emission has potential to suppress SF, particularly in the proximity of the AGN.

On the different levels of dust attenuation to nebular and stellar light in star-forming galaxies

Abstract: As a science verification study of the newly released AKARI/FIS Faint Source Catalog ver.1, this paper discusses the different levels of dust attenuation toward stellar light and nebular emission lines within local star-forming galaxies at 0.02 2,000 sources), we compare the dust attenuation levels toward stellar light (from L(IR)/L(UV) ratio) and nebular emission lines (from H-alpha/H-beta ratio). We find that there is a clear trend that more massive galaxies tend to have higher "extra" attenuation toward nebular regions, while galaxies with higher specific star formation rates tend to have lower extra attenuation. We also confirm these trends by using the WISE mid-infrared photometry with a significantly large sample size of the WISE-SDSS-GALEX galaxies (>50,000 sources). Finally, we study how the levels of extra attenuation toward nebular regions change across the SFR-Mstar plane. We find that, even at a fixed stellar mass, galaxies located below the main sequence tend to have higher levels of extra attenuation toward nebular regions, suggesting the change in dust geometry within the galaxies across the star-forming main sequence during the course of star formation quenching process.

The Hyper Suprime-Cam SSP Transient Survey in COSMOS: Overview

Abstract: We present an overview of a deep transient survey of the COSMOS field with the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77 deg^2 ultra-deep layer and 5.78 deg^2 deep layer in the Subaru Strategic Program over six- and four-month periods from 2016 to 2017, respectively. The ultra-deep layer reaches a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and 24.6 mag in g, r, i, z, and y bands, respectively; the deep layer is ∼0.6 mag shallower. In total, 1824 supernova candidates were identified. Based on light-curve fitting and derived light-curve shape parameter, we classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129 objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects are located at z > 1. Our unique data set doubles the number of Type Ia SNe at z > 1 and enables various time-domain analyses of Type II SNe, high-redshift superluminous SNe, variable stars, and active galactic nuclei.

The W-subtype active contact binary PZ UMa with a possible more massive tertiary component

Abstract: Two sets of multiple-color (B, V, R-C, I-C) light curves of PZ UMa were observed independently with the 2.4 m telescope at the Thai National Observatory and the 1 m telescope at Yunnan Observatories. The light curves were analyzed with the Wilson-Devinney program and the two sets of light curves produced consistent results, which shows that PZ UMa is a W-subtype contact binary with an extreme mass ratio (M-1/M-2 = 0.18). The basic physical parameters of PZ UMa were determined to be M-2 = 0.77(2) M-circle dot, M-1 = 0.14(1) M-circle dot, R-2 = 0.92(1) R-circle dot, R-1 = 0.43(1) R-circle dot, L-2 = 0.46(2) L-circle dot, and L-1 = 0.15(3) L-circle dot. The orbital period analysis of PZ UMa revealed a 13.22 yr periodicity, which implies that there may be a tertiary component orbiting around the binary system. The mass and orbital radius of the tertiary component were calculated to be M-3 = 0.88 M-circle dot and a(3) = 3.67 au, if the orbit was coplanar with the central binary system. It is interesting that the minimum mass of the tertiary was calculated to be M-3min = 0.84 M-circle dot, which means the tertiary component is even larger than the primary star and the secondary one of PZ UMa. PZ UMa is a late-type contact binary with stellar activity. The O'Connell effect appeared on its light curves when it was observed in 2016 April. However, the O'Connell effect reversed when the target was observed again in 2016 December. The changes of the O'Connell effect on such a short time-scale strongly support the occurrence of rapidly changing magnetic activity in this W UMa binary.

Starspots in contact and semi-detached binary systems

Abstract: We investigated the statistical properties of both cool and hot starspots in eclipsing binary stars. The starspot and binary parameters for contact and semi-detached systems were collected from literature, which were determined on the basis of synthetic light-curve analysis. We examined associations between these parameters. As a result, the cool spots in W-type binaries show properties similar to those of sunspots and starspots generated by dynamos, which differs from those of the cool spots in A-type binaries. The properties of hot spots also differ between the W- and A-type samples. From the physical properties of A- and W-type binaries, we infer that mass transfer is a dominant process for forming the hot spots in A-type binaries; and both mass transfer and magnetic activity can contribute to the formation of the hot spots in W-type binaries. Our results also indicate that the hot-spot size in the A-type sample is correlated with the temperature of spotted stars, orbital period, mass ratio, and fill-out factor.

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45 m telescope (FUGIN). V. Dense gas mass fraction of molecular gas in the Galactic plane

Abstract: Recent observations of the nearby Galactic molecular clouds indicate that the dense gas in molecular clouds have quasi-universal properties on star formation, and observational studies of extra galaxies have shown a galactic-scale correlation between the star formation rate (SFR) and surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of the dense gas in molecular clouds f_DG. In particular, for the Milky Way (MW), there are no previous studies resolving the f_DG disk over a scale of several kpc. In this study, the f_DG was measured over 5kpc in the first quadrant of the MW, based on the CO J=1-0 data in l=10-50 deg obtained as part of the FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) project. The total molecular mass was measured using 12CO, and the dense gas mass was estimated using C18O. The fractional masses including f_DG in the region within ~30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3kpc Arm, Norma Arm, Scutum Arm, Sagittarius Arm, and inter-arm regions) were measured. As a result, an averaged f_DG of 2.9^{+2.6}_{-2.6} % was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor of inefficient star formation in galaxies. It was also found that the f_DG shows large variations depending on the structures in the MW disk. The f_DG in the Galactic arms were estimated to be ~4-5%, while those in the bar and inter-arm regions were as small as ~0.1-0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to the differences in SFRs.

Nobeyama 45 m mapping observations toward Orion A. II. Classification of cloud structures and variation of the 13CO/C18O abundance ratio due to far-UV radiation

Abstract: We present results of the classification of cloud structures toward the Orion A Giant Molecular Cloud based on wide-field 12CO (J = 1–0), 13CO (J = 1–0), and C18O (J = 1–0) observations using the Nobeyama 45 m radio telescope. We identified 78 clouds toward Orion A by applying Spectral Clustering for Interstellar Molecular Emission Segmentation (SCIMES) to the data cube of the column density of 13CO. Well-known subregions such as OMC-1, OMC-2/3, OMC-4, OMC-5, NGC 1977, L1641-N, and the dark lane south filament (DLSF) are naturally identified as distinct structures in Orion A. These clouds can also be classified into three groups: the integral-shaped filament, the southern regions of Orion A, and the other filamentary structures in the outer parts of Orion A and the DLSF. These groups show differences in scaling relations between the physical properties of the clouds. We derived the abundance ratio between 13CO and C18O, $X_{^{13}\\mathrm{CO}}/X_{\\mathrm{C}^{18}\\mathrm{O}}$, which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of $X_{^{13}\\mathrm{CO}}/X_{\\mathrm{C}^{18}\\mathrm{O}}$ is also seen within a cloud in both the spatial and velocity directions and the ratio tends to be high at the edge of the cloud. The values of $X_{^{13}\\mathrm{CO}}/X_{\\mathrm{C}^{18}\\mathrm{O}}$ decrease from 17 to 10 with the median of the column densities of the clouds at the column density of $N_{\\mathrm{C^{18}O}} \\gtrsim 1 \\times 10^{15}\\:$cm−2 or visual extinction of AV ≳ 3 mag under the strong far-ultraviolet (FUV) environment of G0 > 103, whereas it is almost independent of the column density in the weak FUV radiation field. These results are explained if the selective photodissociation of C18O is enhanced under a strong FUV environment and it is suppressed in the dense part of the clouds.

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

Abstract: 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.

KOOLS-IFU: Kyoto Okayama Optical Low-dispersion Spectrograph with optical-fiber Integral Field Unit

Abstract: Observations of transient objects, such as short gamma-ray bursts (GRBs) and electromagnetic counterparts of gravitational wave sources, require prompt spectroscopy. To carry out prompt spectroscopy, we developed an optical-fiber integral field unit (IFU) and connected it with an existing optical spectrograph KOOLS. KOOLS with IFU, hereafter KOOLS--IFU, was mounted on the Okayama Astrophysical Observatory 188-cm telescope. The fiber core and cladding diameters of the fiber bundle are 100 $\mu$m and 125 $\mu$m, respectively, and 127 fibers are hexagonally close-packed in the sleeve of the two-dimensional (2D) fiber array. We conducted test observations to measure the KOOLS--IFU performance and obtained the following conclusions: (1) the spatial sampling is 2''.34 $\pm$ 0''.05 per fiber, and the total field of view (FoV) is 30''.4 $\pm$ 0''.65 with 127 fibers; (2) the observable wavelength and the spectral resolving power of the grisms of KOOLS are 4030--7310 A and 400--600; 5020--8830 A and 600--900; 4160--6000 A and 1000--1200; and 6150--7930 A and 1800--2400, respectively; and (3) the estimated limiting magnitude is 18.2--18.7 AB mag during 30-min exposure under the optimal condition.

High-resolution CO images of the Galactic central molecular zone

Abstract: We performed Nyquist-sampled mapping observations of the central molecular zone of our Galaxy in the J = 1–0 lines of CO, 13CO, and C18O using the 45 m telescope at the Nobeyama Radio Observatory. The newly obtained data sets were an improvement by a factor of four in spatial resolution of the CO data previously obtained with the same telescope 22 years ago, providing the highest angular resolution CO atlas of this special area of the Galaxy. The data cover the area: −0${^{\\circ}_{.}}$8 ≤ l ≤ +1${^{\\circ}_{.}}$4 and −0${^{\\circ}_{.}}$35 ≤ b ≤ +0${^{\\circ}_{.}}$35 with a 15″ beamwidth. Total intensity ratios for CO J = 3–2/J = 1–0, 13CO/CO J = 1–0 and C18O/13CO J = 1–0, are 0.70 ± 0.06, 0.12 ± 0.01, and 0.14 ± 0.01, respectively. The high-resolution CO images show the fine structure of the molecular gas and enable us to identify a number of compact clouds with broad velocity widths, i.e., high-velocity compact clouds. We conducted a detailed comparison of our CO J = 1–0 data with the CO J = 3–2 data obtained with the James Clerk Maxwell Telescope to derive the distribution and kinematics of the highly excited gas. Three, out of four, of the previously identified high CO J = 3–2/J = 1–0 ratio areas at l = +1${^{\\circ}_{.}}$3, 0${^{\\circ}_{.}}$0, and −0${^{\\circ}_{.}}$4 were confirmed with a higher spatial resolution. In addition to these, we identified several very compact, high CO J = 3–2/J = 1–0 spots with broad velocity widths for the first time. These are candidates for accelerated gas in the vicinity of invisible, point-like massive objects.