Naoki Yoshida

The University of Tokyo, Tōkyō, Japan

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Publications (132)614.05 Total impact

  • Gen Chiaki, Naoki Yoshida
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    ABSTRACT: We present a novel method for particle splitting in smoothed particle hydrodynamics simulations. Our method utilizes the Voronoi diagram for a given particle set to determine the position of fine daughter particles. We perform several test simulations to compare our method with a conventional splitting method in which the daughter particles are placed isotropically over the local smoothing length. We show that, with our method, the density deviation after splitting is reduced by a factor of about two compared with the conventional method. Splitting would smooth out the anisotropic density structure if the daughters are distributed isotropically, but our scheme allows the daughter particles to trace the original density distribution with length scales of the mean separation of their parent. We apply the particle splitting to simulations of the primordial gas cloud collapse. The thermal evolution is accurately followed to the hydrogen number density of 10^12 /cc. With the effective mass resolution of ~10^-4 Msun after the multi-step particle splitting, the protostellar disk structure is well resolved. We conclude that the method offers an efficient way to simulate the evolution of an interstellar gas and the formation of stars.
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    ABSTRACT: Weak gravitational lensing enables us to search clusters without the conventional assumption on the relation between visible and dark matter. We explore a variety of statistics of clusters selected with cosmic shear measurement by utilizing both analytic models and large numerical simulations. We first develop a halo model to predict the abundance and the clustering of weak lensing selected clusters. Observational effects such as galaxy shape noise are included in our model. We then generate realistic mock weak lensing catalogs to test the accuracy of our analytic model. To this end, we perform full-sky ray-tracing simulations that allow us to have multiple realizations of a large continuous area. We model the masked regions on the sky using the actual positions of bright stars, and generate 200 mock weak lensing catalogs with sky coverage of $\sim$1000 squared degrees. We utilize the large set of mock catalogs to evaluate the covariance matrices between the local and non-local statistics. We show that our theoretical model agrees well with the ensemble average of statistics and their covariances calculated directly from the mock catalogues. With a typical selection threshold, ignoring shape noise correction causes overestimation of the clustering of weak lensing selected clusters with a level of about $10\%$, and shape noise correction boosts the cluster abundance by a factor of a few. We calculate the cross-covariances using the halo model with accounting for the effective reduction of the survey area due to masks. The covariance of the cosmic shear auto power spectrum is affected by the mode-coupling effect that originates from sky masking. Our model and the results can be readily used for cosmological analysis with ongoing and future weak lensing surveys.
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    ABSTRACT: While observations of large-scale structure and the cosmic microwave background (CMB) provide strong constraints on the amplitude of the primordial power spectrum (PPS) on scales larger than 10 Mpc, the amplitude of the power spectrum on sub-galactic length scales is much more poorly constrained. We study early structure formation in a cosmological model with a blue-tilted PPS. We assume that the standard scale-invariant PPS is modified at small length scales as $P(k) \sim k^{m_{\rm s}}$ with $m_{\rm s} > 1$. We run a series of cosmological hydrodynamic simulations to examine the dependence of the formation epoch and the characteristic mass of primordial stars on the tilt of the PPS. In models with $m_{\rm s} > 1$, star-forming gas clouds are formed at $z > 100$, when formation of hydrogen molecules is inefficient because the intense CMB radiation destroys chemical intermediates. Without efficient coolant, the gas clouds gravitationally contract while keeping a high temperature. The protostars formed in such "hot" clouds grow very rapidly by accretion to become extremely massive stars that may leave massive black holes with a few hundred solar-masses at $z > 100$. The shape of the PPS critically affects the properties and the formation epoch of the first generation of stars. Future experiments of the CMB polarization and the spectrum distortion may provide important information on the nature of the first stars and their formation epoch, and hence on the shape of the small-scale power spectrum.
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    Aravind Natarajan, Nick Zhu, Naoki Yoshida
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    ABSTRACT: Recent observations of the cosmic microwave background (CMB) anisotropies and the distribution of galaxies, galaxy clusters, and the Lyman Alpha forest have constrained the shape of the power spectrum of matter fluctuations on large scales k < few h/Mpc. We explore a new technique to constrain the matter power spectrum on smaller scales, assuming the dark matter is a Weakly Interacting Massive Particle (WIMP) that annihilates at early epochs. Energy released by dark matter annihilation can modify the spectrum of CMB temperature fluctuations and thus CMB experiments such as Planck have been able to constrain the quantity f /m < 1/88 picobarn c / GeV, where f is the fraction of energy absorbed by gas, is the annihilation rate assumed constant, and m is the particle mass. We assume the standard scale-invariant primordial matter power spectrum of P_prim(k) ~ k^{n_s} at large scales k < k_p, while we adopt the modified power law of P_prim(k) ~ k_p^{n_s} (k/k_p)^{m_s} at small scales. We then aim at deriving constraints on m_s. For m_s > n_s, the excess small-scale power results in a much larger number of nonlinear small mass halos, particularly at high redshifts. Dark matter annihilation in these halos releases sufficient energy to partially ionize the gas, and consequently modify the spectrum of CMB fluctuations. We show that the recent Planck data can already be used to constrain the power spectrum on small scales. For a simple model with an NFW profile with halo concentration parameter c_200 = 5 and f / m = 1/100 picobarn c / GeV, we can limit the mass variance sigma_{max} < 100 at the 95% confidence level, corresponding to a power law index m_s < 1.43 (1.63) for k_p = 100 (1000) h/Mpc. Our results are also relevant to theories that feature a running spectral index.
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    Ken Osato, Masato Shirasaki, Naoki Yoshida
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    ABSTRACT: We study the impact of baryonic physics on cosmological parameter estimation with weak lensing surveys. We run a set of cosmological hydrodynamics simulations with different galaxy formation models. We then perform ray-tracing simulations through the total matter density field to generate 100 independent convergence maps of 25 deg$^2$ field-of-view, and use them to examine the ability of the following three lensing statistics as cosmological probes; power spectrum, peak counts, and Minkowski Functionals. For the upcoming wide-field observations such as Subaru Hyper Suprime-Cam (HSC) survey with a sky coverage of 1400 deg$^2$, the higher-order statistics provide tight constraints on the matter density, density fluctuation amplitude, and dark energy equation of state, but appreciable parameter bias is induced by the baryonic processes such as gas cooling and stellar feedback. When we use power spectrum, peak counts, and Minkowski Functionals, the relative bias in the dark energy equation of state parameter $w$ is at a level of, respectively, $\sim0.06\sigma$, $0.5-0.6\sigma$, and $0.01-0.1\sigma$ where $\sigma$ is the overall error derived from Fisher analysis. We find the bias is induced in different directions in the parameter space depending on the statistics employed. While the two-point statistics, i.e. power spectrum, yield robust results against baryonic effects, the overall constraining power is weak compared with the other higher-order statistics. On the other hand, using higher-order statistics alone results in significantly biased parameter estimate. We suggest to use an optimized combination of, for example, power spectrum and higher-order statistics so that the baryonic effects on parameter estimation are mitigated. Such `calibrated' combination can place stringent and robust constraints on cosmological parameters.
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    ABSTRACT: We perform a large set of cosmological simulations of early structure formation and follow the formation and evolution of 1540 star-forming gas clouds to derive the mass distribution of primordial stars. The star formation in our cosmological simulations is characterized by two distinct populations, the so-called Population III.1 stars and primordial stars formed under the influence of far ultraviolet (FUV) radiation (Population III.2D stars). In this work, we determine the stellar masses by using the dependences on the physical properties of star-forming cloud and/or the external photodissociating intensity from nearby primordial stars, which are derived from the results of two-dimensional radiation hydrodynamic simulations of protostellar feedback. The characteristic mass of the Pop III stars is found to be a few hundred solar masses at z ~ 25, and it gradually shifts to lower masses with decreasing redshift. At high redshifts z > 20, about half of the star-forming gas clouds are exposed to intense FUV radiation and thus give birth to massive Pop III.2D stars. However, the local FUV radiation by nearby Pop III stars becomes weaker at lower redshifts, when typical Pop III stars have smaller masses and the mean physical separation between the stars becomes large owing to cosmic expansion. Therefore, at z < 20, a large fraction of the primordial gas clouds host Pop III.1 stars. At z =< 15, the Pop III.1 stars are formed in relatively cool gas clouds due to efficient radiative cooling by H_2 and HD molecules; such stars have masses of a few x 10 Msun. Since the stellar evolution and the final fate are determined by the stellar mass, Pop III stars formed at different epochs play different roles in the early universe.
    Monthly Notices of the Royal Astronomical Society 01/2015; 448(1). DOI:10.1093/mnras/stv044 · 5.23 Impact Factor
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    Akira Harada, Ayuki Kamada, Naoki Yoshida
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    ABSTRACT: We perform a set of cosmological simulations of structure formation in a mixed dark matter (MDM) model. Our model is motivated by the recently identified $3.5\,{\rm keV}$ X-ray line that can be explained by the decay of non-resonantly produced sterile neutrinos, if they account for $10$-$60\%$ of the dark matter in the Universe. The non-resonantly produced sterile neutrino has sizable free-streaming length and hence behaves effectively as warm dark matter (WDM). Assuming the rest of dark matter is composed of some stable and cold particles, i.e. cold dark matter (CDM), we follow the coevolution of the CDM and WDM density perturbations. Specifically, we consider the models with the warm component fraction of $r_{\rm warm}=0.25$ and $0.5$. Our MDM model predicts that the comoving Jeans length at the matter-radiation equality is close to that of the thermally produced warm dark matter model with particle mass $m_{\rm WDM}=2.4\,{\rm keV}$ but that the suppression in the fluctuation power spectrum is weaker. We perform large $N$-body simulations to study the structure of nonlinear dark halos in the MDM model. The abundance of substructure is significantly reduced in the MDM model, and hence the so-called small scale crisis is mitigated. The cumulative maximum circular velocity function (CVF) of at least one halo in the MDM models is in good agreement with the CVFs of the observed satellites in the Milky Way and Andromeda. We argue that the MDM models open an interesting possibility to reconcile the reported $3.5\,{\rm keV}$ line and the internal structure of galaxies.
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    ABSTRACT: We investigate the condition for the formation of low-mass second-generation stars in the early universe. It has been proposed that gas cooling by dust thermal emission can trigger fragmentation of a low-metallicity star-forming gas cloud. In order to determine the critical condition in which dust cooling induces the formation of low-mass stars, we follow the thermal evolution of a collapsing cloud by a one-zone semi-analytic collapse model. Earlier studies assume the dust amount in the local universe, where all refractory elements are depleted onto grains, and/or assume the constant dust amount during gas collapse. In this paper, we employ the models of dust formation and destruction in early supernovae to derive the realistic dust compositions and size distributions for multiple species as the initial conditions of our collapse calculations. We also follow accretion of heavy elements in the gas phase onto dust grains, i.e., grain growth, during gas contraction. We find that grain growth well alters the fragmentation property of the clouds, and that this still does not approach to the value in the local universe. The critical conditions can be written by the gas metallicity Zcr and the initial depletion efficiency fdep,0 of gas-phase metal onto grains, or dust-to-metal mass ratio, as (Zcr/10^{-5.5} Zsun) = (fdep,0/0.18)^{-0.44} with small scatters in the range of Zcr = [0.06--3.2]x10^{-5} Zsun. We also show that the initial dust composition and size distribution are important to determine Zcr.
    Monthly Notices of the Royal Astronomical Society 10/2014; 446(3). DOI:10.1093/mnras/stu2298 · 5.23 Impact Factor
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    ABSTRACT: We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting from the recently discovered $\rm [Fe/H]<-7.1$ star SMSS J031300 (Keller et al. 2014). We show that the elemental abundances observed on the surface of SMSS J031300 can be well fit by the yields of faint, metal free, supernovae. Using properly calibrated faint supernova explosion models, we study, for the first time, the formation of dust grains in such carbon-rich, iron-poor supernova ejecta. Calculations are performed assuming both unmixed and uniformly mixed ejecta and taking into account the partial destruction by the supernova reverse shock. We find that, due to the paucity of refractory elements beside carbon, amorphous carbon is the only grain species to form, with carbon condensation efficiencies that range between (0.15-0.84), resulting in dust yields in the range (0.025-2.25)M$_{\odot}$. We follow the collapse and fragmentation of a star forming cloud enriched by the products of these faint supernova explosions and we explore the role played by fine structure line cooling and dust cooling. We show that even if grain growth during the collapse has a minor effect of the dust-to-gas ratio, due to C depletion into CO molecules at an early stage of the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could be triggered by dust cooling and fragmentation. A comparison between model predictions and observations of a sample of C-normal and C-rich metal-poor stars supports the idea that a single common pathway may be responsible for the formation of the first low-mass stars.
    The Astrophysical Journal 09/2014; 794(2). DOI:10.1088/0004-637X/794/2/100 · 6.28 Impact Factor
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    Takashi Okamoto, Ikkoh Shimizu, Naoki Yoshida
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    ABSTRACT: We present cosmological hydrodynamic simulations performed to study evolution of galaxy population. The simulations follow timed release of mass, energy, and metals by stellar evolution and employ phenomenological treatments of supernova feedback, pre-supernova feedback modeled as feedback by radiation pressure from massive stars, and quenching of gas cooling in large halos. We construct the fiducial model so that it reproduces the observationally estimated galaxy stellar mass functions and the relationships between the galaxy stellar mass and the host halo mass from $z = 4$ to 0. We find that the fiducial model constructed this way naturally explains the cosmic star formation history, the galaxy downsizing, and the star formation rate and metallicity of the star-forming galaxies. The simulations without the quenching of the gas cooling in large halos overproduce massive galaxies at $z < 2$ and fail to reproduce galaxy downsizing. The simulations that do not employ the radiation pressure feedback from young stars predict too strong redshift evolution of the mass-metallicity relation. Furthermore, the slope of the relation becomes too steep at low redshift without the radiation pressure feedback. The metallicity dependence in the radiation pressure feedback is a key to explain the observed mass-metallicity relation. These facts indicate that these two processes in addition to supernova feedback are essential for galaxy evolution. Our simple phenomenological model is suitable to construct a mock galaxy sample to study physical properties of observed galaxy populations.
    Publications- Astronomical Society of Japan 04/2014; 66(4). DOI:10.1093/pasj/psu046 · 2.01 Impact Factor
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    Aravind Natarajan, Naoki Yoshida
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    ABSTRACT: One of the milestones in the cosmic history is the formation of the first luminous objects and Hydrogen reionization. The standard theory of cosmic structure formation predicts that the first generation of stars were born about a few hundred million years after the Big Bang. The dark Universe was then lit up once again, and eventually filled with ultraviolet photons emitted from stars, galaxies, and quasars. The exact epoch of the cosmic reionization and the details of the process, even the dominant sources, are not known except the fact that the universe was reionized early on. Signatures of reionization are expected to be imprinted in the cosmic microwave background radiation, especially in its large scale polarization. Future CMB experiments, together with other probes such as 21 cm surveys, will provide rich information on the process of reionization. We review recent studies on reionization. The implications from available observations in a wide range of wavelengths are discussed. Results from state-of-the-art computer simulations are presented. Finally, we discuss prospects for exploring the first few hundred million years of the cosmic history.
    04/2014; 2014(6). DOI:10.1093/ptep/ptu067
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    ABSTRACT: We present the first measurement of the cross-correlation of weak gravitational lensing and the extragalactic gamma-ray background emission using data from the Canada-France-Hawaii Lensing Survey and the Fermi Large Area Telescope. The cross-correlation is a powerful probe of signatures of dark matter annihilation, because both cosmic shear and gamma-ray emission originate directly from the same DM distribution in the universe, and it can be used to derive constraints on dark matter annihilation cross-section. We show that the measured lensing-gamma correlation is consistent with a null signal. Comparing the result to theoretical predictions, we exclude dark matter annihilation cross sections of =10^{-24}-10^{-25} cm^3 s^-1 for a 100 GeV dark matter. If dark matter halos exist down to the mass scale of 10^-6 M_sun, we are able to place constraints on the thermal cross sections ~ 3 x 10^{-26} cm^3 s^-1 for a 10 GeV dark matter annihilation into tau^{+} tau^{-}. Future gravitational lensing surveys will increase sensitivity to probe annihilation cross sections of ~ 3 x 10^{-26} cm^3 s^-1 even for a 100 GeV dark matter. Detailed modeling of the contributions from astrophysical sources to the cross correlation signal could further improve the constraints by ~ 40-70 %.
    Physical Review D 04/2014; 90(6). DOI:10.1103/PhysRevD.90.063502 · 4.86 Impact Factor
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    ABSTRACT: Dust grains in low-metallicity star-forming regions may be responsible for the formation of the first low-mass stars. The minimal conditions to activate dust-induced fragmentation require the gas to be pre-enriched above a critical dust-to-gas mass ratio Dcr=[2.6--6.3]x10^-9 with the spread reflecting the dependence on the grain properties. The recently discovered Galactic halo star SDSS J102915+172927 has a stellar mass of 0.8 Msun and a metallicity of Z=4.5x10^-5 Zsun and represents an optimal candidate for the dust-induced low-mass star formation. Indeed, for the two most plausible Population III supernova progenitors, with 20 Msun and 35 Msun, the critical dust-to-gas mass ratio can be overcome provided that at least 0.4 Msun of dust condenses in the ejecta, allowing for moderate destruction by the reverse shock. Here we show that even if dust formation in the first supernovae is less efficient or strong dust destruction does occur, grain growth during the collapse of the parent gas cloud is sufficiently rapid to activate dust cooling and likely fragmentation into low-mass and long-lived stars. Silicates and magnetite grains can experience significant grain growth in the density range 10^9 /cc < nH<10^12 /cc by accreting gas-phase species (SiO, SiO2, and Fe) until their gas-phase abundance drops to zero, reaching condensation efficiencies =1. The corresponding increase in the dust-to-gas mass ratio allows dust-induced cooling and fragmentation to be activated at 10^12 /cc < nH < 10^14 /cc, before the collapsing cloud becomes optically thick to continuum radiation. We show that for all the initial conditions that apply to the parent cloud of SDSS J102915+172927, dust-driven fragmentation is able to account for the formation of the star.
    Monthly Notices of the Royal Astronomical Society 01/2014; 439(3). DOI:10.1093/mnras/stu178 · 5.23 Impact Factor
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    ABSTRACT: Origins of superluminous supernovae (SLSNe) discovered by recent SN surveys are still not known well. One idea to explain the huge luminosity is the collision of dense CSM and SN ejecta. If SN ejecta is surrounded by dense CSM, the kinetic energy of SN ejecta is efficiently converted to radiation energy, making them very bright. To see how well this idea works quantitatively, we performed numerical simulations of collisions of SN ejecta and dense CSM by using one-dimensional radiation hydrodynamics code STELLA and obtained light curves (LCs) resulting from the collision. First, we show the results of our LC modeling of SLSN 2006gy. We find that physical parameters of dense CSM estimated by using the idea of shock breakout in dense CSM (e.g., Chevalier & Irwin 2011, Moriya & Tominaga 2012) can explain the LC properties of SN 2006gy well. The dense CSM's radius is about 1016 cm and its mass about 15 M ☉. It should be ejected within a few decades before the explosion of the progenitor. We also discuss how LCs change with different CSM and SN ejecta properties and origins of the diversity of H-rich SLSNe. This can potentially be a probe to see diversities in mass-loss properties of the progenitors. Finally, we also discuss a possible signature of SN ejecta-CSM interaction which can be found in H-poor SLSN.
    Proceedings of the International Astronomical Union 12/2013; 9(S296). DOI:10.1017/S1743921313009277
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    Masato Shirasaki, Naoki Yoshida
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    ABSTRACT: Measurement of cosmic shear using weak gravitational lensing is a challenging task that involves a number of complicated procedures. We study in detail the systematic errors in measurement of weak lensing Minkowski Functionals (MFs). Specifically, we focus on systematics associated with galaxy shape measurement, photometric redshift errors, and shear calibration correction. We first generate mock weak lensing catalogues that directly incorporate the actual observational characteristics of the Canada-France-Hawaii Lensing Survey (CFHTLenS). We then perform the Fisher analysis using the large set of mock catalogues for various cosmological models. We find that the statistical error associated with the observational effects degrades the cosmological parameter constraints by a factor of a few. Subaru Hyper Suprime-Cam (HSC) survey with the sky coverage of ~1400 deg2 will constrain the dark energy equation of state parameter with an error of Delta w_0 ~ 0.25 by the lensing MFs alone, but biases induced by the systematics can be comparable to the 1-sigma error. We conclude that the lensing MFs are powerful statistics beyond the two-point statistics, only if well calibrated measurement of both the redshifts and the shapes of source galaxies is performed. Finally, we analyse the CFHTLenS data to explore the ability of the MFs to break degeneracies between a few cosmological parameters. Using a combined analysis of the MFs and the shear correlation function, we derive the matter density Omega_m0= 0.256+0.054-0.046.
    The Astrophysical Journal 12/2013; 786(1). DOI:10.1088/0004-637X/786/1/43 · 6.28 Impact Factor
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    ABSTRACT: We investigate the potential use of nebular emission lines in the rest-frame far-infrared (FIR) for determining spectroscopic redshift of z>8 galaxies with the Atacama Large Millimeter/sub-millimeter Array (ALMA). After making a line emissivity model as a function of metallicity, especially for the [O III] 88 micron line which is likely to be the strongest FIR line from H II regions, we predict the line fluxes from high-z galaxies based on a cosmological hydrodynamics simulation of galaxy formation. Since the metallicity of galaxies reaches at ~0.2 Zsun even at z>8 in our simulation, we expect the [O III] 88 micron line as strong as 1.3 mJy for 27 AB objects, which is detectable at a high significance by <1 hour integration with ALMA. Therefore, the [O III] 88 micron line would be the best tool to confirm the spectroscopic redshifts beyond z=8.
    The Astrophysical Journal 12/2013; 780(2). DOI:10.1088/2041-8205/780/2/L18 · 6.28 Impact Factor
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    Ayuki Kamada, Masato Shirasaki, Naoki Yoshida
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    ABSTRACT: We explore the discovery potential of light gravitino mass m_{3/2} by combining future cosmology surveys and collider experiments. The former probe the imprint of light gravitinos in the cosmic matter density field, whereas the latter search signatures of a supersymmetry breaking mechanism. Free-streaming of light gravitinos suppresses the density fluctuations at galactic and sub-galactic length scales, where weak gravitational lensing can be used as a powerful probe. We perform numerical simulations of structure formation to quantify the effect. We then run realistic ray-tracing simulations of gravitational lensing to measure the cosmic shear in models with light gravitino. We forecast the possible reach of future wide-field surveys by Fisher analysis; the light gravitino mass can be determined with an accuracy of m_{3/2}=4\pm 1 eV by a combination of the Hyper Suprime Cam survey and cosmic microwave background anisotropy data obtained by Planck satellite. The corresponding accuracy to be obtained by the future Large Synoptic Survey Telescope is \delta m_{3/2}=0.6 eV. Data from experiments at Large Hadron Collider at 14 TeV will provide constraint at m_{3/2} \simeq 5 eV in the minimal framework of gauge-mediated supersymmetry breaking (GMSB) model. We conclude that a large class of the GMSB model can be tested by combining the cosmological observations and the collider experiments.
    Journal of High Energy Physics 11/2013; 2014(6). DOI:10.1007/JHEP06(2014)162 · 6.22 Impact Factor
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    ABSTRACT: We have performed a large cosmological SPH simulation tailored to the deep survey in the UDF with the HST made in 2012, the so-called UDF12 campaign. After making a light-cone output of our simulation, we have applied the exactly same color selection criteria as the UDF12 campaign to select galaxies from our simulation, and then, have examined the physical properties of them as a proxy of the real observed UDF12 galaxies at z > 7. As a result, we find that the halo mass and the stellar mass are linearly proportional to the observed ultraviolet (UV) luminosity. The dust attenuation well correlates with the observed UV luminosity, but shows a large dispersion. The UV slope also correlates with the UV luminosity, which is consistent with observations quantitatively. The star formation rate (SFR) also proportional to the stellar mass. The specific SFR shows only a weak dependency on the mass. These relations can be explained quantitatively if the SFR is determined by the halo accretion rate. The SFRs of our high-z galaxies increase with time in contrast to usual assumptions made in the SED fitting method. The typical increasing timescales of the galaxies at z ~ 7, 8, 9 and z ~ 10 are 250, 180, 100 and 50 Myr, respectively. We also find that an average metallicity weighted by the Lyman continuum luminosity measured by emission line ratios, reaches at 0.1 to 0.5 Solar for some galaxies even at z ~ 10, suggesting a rapid metal enrichment. We also expect > 0.1 mJy at 350 GHz of the dust thermal emission from the galaxies with H160 < 27, which can be detectable with the ALMA. In our simulation, the galaxies selected by the UDF12 survey contribute to only 43%, 33%, 23% and 9% of the cosmic SFR density at z ~ 7, 8, 9 and z ~10, respectively. The rest mainly comes from galaxies too faint to be detected by the current survey. The JWST will push the detected fraction up to 72-56%.
    Monthly Notices of the Royal Astronomical Society 09/2013; 440(1). DOI:10.1093/mnras/stu265 · 5.23 Impact Factor
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    ABSTRACT: Redshift space distortion (RSD) observed in galaxy redshift surveys is a powerful tool to test gravity theories on cosmological scales, but the systematic uncertainties must carefully be examined for future surveys with large statistics. Here we employ various analytic models of RSD and estimate the systematic errors on measurements of the structure growth-rate parameter, f\sigma_8, induced by non-linear effects and the halo bias with respect to the dark matter distribution, by using halo catalogues from 40 realisations of 3.4 \times 10^8 comoving h^{-3}Mpc^3 cosmological N-body simulations. We consider hypothetical redshift surveys at redshifts z=0.5, 1.35 and 2, and different minimum halo mass thresholds in the range of 5.0 \times 10^{11} -- 2.0 \times 10^{13} h^{-1} M_\odot. We find that the systematic error of f\sigma_8 is greatly reduced to ~4 per cent level, when a recently proposed analytical formula of RSD that takes into account the higher-order coupling between the density and velocity fields is adopted, with a scale-dependent parametric bias model. Dependence of the systematic error on the halo mass, the redshift, and the maximum wavenumber used in the analysis is discussed. We also find that the Wilson-Hilferty transformation is useful to improve the accuracy of likelihood analysis when only a small number of modes are available in power spectrum measurements.
    Monthly Notices of the Royal Astronomical Society 08/2013; 443(4). DOI:10.1093/mnras/stu1382 · 5.23 Impact Factor
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    ABSTRACT: Supermassive stars (SMSs) forming via very rapid mass accretion (Mdot >~ 0.1 Msun/yr) could be precursors of supermassive black holes observed beyond redshift of about 6. Extending our previous work, we here study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10^{4 - 5} Msun. Our stellar evolution calculations show that a star becomes supermassive while passing through the "supergiant protostar" stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass, until =~ 100 AU for M_* >~ 10^4 Msun, after which the star begins to slowly contract. Because of the large radius the effective temperature is always less than 10^4 K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M_* >~ 10^5 Msun can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10^5 Msun. An extremely massive black hole should form after the collapse of the inner core.
    The Astrophysical Journal 08/2013; 778(2). DOI:10.1088/0004-637X/778/2/178 · 6.28 Impact Factor

Publication Stats

8k Citations
614.05 Total Impact Points

Institutions

  • 2008–2015
    • The University of Tokyo
      • • Department of Physics
      • • Institute for the Physics and Mathematics of the Universe (IPMU)
      Tōkyō, Japan
  • 2003–2014
    • National Astronomical Observatory of Japan
      • Division of Theoretical Astronomy
      Edo, Tōkyō, Japan
  • 2010
    • Kyoto University
      • Department of Physics II
      Kioto, Kyōto, Japan
  • 2004–2009
    • Nagoya University
      Nagoya, Aichi, Japan
  • 2002–2004
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2001
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 2000–2001
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
    • University of Padova
      • Department of Physics and Astronomy "Galileo Galilei"
      Padua, Veneto, Italy