Showing papers in "Publications of the Astronomical Society of Japan in 2016"

The Subaru FMOS galaxy redshift survey (FastSound). IV. New constraint on gravity theory from redshift space distortions at z ∼ 1.4

Abstract: We measure the redshift-space correlation function from a spectroscopic sample of 2783 emission line galaxies from the FastSound survey. The survey, which uses the Subaru Telescope and covers the redshift ranges of $1.19<$z$<1.55$, is the first cosmological study at such high redshifts. We detect clear anisotropy due to redshift-space distortions (RSD) both in the correlation function as a function of separations parallel and perpendicular to the line of sight and its quadrupole moment. RSD has been extensively used to test general relativity on cosmological scales at $z<1$. Adopting a LCDM cosmology with the fixed expansion history and no velocity dispersion $\sigma_{\rm v}=0$, and using the RSD measurements on scales above 8Mpc/h, we obtain the first constraint on the growth rate at the redshift, $f(z)\sigma_8(z)=0.482\pm 0.116$ at $z\sim 1.4$ after marginalizing over the galaxy bias parameter $b(z)\sigma_8(z)$. This corresponds to $4.2\sigma$ detection of RSD. Our constraint is consistent with the prediction of general relativity $f\sigma_8\sim 0.392$ within the $1-\sigma$ confidence level. When we allow $\sigma_{\rm v}$ to vary and marginalize it over, the growth rate constraint becomes $f\sigma_8=0.494^{+0.126}_{-0.120}$. We also demonstrate that by combining with the low-z constraints on $f\sigma_8$, high-z galaxy surveys like the FastSound can be useful to distinguish modified gravity models without relying on CMB anisotropy experiments.

Three-dimensional distribution of the ISM in the Milky Way galaxy. III. The total neutral gas disk

Abstract: We present newly obtained three-dimensional gaseous maps of the Milky Way Galaxy; HI, H$_2$ and total-gas (HI plus H$_2$) maps, which were derived from the HI and $^{12}$CO($J=1$--0) survey data and rotation curves based on the kinematic distance. The HI and H$_2$ face-on maps show that the HI disk is extended to the radius of 15--20 kpc and its outskirt is asymmetric to the Galactic center, while most of the H$_2$ gas is distributed inside the solar circle. The total gas mass within radius 30 kpc amounts to $8.0\times 10^9$ M$_\odot$, 89\% and 11\% of which are HI and H$_2$, {respectively}. The vertical slices show that the outer HI disk is strongly warped and the inner HI and H$_2$ disks are corrugated. The total gas map is advantageous to trace spiral structure from the inner to outer disk. Spiral structures such as the Norma-Cygnus, the Perseus, the Sagittarius-Carina, the Scutum-Crux, and the Orion arms are more clearly traced in the total gas map than ever. All the spiral arms are well explained with logarithmic spiral arms with pitch angle of $11\degree$ -- $15\degree$. The molecular fraction to the total gas is high near the Galactic center and decreases with the Galactocentric distance. The molecular fraction also locally enhanced at the spiral arms compared with the inter-arm regions.

Mass constraint for a planet in a protoplanetary disk from the gap width

Abstract: A giant planet creates a gap in a protoplanetary disk, which might explain the observed gaps in protoplanetary disks. The width and depth of the gaps depend on the planet mass and disk properties. We have performed two-dimensional hydrodynamic simulations for various planet masses, disk aspect ratios, and viscosities, to obtain an empirical formula for the gap width. The gap width is proportional to the square root of the planet mass, −3/4 the power of the disk aspect ratio and −1/4 the power of the viscosity. This empirical formula enables us to estimate the mass of a planet embedded in the disk from the width of an observed gap. We have applied the empirical formula for the gap width to the disk around HL Tau, assuming that each gap observed by the Atacama Large Millimeter/submillimeter Array (ALMA) observations is produced by planets, and discussed the planet masses within the gaps. The estimate of planet masses from the gap widths is less affected by the observational resolution and dust filtration than that by the gap depth.

SXDF-ALMA 2-arcmin2 deep survey: 1.1-mm number counts

Abstract: We report 1.1-mm number counts revealed with the Atacama Large Millimeter/submillimeter Array (ALMA) in the Subaru/XMM-Newton Deep Survey Field (SXDF). The advent of ALMA enables us to reveal millimeter-wavelength number counts down to the faint end without source confusion. However, previous studies are based on the ensemble of serendipitously detected sources in fields originally targeting different sources and could be biased due to the clustering of sources around the targets. We derive number counts in the flux range of 0.2-2 mJy by using 23 (≥4σ) sources detected in a continuous 2.0-arcmin2 area of the SXDF. The number counts are consistent with previous results within errors, suggesting that the counts derived from serendipitously detected sources are not significantly biased, although there could be field-to-field variation due to the small survey area. By using the best-fitting function of the number counts, we find that ˜40% of the extragalactic background light at 1.1 mm is resolved at S1.1mm > 0.2 mJy.

East Asian observations of low-latitude aurora during the Carrington magnetic storm

Abstract: The magnetic storm around 1859 September 2, caused by so-called Carrington flare, was the most intense in the history of modern scientific observations, and hence is considered to be the benchmark event for space weather. The magnetic storm caused worldwide observations of auroras even at very low latitudes such as Hawaii, Panama, or Santiago, and the available magnetic field measurement at Bombay, India, showed two peaks: the main was the Carrington event which occurred in day time in East Asia, and a second storm after the Carrington event which occurred at night in East Asia. In this paper, we present a result from surveys of aurora records in East Asia, which provides new information of the aurora activity of this important event. We found some new East Asian records of low latitude aurora observations caused by the storm which occurred after the Carrington event. The size of the aurora belt of the second peak of the Carrington magnetic storm was even wider than usual low-latitude aurora events.

A search for a keV signature of radiatively decaying dark matter with Suzaku XIS observations of the X-ray diffuse background

Abstract: We performed the deepest search for an X-ray emission line between 0.5 and 7 keV from non-baryonic dark matter with the Suzaku XIS. Dark matter associated with the Milky Way galaxy was selected as the target to obtain the best signal-to-noise ratio. From the Suzaku archive, we selected 187 data sets of blank sky regions which were dominated by the X-ray diffuse background. The data sets were from 2005 to 2013. Instrumental responses were adjusted by multiple calibration data sets of the Crab Nebula. We also improved the technique of subtracting lines of instrumental origin. These energy spectra were well described by X-ray emission due to charge exchange around the Solar System, hot plasma in and around the Milky Way and superposition of extra-galactic point sources. A signal of a narrow emission line was searched for, and the significance of detection was evaluated in consideration of the blind search method (the Look-elsewhere Effect). Our results exhibited no significant detection of an emission line feature from dark matter. The 3$\sigma$ upper limit for the emission line intensity between 1 and 7 keV was $\sim10^{-2}$ photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$, or $\sim 5\times10^{-4}$ photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ per $M_\odot$ pc$^{-2}$, assuming a dark matter distribution with the Galactic rotation curve. The parameters of sterile neutrinos as candidates of dark matter were also constrained.

Radiation hydrodynamic simulations of line-driven disk winds for ultra-fast outflows

Abstract: Using two-dimensional radiation hydrodynamic simulations, we investigate origin of the ultra fast outflows (UFOs) that are often observed in luminous active galactic nuclei (AGNs). We found that the radiation force due to the spectral lines generates strong winds (line-driven disk winds) that are launched from the inner region of accretion disks (~30 Schwarzschild radii). A wide range of black hole masses ($M_{\rm BH}$) and Eddington ratios ($\varepsilon$) was investigated to study conditions for causing the line-driven winds. For $M_{\rm BH} = 10^6-10^9 M_\odot$ and $\varepsilon = 0.1-0.7$, funnel-shaped disk winds appear, in which dense matter is accelerated outward with an opening angle of 70-80 deg and with 10% of the light speed. If we observe the wind along its direction, the velocity, the column density, and the ionization state are consistent with those of the observed UFOs. As long as the obscuration by the torus does not affect the observations of X-ray bands, the UFOs could be statistically observed in about 13-28% of the luminous AGNs, which is not inconsistent with the observed ratio (~40%). We also found that the results are insensitive to the X-ray luminosity and the density of the disk surface. Thus, we can conclude that the UFOs could exist in any luminous AGNs, such as narrow-line Seyfert 1s (NLS1s) and quasars with $\varepsilon > 0.1$, in which fast line-driven winds are associated.

The New Numerical Galaxy Catalog (ν2GC): An updated semi-analytic model of galaxy and active galactic nucleus formation with large cosmological N-body simulations

Abstract: We present a new cosmological galaxy formation model, $ u^2$GC, as an updated version of our previous model $ u$GC. We adopt the so-called "semi-analytic" approach, in which the formation history of dark matter halos is computed by ${\it N}$-body simulations, while the baryon physics such as gas cooling, star formation and supernova feedback are simply modeled by phenomenological equations. Major updates of the model are as follows: (1) the merger trees of dark matter halos are constructed in state-of-the-art ${\it N}$-body simulations, (2) we introduce the formation and evolution process of supermassive black holes and the suppression of gas cooling due to active galactic nucleus (AGN) activity, (3) we include heating of the intergalactic gas by the cosmic UV background, and (4) we tune some free parameters related to the astrophysical processes using a Markov chain Monte Carlo method. Our ${\it N}$-body simulations of dark matter halos have unprecedented box size and mass resolution (the largest simulation contains 550 billion particles in a 1.12 Gpc/h box), enabling the study of much smaller and rarer objects. The model was tuned to fit the luminosity functions of local galaxies and mass function of neutral hydrogen. Local observations, such as the Tully-Fisher relation, size-magnitude relation of spiral galaxies and scaling relation between the bulge mass and black hole mass were well reproduced by the model. Moreover, the model also well reproduced the cosmic star formation history and the redshift evolution of rest-frame ${\it K}$-band luminosity functions. The numerical catalog of the simulated galaxies and AGNs is publicly available on the web.

A radiation-hydrodynamics model of accretion columns for ultra-luminous X-ray pulsars

Abstract: Prompted by the recent discovery of pulsed emission from an ultra-luminous X-ray source, M82 X-2 (\"ULX-pulsar\"), we perform a two-dimensional radiation-hydrodynamic simulation of a super-critical accretion flow onto a neutron star through a narrow accretion column. We set an accretion column with a cone shape filled with tenuous gas with density of $10^{-4} {\\rm g}~ {\\rm cm}^{-3}$ above a neutron star and solve the two dimensional gas motion and radiative transfer within the column. The side boundaries are set such that radiation can freely escape, while gas cannot. Since the initial gas layer is not in a hydrostatic balance, the column gas falls onto the neutron-star surface, thereby a shock being generated. As a result, the accretion column is composed of two regions: an upper, nearly free-fall region and a lower settling region, as was noted by Basko \\& Sunyaev (1976). The average accretion rate is very high; ${\\dot M}\\sim 10^{2-3} L_{\\rm E}/c^2$ (with $L_{\\rm E}$ being the Eddington luminosity), and so radiation energy dominates over gas internal energy entirely within the column. Despite the high accretion rate, the radiation flux in the laboratory frame is kept barely below $L_{\\rm E}/(4\\pi r^2)$ at a distance $r$ in the settling region so that matter can slowly accrete. This adjustment is made possible, since large amount of photons produced via dissipation of kinetic energy of matter can escape through the side boundaries. The total luminosity can greatly exceed $L_{\\rm E}$ by several orders of magnitude, whereas the apparent luminosity observed from the top of the column is much less. Due to such highly anisotropic radiation fields, observed flux should exhibit periodic variations with the rotation period, provided that the rotation and magnetic axes are misaligned.

Rotation curve decomposition for size–mass relations of bulge, disk, and dark halo components in spiral galaxies

Abstract: Rotation curves of more than one hundred spiral galaxies were compiled from the literature, and deconvolved into bulge, disk, and dark halo using χ fitting in order to determine their scale radii and masses. Correlation analyses were obtained of the fitting parameters for galaxies that satisfied selection and accuracy criteria. Size-mass relations indicate that the sizes and masses are positively correlated among different components in such a way that the larger or more massive is the dark halo, the larger or more massive are the disk and bulge. Empirical size-mass relations were obtained for bulge, disk and dark halo by the least-squares fitting. The disk-to-halo mass ratio was found to be systematically greater by a factor of three than that predicted by cosmological simulations combined with photometry. A preliminary mass function for dark halo was obtained, which is represented by the Schechter function followed by a power law.

J-GEM follow-up observations to search for an optical counterpart of the first gravitational wave source GW150914

Abstract: We present our optical follow-up observations to search for an electromagnetic counterpart of the first gravitational wave source GW150914 in the framework of the Japanese collaboration for Gravitational wave ElectroMagnetic follow-up (J-GEM), which is an observing group utilizing optical and radio telescopes in Japan, as well as those in New Zealand, China, South Africa, Chile, and Hawaii. We carried out a wide-field imaging survey with Kiso Wide Field Camera (KWFC) on the 1.05-m Kiso Schmidt telescope in Japan and a galaxy-targeted survey with Tripole5 on the B&C 61-cm telescope in New Zealand. Approximately 24 deg2 regions in total were surveyed in i-band with KWFC and 18 nearby galaxies were observed with Tripole5 in g-, r-, and i-bands 4-12 days after the gravitational wave detection. Median 5-sigma depths are i~18.9 mag for the KWFC data and g~18.9 mag, r~18.7 mag, and i~18.3 mag for the Tripole5 data. Probability for a counterpart to be in the observed area is 1.2% in the initial skymap and 0.1% in the final skymap. We do not find any transient source associated to an external galaxy with spatial offset from its center, which is consistent with the local supernova rate. We summarize future prospects and ongoing efforts to pin down electromagnetic counterparts of binary black hole mergers as well as neutron star mergers.

Survey of period variations of superhumps in SU UMa-type dwarf novae. VIII. The eighth year (2015–2016)

Abstract: This work was supported by the Grant-in-Aid “Initiative for High-Dimensional Data-Driven Science through Deepening of Sparse Modeling” (25120007) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

Machine-learning selection of optical transients in the Subaru/Hyper Suprime-Cam survey

Abstract: We present an application of machine-learning (ML) techniques to source selection in the optical transient survey data with Hyper Suprime-Cam (HSC) on the Subaru telescope. Our goal is to select real transient events accurately and in a timely manner out of a large number of false candidates, obtained with the standard difference-imaging method. We have developed the transient selector which is based on majority voting of three ML machines of AUC Boosting, Random Forest, and Deep Neural Network. We applied it to our observing runs of Subaru-HSC in 2015 May and August, and proved it to be efficient in selecting optical transients. The false positive rate was 1.0% at the true positive rate of 90% in the magnitude range of 22.0–25.0 mag for the former data. For the latter run, we successfully detected and reported ten candidates of supernovae within the same day as the observation. From these runs, we learned the following lessons: (1) the training using artificial objects is effective in filtering out false candidates, especially for faint objects, and (2) combination of ML by majority voting is advantageous.

Suzaku spectra of a Type-II supernova remnant, Kes 79

Abstract: This paper reports results of a Suzaku observation of the supernova remnant (SNR) Kes 79 (G33.6+0.1). The X-ray spectrum is best fitted by a two-temperature model: a non-equilibrium ionization (NEI) plasma and a collisional ionization equilibrium (CIE) plasma. The NEI plasma is spatially confined within the inner radio shell with kT~0.8 keV, while the CIE plasma is found in more spatially extended regions associated with the outer radio shell with kT~0.2 keV and solar abundance. Therefore, the NEI plasma is attributable to the SN ejecta and the CIE plasma is forward shocked interstellar medium. In the NEI plasma, we discovered K-shell line of Al, Ar and Ca for the first time. The abundance pattern and estimated mass of the ejecta are consistent with the core-collapse supernova explosion of a ~30-40 solar mass progenitor star. An Fe line with center energy of ~6.4 keV is also found in the southeast (SE) portion of the SNR, a close peripheral region around dense molecular clouds. One possibility is that the line is associated with the ejecta. However, the centroid energy of ~6.4 keV and the spatial distribution of enhancement near the SE peripheral do not favor this scenario. Since the ~6.4 keV emitting region coincides to the molecular clouds, we propose another possibility that the Fe line is due to K-shell ionization of neutral Fe by the interaction of locally accelerated protons (LECRp) with the surrounding molecular cloud. Both these possibilities, heated ejecta or LECRp origin, are discussed based on the observational facts.

Scale heights and equivalent widths of the iron K-shell lines in the Galactic diffuse X-ray emission

Abstract: This paper reports the analysis of the X-ray spectra of the Galactic diffuse X-ray emission (GDXE) in the Suzaku archive. The fluxes of the Fe I K alpha (6.4 keV), Fe XXV,He alpha (6.7 keV) and Fe XXVI Ly alpha (6.97 keV) lines are separately determined. From the latitude distributions, we confirm that the GDXE is decomposed into the Galactic center (GCXE), the Galactic bulge (GBXE) and the Galactic ridge (GRXE) X-ray emissions. The scale heights (SHs) of the Fe XXV He alpha line of the GCXE, GBXE and GRXE are determined to be ~40, ~310 and ~140 pc, while those of the Fe I K alpha line are ~30, ~160 and ~70 pc, respectively. The mean equivalent widths (EWs) of the sum of the Fe XXV He alpha and Fe XXVI Ly alpha lines are ~750 eV, ~600 eV and ~550 eV, while those of the Fe I K alpha line are ~150~eV, ~60~eV and ~100~eV for the GCXE, GBXE and GRXE, respectively. The origin of the GBXE, GRXE and GCXE is separately discussed based on the new results of the SHs and EWs, in comparison with those of the Cataclysmic Variables (CVs), Active Binaries (ABs) and Coronal Active stars (CAs).

Large X-ray flares on stars detected with MAXI/GSC: A universal correlation between the duration of a flare and its X-ray luminosity

Abstract: 23 giant flares from 13 active stars (eight RS CVn systems, one Algol system, three dMe stars and one YSO) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 10^(31-34) ergs s-1 in the 2-20 keV band, the emission measure of 10^(54-57) cm-3, the e-folding time of 1 hour to 1.5 days, and the total radiative energy released during the flare of 10^(34-39) ergs. Notably, the peak X-ray luminosity of 5(3-9)*10^33 ergs s-1 in the 2-20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest ever observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and our own data, we discovered a universal correlation of tau~L_X^0.2 for the flare duration tau and the intrinsic X-ray luminosity L_X in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in tau and L_X, respectively. The MAXI/GSC sample is located at the highest ends on the correlation.

Angular momentum of the N2H+ cores in the Orion A cloud

Abstract: We have analyzed the angular momentum of the molecular cloud cores in the Orion A giant molecular cloud observed in the N 2 H + J = 1–0 line with the Nobeyama 45 m radio telescope. We have measured the velocity gradient using position–velocity diagrams passing through core centers, and made sinusoidal fits against the position angle. Twenty-seven out of 34 N 2 H + cores allowed us to measure the velocity gradient without serious confusion. The derived velocity gradient ranges from 0.5 to 7.8 km s −1 pc −1 . We marginally found that the specific angular momentum J/M (against the core radius R) of the Orion N 2 H + cores tends to be systematically larger than that of molecular cloud cores in cold dark clouds obtained by Goodman et al., in the J/M–R relation. The ratio β of rotational to gravitational energy is derived to be β = 10 −2.3±0.7 , and is similar to that obtained for cold dark cloud cores in a consistent definition. The large-scale rotation of the ∫-shaped filament of the Orion A giant molecular cloud does not likely govern the core rotation at smaller scales.

V1006 Cygni: Dwarf nova showing three types of outbursts and simulating some features of the WZ Sge-type behavior

Abstract: We observed the 2015 July-August long outburst of V1006 Cyg and established this object to be an SU UMa-type dwarf nova in the period gap. Our observations have confirmed that V1006 Cyg is the second established object showing three types of outbursts (normal, long normal and superoutbursts) after TU Men. We have succeeded in recording the growing stage of superhumps (stage A superhumps) and obtained a mass ratio of 0.26-0.33, which is close to the stability limit of tidal instability. This identification of stage A superhumps demonstrated that superhumps indeed slowly grow in systems near the stability limit, the idea first introduced by Kato et al. 2014, arXiv:1406.6428). The superoutburst showed a temporary dip followed by a rebrightening. The moment of the dip coincided with the stage transition of superhumps, and we suggest that stage C superhumps is related to the start of the cooling wave in the accretion disk. We interpret that the tidal instability was not strong enough to maintain the disk in the hot state when the cooling wave started. We propose that the properties commonly seen in the extreme ends of mass ratios (WZ Sge-type objects and long-period systems) can be understood as a result of weak tidal effect.

Extreme asymmetry in the polarized disk of V1247 Orionis

Abstract: We present the first near-infrared scattered-light detection of the transitional disk around V1247 Ori, which was obtained using high-resolution polarimetric differential imaging observations with Subaru/HiCIAO. Our imaging in the H band reveals the disk morphology at separations of approx.0.14-0.86 (54-330 au) from the central star. The polarized intensity image shows a remarkable arc-like structure toward the southeast of the star, whereas the fainter northwest region does not exhibit any notable features. The shape of the arm is consistent with an arc of 0.28 +/- 0.09 in radius (108 au from the star), although the possibility of a spiral arm with a small pitch angle cannot be excluded. V1247 Ori features an exceptionally large azimuthal contrast in scattered, polarized light; the radial peak of the southeastern arc is about three times brighter than the northwestern disk measured at the same distance from the star. Combined with the previous indication of an inhomogeneous density distribution in the gap at 46 au, the notable asymmetry in the outer disk suggests the presence of unseen companions and/or planet-forming processes ongoing in the arc.

Rapid merger of binary primordial black holes: An implication for GW150914

Abstract: We propose a new scenario for the evolution of a binary of primordial black holes (PBHs). We consider a dynamical friction by ambient dark matter, scattering of dark matter particles with a highly eccentric orbit besides the standard two-body relaxation process to refill the loss cone, and interaction between the binary and a circumbinary disk, assuming that PBHs do not constitute the bulk of dark matter. Binary PBHs lose the energy and angular momentum by these processes, which could be sufficiently efficient for a typical configuration. Such a binary coalesces due to the gravitational wave emission in a time scale much shorter than the age of the universe. We estimate the density parameter of the resultant gravitational wave background. Astrophysical implication concerning the formation of intermediate-mass to supermassive black holes is also discussed.

CO Multi-line Imaging of Nearby Galaxies (COMING). I. Physical properties of molecular gas in the barred spiral galaxy NGC 2903

Abstract: We present simultaneous mappings of J=1-0 emission of 12CO, 13CO, and C18O molecules toward the whole disk (8' x 5' or 20.8 kpc x 13.0 kpc) of the nearby barred spiral galaxy NGC 2903 with the Nobeyama Radio Observatory 45-m telescope at an effective angular resolution of 20" (or 870 pc). We detected 12CO(J=1-0) emission over the disk of NGC 2903. In addition, significant 13CO(J=1-0) emission was found at the center and bar-ends, whereas we could not detect any significant C18O(J=1-0) emission. In order to improve the signal-to-noise ratio of CO emission and to obtain accurate line ratios of 12CO(J=2-1)/12CO(J=1-0) ($R_{2-1/1-0}$) and 13CO(J=1-0)/12CO(J=1-0) ($R_{13/12}$), we performed the stacking analysis for our 12CO(J=1-0), 13CO(J=1-0), and archival 12CO(J=2-1) spectra with velocity-axis alignment in nine representative regions of NGC 2903. We successfully obtained the stacked spectra of the three CO lines, and could measure averaged $R_{2-1/1-0}$ and $R_{13/12}$ with high significance for all the regions. We found that both $R_{2-1/1-0}$ and $R_{13/12}$ differ according to the regions, which reflects the difference in the physical properties of molecular gas; i.e., density ($n_{\rm H_2}$) and kinetic temperature ($T_K$). We determined $n_{\rm H_2}$ and $T_K$ using $R_{2-1/1-0}$ and $R_{13/12}$ based on the large velocity gradient approximation. The derived $n_{\rm H_2}$ ranges from ~ 1000 cm$^{-3}$ (in the bar, bar-ends, and spiral arms) to 3700 cm$^{-3}$ (at the center) and the derived $T_K$ ranges from 10 K (in the bar and spiral arms) to 30 K (at the center). We examined the dependence of star formation efficiencies (SFEs) on $n_{\rm H_2}$ and $T_K$, and found the positive correlation between SFE and $n_{\rm H_2}$ with the correlation coefficient for the least-square power-law fit $R^2$ of 0.50. This suggests that molecular gas density governs the spatial variations in SFEs.

Galactic center mini-spiral by ALMA: Possible origin of the central cluster

Abstract: We present continuum images of the "Galactic Center Mini-spiral" of 100, 250, and 340 GHz bands with the analysis of the Cy.0 data acquired from the Atacama Large Millimeter/submillimeter Array (ALMA) archive. Pretty good UV coverage of the data and the "self-calibration" method give us an opportunity to obtain dynamic ranges of over 2x10^4 in the resultant maps of the 250 and 340 GHz bands. In particular the image of the 340 GHz band has high dynamic ranges unprecedented in sub-millimeter wave. The angular resolutions attain to 1.57"x1.33" in the 100 GHz band, 0.63"x0.53" in the 250 GHz band, and 0.44"x0.38" in the 340 GHz band, respectively. The continuum images clearly depict the "Mini-spiral", which is an ionized gas stream in the vicinity of Sgr A*. We found the tight correlation between the dust emission peaks and the OB/WR stars in the Northern-arm of the "Mini-spiral". The core mass function of the dust core identified by the clumpfind algorithm would obey the flat power-law dN/dM=aM^-1.5+/-0.4 on the high-mass side. These support the scenario that the star forming cloud has fallen into the immediate vicinity of Sgr A* for the origin of the Central cluster.

Evolution and Statistics of Non-Sphericity of Dark Matter Halos from Cosmological N-Body Simulation

Abstract: We revisit the non-sphericity of cluster-mass scale halos from cosmological N-body simulation on the basis of triaxial modelling. In order to understand the difference between the simulation results and the conventional ellipsoidal collapse model (EC), we first consider the evolution of individual simulated halos. The major difference between EC and the simulation becomes appreciable after the turn-around epoch. Moreover, it is sensitive to the individual evolution history of each halo. Despite such strong dependence on individual halos, the resulting nonsphericity of halos exhibits weak but robust mass dependence in a statistical fashion; massive halos are more spherical up to the turn-around, but gradually become less spherical by z = 0. This is clearly inconsistent with the EC prediction; massive halos are usually more spherical. In addition, at z=0, inner regions of the halos are less spherical than outer regions, i.e., the density distribution inside the halos is highly inhomogeneous and therefore not self-similar. Since most of previous fitting formulae for the PDF of axis ratio of triaxial ellipsoids have been constructed under the self-similarity assumption, they are not accurate. Indeed, we compute the PDF of projected axis ratio a1/a2 directly from the simulation data without the self-similarity assumption, and find that it is very sensitive to the assumption. The latter needs to be carefully taken into account in direct comparison with observations, and therefore we provide an empirical fitting formula for the PDF of a1/a2. Our preliminary analysis suggests that the derived PDF of a1/a2 roughly agrees with the current weak-lensing observations. More importantly, the present results will be useful in future exploration of the non-sphericity of clusters in X-ray and optical observations.

Millimeter-sized grains in the protostellar envelopes: Where do they come from?

Abstract: Grain growth during star formation affects the physical and chemical processes in the evolution of star-forming clouds. We investigate the origin of the millimeter (mm)-sized grains recently observed in Class I protostellar envelopes. We use the coagulation model developed in our previous paper and find that a hydrogen number density of as high as $10^{10}~{\rm cm^{-3}}$, instead of the typical density $10^5~{\rm cm^{-3}}$, is necessary for the formation of mm-sized grains. Thus, we test a hypothesis that such large grains are transported to the envelope from the inner, denser parts, finding that gas drag by outflow efficiently "launches" the large grains as long as the central object has not grown to $\gtrsim 0.1$ M$_{\odot}$. By investigating the shattering effect on the mm-sized grains, we ensure that the large grains are not significantly fragmented after being injected in the envelope. We conclude that the mm-sized grains observed in the protostellar envelopes are not formed in the envelopes but formed in the inner parts of the star-forming regions and transported to the envelopes before a significant mass growth of the central object, and that they survive in the envelopes.