arXiv subjects

Atsushi Taruya

Publications and source records attributed to Atsushi Taruya.

At least 163 records · Page 9Linked to original sources

arXiv2005

Minkowski Functionals of SDSS galaxies I : Analysis of Excursion Sets

We present a first morphometric investigation of a preliminary sample from the SDSS of 154287 galaxies with apparent magnitude 14.5<m_r<17.5 and redshift 0.001<z<0.4. We measure the Minkowski Functionals, which are a complete set of morphological descriptors. To account for the complicated wedge--like geometry of the present survey data, we construct isodensity contour surfaces from the galaxy positions in redshift space and employ two complementary methods of computing the Minkowski Functionals. We find that the observed Minkowski Functionals for SDSS galaxies are consistent with the prediction of a Lambda--dominated spatially--flat Cold Dark Matter model with random--Gaussian initial conditions, within the cosmic variance estimated from the corresponding mock catalogue. We expect that future releases of the SDSS survey will allow us to distinguish morphological differences in the galaxy distribution with regard to different morphological type and luminosity ranges.

astro-ph↗

arXiv2005

The Rossiter-McLaughlin effect and analytic radial velocity curves for transiting extrasolar planetary systems

A transiting extrasolar planet sequentially blocks off the light coming from the different parts of the disk of the host star in a time dependent manner. Due to the spin of the star, this produces an asymmetric distortion in the line profiles of the stellar spectrum, leading to an apparent anomaly of the radial velocity curves, known as the Rossiter - McLaughlin effect. Here, we derive approximate but accurate analytic formulae for the anomaly of radial velocity curves taking account of the stellar limb darkening. The formulae are particularly useful in extracting information of the projected angle between the planetary orbit axis and the stellar spin axis, λ, and the projected stellar spin velocity, V sin I_s. We create mock samples for the radial curves for the transiting extrasolar system HD209458, and demonstrate that constraints on the spin parameters (V sin I_s, λ) may be significantly improved by combining our analytic template formulae and the precision velocity curves from high-resolution spectroscopic observations with 8-10 m class telescopes. Thus future observational exploration of transiting systems using the Rossiter - McLaughlin effect is one of the most important probes to better understanding of the origin of extrasolar planetary systems, especially the origin of their angular momentum.

astro-ph↗

arXiv2005

Probing anisotropies of gravitational-wave backgrounds with a space-based interferometer: geometric properties of antenna patterns and their angular power

We discuss the sensitivity to anisotropies of stochastic gravitational-wave backgrounds (GWBs) observed via space-based interferometer. In addition to the unresolved galactic binaries as the most promising GWB source of the planned Laser Interferometer Space Antenna (LISA), the extragalactic sources for GWBs might be detected in the future space missions. The anisotropies of the GWBs thus play a crucial role to discriminate various components of the GWBs. We study general features of antenna pattern sensitivity to the anisotropies of GWBs beyond the low-frequency approximation. We show that the sensitivity of space-based interferometer to GWBs is severely restricted by the data combinations and the symmetries of the detector configuration. The spherical harmonic analysis of the antenna pattern functions reveals that the angular power of the detector response increases with frequency and the detectable multipole moments with effective sensitivity h_{eff} \sim 10^{-20} Hz^{-1/2} may reach $\ell \sim$ 8-10 at $f \sim f_*=10$ mHz in the case of the single LISA detector. However, the cross correlation of optimal interferometric variables is blind to the monopole (\ell=0) intensity anisotropy, and also to the dipole (\ell=1) in some case, irrespective of the frequency band. Besides, all the self-correlated signals are shown to be blind to the odd multipole moments (\ell=odd), independently of the frequency band.

gr-qc↗

arXiv2004

Evolution of gravitational waves in the high-energy regime of brane-world cosmology

We discuss the cosmological evolution of gravitational waves (GWs) after inflation in a brane-world cosmology embedded in five-dimensional anti-de Sitter (AdS_5) bulk spacetime. In a brane-world scenario, the evolution of GWs is affected by the non-standard cosmological expansion and the excitation of the Kaluza-Klein modes (KK-modes), which are significant in the high-energy regime of the universe. We numerically solve the wave equation of GWs in the Poincare coordinates of the AdS_5 spacetime. Using a plausible initial condition from inflation, we find that, while the behavior of GWs in the bulk is sensitive to the transition time from inflation to the radiation dominated epoch, the amplitude of GWs on the brane is insensitive to this time if the transition occurs early enough before horizon re-entry. As a result, the amplitude of GWs is suppressed by the excitation of KK-modes which escape from the brane into the bulk, and the effect may compensate the enhancement of the GWs by the non-standard cosmological expansion. Based on this, the influence of the high-energy effects on the stochastic background of GWs is discussed.

hep-th↗

arXiv2004

Self-gravitating Stellar Systems and Non-extensive Thermostatistics

After introducing the fundamental properties of self-gravitating systems, we present an application of Tsallis' generalized entropy to the analysis of their thermodynamic nature. By extremizing the Tsallis entropy, we obtain an equation of state known as the stellar polytrope. For a self-gravitating stellar system confined within a perfectly reflecting wall, we discuss the thermodynamic instability caused by its negative specific heat. The role of the extremum as a quasi-equilibrium is also demonstrated from the results of N-body simulations.

cond-mat.stat-mech↗

arXiv2004

Cosmological Density Distribution Function from the Ellipsoidal Collapse Model in Real Space

We calculate the one-point probability distribution function (PDF) for cosmic density in non-linear regime of the gravitational evolution. Under the local approximation that the evolution of cosmic fluid fields can be characterized by the Lagrangian local dynamics with finite degrees of freedom, the analytic expressions of PDF are derived taking account of the smoothing effect. The validity and the usefulness of the local approximation are then discussed comparing those results with N-body simulations in a Gaussian initial condition. Adopting the ellipsoidal collapse model (ECM) and the spherical collapse model (SCM) as Lagrangian local dynamics, we found that the PDFs from the local approximation excellently match the simulation results in the case of the cold dark matter initial spectrum. As for the scale-free initial spectra given by $P(k)\propto k^n$, N-body result suffers from spurious numerical effects, which prevent us to give a detailed comparison. Nevertheless, at the quality of N-body data, the model predictions based on the ECM and the SCM quantitatively agree with N-body results in cases with spectral index n<0. For the index n>=0, choice of the Lagrangian local dynamics becomes crucial for an accurate prediction and a more delicate modeling is required, however, we find that the model prediction based on the ECM provides a better approximation to the N-body results of cumulants and PDFs.

astro-ph↗

arXiv2003

Evolution of gravitational waves from inflationary brane-world : numerical study of high-energy effects

We study the evolution of gravitational waves(GWs) after inflation in a brane-world cosmology embedded in five-dimensional anti-de Sitter spacetime. Contrary to the standard four-dimensional results, the GWs at the high-energy regime in brane-world model suffer from the effects of the non-standard cosmological expansion and the excitation of the Kaluza-Klein modes(KK-modes), which can affect the amplitude of stochastic gravitational wave background significantly. To investigate these two high-energy effects quantitatively, we numerically solve the wave equation of the GWs in the radiation dominated epoch at relatively low-energy scales. We show that the resultant GWs is suppressed by the excitation of the KK modes. The created KK modes are rather soft and escape away from the brane to the bulk gravitational field. The results are also compared to the semi-analytic prediction from the low-energy approximation and the evolved amplitude of GWs on the brane reasonably matches the numerical simulations.

hep-th↗

arXiv2003

Long-term Evolution of Stellar Self-Gravitating System away from the Thermal Equilibrium: connection with non-extensive statistics

With particular attention to the recently postulated introduction of a non-extensive generalization of Boltzmann-Gibbs statistics, we study the long-term stellar dynamical evolution of self-gravitating systems on timescales much longer than the two-body relaxation time. In a self-gravitating N-body system confined in an adiabatic wall, we show that the quasi-equilibrium sequence arising from the Tsallis entropy, so-called stellar polytropes, plays an important role in characterizing the transient states away from the Boltzmann-Gibbs equilibrium state.

astro-ph↗

arXiv2003

Biasing and Genus Statistics of Dark Matter Halos in the Hubble Volume Simulation

We present a numerical analysis of genus statistics for dark matter halo catalogs from the Hubble volume simulation. The huge box-size of the Hubble volume simulation enables us to carry out a reliable statistical analysis of the biasing properties of halos at a Gaussian smoothing scale of R_G>30Mpc/h with a cluster-mass scale of between 7*10^{13}Msolar/h and 6*10^{15}Msolar/h. A detailed comparison of the genus for dark matter halos with that for the mass distribution shows that the non-Gaussianity induced by the halo biasing is comparable to that by nonlinear gravitational evolution, and both the shape and the amplitude of the genus are almost insensitive to the halo mass at R_G>30Mpc/h. In order to characterize the biasing effect on the genus, we apply a perturbative formula developed by Matsubara (1994). We find that the perturbative formula well describes the simulated halo genus at R_G>50Mpc/h. The result indicates that the biasing effect on the halo genus is well approximated by nonlinear deterministic biasing up to the second-order term in the mass density fluctuation. The two parameters describing the linear and quadratic terms in the nonlinear biasing accurately specify the genus for galaxy clusters.

astro-ph↗

arXiv2003

Evolution of Cosmological Density Distribution Function from the Local Collapse Model

We present a general framework to treat the evolution of one-point probability distribution function (PDF) for cosmic density $δ$ and velocity-divergence fields $θ$. In particular, we derive an evolution equation for the one-point PDFs and consider the stochastic nature associated with these quantities. Under the local approximation that the evolution of cosmic fluid fields can be characterized by the Lagrangian local dynamics with finite degrees of freedom, evolution equation for PDFs becomes a closed form and consistent formal solutions are constructed. Adopting this local approximation, we explicitly evaluate the one-point PDFs $P(δ)$ and $P(θ)$ from the spherical and the ellipsoidal collapse models as the representative Lagrangian local dynamics. In a Gaussian initial condition, while the local density PDF from the ellipsoidal model almost coincides with the that of the spherical model, differences between spherical and ellipsoidal collapse model are found in the velocity-divergence PDF. Importantly, the joint PDF of local density, $P(δ,t;δ',t')$, evaluated at the same Lagrangian position but at the different times $t$ and $t'$ from the ellipsoidal collapse model exhibits a large amount of scatter. The mean relation between $δ$ and $δ'$ does fail to match the one-to-one mapping obtained from spherical collapse model. Moreover, the joint PDF $P(δ;θ)$ from the ellipsoidal collapse model shows a similar stochastic feature, both of which are indeed consistent with the recent result from N-body simulations.

astro-ph↗

arXiv2003

Gravothermal Catastrophe and Tsallis' Generalized Entropy of Self-Gravitating Systems III. quasi-equilibrium structure using normalized q-values

We revisit the issues on the thermodynamic property of stellar self-gravitating system arising from Tsallis' non-extensive entropy. Previous papers (Taruya & Sakagami, Physica A 307 (2002) 185 (cond-mat/0107494); ibid. (2002) in press (cond-mat/0204315)) have revealed that the extremum-state of Tsallis entropy characterized by the so-called stellar polytrope has consistent thermodynamic structure, which predicts the thermodynamic instability due to the negative specific heat. However, their analyses heavily relies on the old Tsallis formalism using standard linear mean values. In this paper, extending our previous study, we focus on the equilibrium structure based on the standard framework by means of the normalized q-expectation values. It then turns out that the new extremum-state of Tsallis entropy essentially remains unchanged from the previous result, i.e., the stellar quasi-equilibrium distribution can be described by the stellar polytrope. While the thermodynamic stability for a system confined in an adiabatic wall completely agrees with the previous study and thereby the stability/instability criterion remains unchanged, the stability analysis reveals a new equilibrium property for the system surrounded by a thermal bath. In any case, the stability/instability criteria are consistently explained from the presence of negative specific heat and within the formalism, the stellar polytrope is characterized as a plausible non-extensive meta-equilibrium state.

cond-mat.stat-mech↗

arXiv2002

Non-Gaussian Tails of Cosmological Density Distribution Function from Dark Halo Approach

We present a simple model based on the dark halo approach which provides a useful way to understand key points determining the shape of the non-Gaussian tails of the dark matter one-point probability distribution function(PDF). In particular, using the scale-free models with power-law profile of dark halos, we derive a simple analytic expression for the one-point PDF. It is found that the shape of the PDF changes at the characteristic value of $δ_*$ which is defined by the smoothed density of a halo with the characteristic mass $M_*$ at the epoch. In cold dark matter models with top-hat smoothing filters, the characteristic smoothed density at present time typically takes the value $δ_*\gg 1$ for a small smoothing scale $\rth\sim 1$Mpc$/h$ and conversely $δ_*\ll 1$ for a large smoothing scale $\rth > 10$Mpc$/h$. On the range $δ/δ_*<1$, the shape of the PDF is almost solely determined by the outer slope of halos and scales as a power-law. The resultant non-Gaussian tails of PDF then resemble the log-normal PDFs in that range and show a good agreement with N-body simulations, which can be ascribed to the universality of the outer slope of the halo profile. In contrast, tails of one-point PDF in the range $δ/δ_*>1$ basically follow the steep exponential tails of the halo mass function, which exhibit a strong sensitivity to both the outer slope of the halo profile and the initial power spectrum. Based on these results, the discussion on the PDF of galaxy distribution and the application to the weak lensing statistics are also presented.

astro-ph↗

arXiv2002

Three-Dimensional Genus Statistics of Galaxies in the SDSS Early Data Release

We present the first analysis of three-dimensional genus statistics for the SDSS EDR galaxy sample. Due to the complicated survey volume and the selection function, analytic predictions of the genus statistics for this sample are not feasible, therefore we construct extensive mock catalogs from N-body simulations in order to compare the observed data with model predictions. This comparison allows us to evaluate the effects of a variety of observational systematics on the estimated genus for the SDSS sample, including the shape of the survey volume, the redshift distortion effect, and the radial selection function due to the magnitude limit. The observed genus for the SDSS EDR galaxy sample is consistent with that predicted by simulations of a $Λ$-dominated spatially-flat cold dark matter model. Standard ($Ω_0=1$) cold dark matter model predictions do not match the observations. We discuss how future SDSS galaxy samples will yield improved estimates of the genus.

astro-ph↗

arXiv2002

Modeling Pairwise Peculiar Velocity Distribution Function of Dark Matter from Halo Density Profiles

We derive the pairwise peculiar velocity distribution function of dark matter particles applying the dark matter halo approach. Unlike the previous work, we do not assume a Gaussian velocity distribution function of dark matter in a single halo, but compute it self-consistently with the assumed density profile for dark matter halo. The resulting distribution function is well approximated by an exponential distribution which is consistent with the previous observational, numerical and theoretical results. We also compute the pairwise peculiar velocity dispersion for different density profiles, and provide a practical fitting formula. We apply an empirical biasing scheme into our model and present prediction for pairwise peculiar velocity dispersion of {\it galaxies}, and reproduce the previous results of simulations using our semi-analytical method.

astro-ph↗

arXiv2001

Strong Gravitational Lensing Time Delay Statistics and the Density Profile of Dark Halos

The distribution of differential time delays Δt between images produced by strong gravitational lensing contains information on the mass distributions in the lensing objects as well as on cosmological parameters such as H_0. We derive an explicit expression for the conditional probability distribution function of time delays P(Δt|θ), given an image separation between multiple images θ, and related statistics. We consider lensing halos described by the singular isothermal sphere (SIS) approximation and by its generalization as proposed by Navarro, Frenk, & White (NFW) which has a density profile ρ\propto r^{-α} in the innermost region. The time delay distribution is very sensitive to these profiles; steeper inner slopes tend to produce larger time delays. For example, if H_0=70 km/s/Mpc, a Λ-dominated cosmology and a source redshift z_S=1.27 are assumed, lenses with θ=5'' produce a time delay of Δt[yr]=1.5^{+1.7}_{-0.9}, 0.39^{+0.37}_{-0.22}, 0.15^{+0.11}_{-0.09}, and 0.071^{+0.054}_{-0.038} (50% confidence interval), for SIS, generalized NFW with α=1.5, α=1.0, and α=0.5, respectively. At a fixed image separation, the time delay is determined by the difference in the lensing potential between the position of the two images, which typically occur at different impact parameters. Although the values of Δt are proportional to the inverse of H_0, P(Δt|θ) is rather insensitive to all other cosmological model parameters, source redshifts, magnification biases and so on. A knowledge of P(Δt|θ) will also be useful in designing the observing program of future large scale synoptic variability surveys and for evaluating possible selection biases operating against large splitting lens systems.

astro-ph↗

arXiv2001

Gravothermal Catastrophe and Generalized Entropy of Self-Gravitating Systems

We present a first physical application of Tsallis' generalized entropy to the thermodynamics of self-gravitating systems. The stellar system confined in a spherical cavity of radius $r_e$ exhibits an instability, so-called gravothermal catastrophe, which has been originally investigated by Antonov (1962) and Lynden-Bell & Wood (1968) on the basis of the maximum entropy principle for the phase-space distribution function. In contrast to previous analyses using the Boltzmann-Gibbs entropy, we apply the Tsallis-type generalized entropy to seek the equilibrium criteria. Then the distribution function of Vlassov-Poisson system can be reduced to the stellar polytrope system. Evaluating the second variation of Tsallis entropy and solving the zero eigenvalue problem explicitly, we find that the gravothermal instability appears in cases with polytrope index $n>5$. The critical point characterizing the onset of instability are obtained, which exactly matches with the results derived from the standard turning-point analysis. The results give an important suggestion that the Tsallis entropy is indeed applicable and viable to the long-range nature of the self-gravitating system.

cond-mat.stat-mech↗

arXiv2001

Quasi-self-similar evolution of the two-point correlation function: strongly nonlinear regime in Omega_0 < 1 universes

The well-known self-similar solution for the two-point correlation function of the density field is valid only in an Einstein-de Sitter universe. We attempt to extend the solution for non-Einstein-de Sitter universes. For this purpose we introduce an idea of quasi-self-similar evolution; this approach is based on the assumption that the evolution of the two-point correlation is a succession of stages of evolution, each of which spans a short enough period to be considered approximately self-similar. In addition we assume that clustering is stable on scales where a physically motivated `virialization condition' is satisfied. These assumptions lead to a definite prediction for the behavior of the two-point correlation function in the strongly nonlinear regime. We show that the prediction agrees well with N-body simulations in non-Einstein-de Sitter cases, and discuss some remaining problems.

astro-ph↗

arXiv2001

Probability Distribution Function of Cosmological Density Fluctuations from Gaussian Initial Condition: Comparison of One- and Two-point Log-normal Model Predictions with N-body Simulations

We quantitatively study the probability distribution function (PDF) of cosmological nonlinear density fluctuations from N-body simulations with Gaussian initial condition. In particular, we examine the validity and limitations of one-point and two-point log-normal PDF models against those directly estimated from the simulations. We find that the one-point log-normal PDF describes very accurately the cosmological density distribution even in the nonlinear regime (the rms variance σ_{nl} \simlt 4 and the over-density δ\simlt 100). Furthermore the two-point log-normal PDFs are also in good agreement with the simulation data from linear to fairly nonlinear regime, while slightly deviate from them for δ\simlt -0.5. Thus the log-normal PDF can be used as a useful empirical model for the cosmological density fluctuations. While this conclusion is fairly insensitive to the shape of the underlying power spectrum of density fluctuations P(k), models with substantial power on large scales, i.e., n\equiv d\ln P(k)/d \ln k \simlt -1, are better described by the log-normal PDF. On the other hand, we note that the one-to-one mapping of the initial and the evolved density fields consistent with the log-normal model does not approximate the broad distribution of their mutual correlation even on average. Thus the origin of the phenomenological log-normal PDF approximation still remains to be understood.

astro-ph↗