arXiv subjects

Taihei Yano

Publications and source records attributed to Taihei Yano.

15 recordsLinked to original sources

arXiv2026

GREX-PLUS Science Book v2

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA strategic L-class mission to be launched in the 2030s. Its primary science goals are two-fold: galaxy formation and evolution, and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a telescope with a 1 m primary mirror aperture cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2--8 $\mu$m wavelength band and a high-resolution spectrometer with a wavelength resolution of 30,000 in the 10--18 $\mu$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high-resolution spectrometer aims to identify the location of the water ``snowline'' in protoplanetary disks. Both instruments will provide unique datasets for a broad range of scientific topics, including galaxy mass assembly, the origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy of exoplanet atmospheres, planetary atmospheres in the Solar System, and so on. This document is the second version of a collection of scientific themes that can be achieved with GREX-PLUS. Each section in Chapters~2 and 3 is based on presentations at several GREX-PLUS Science Workshops.

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arXiv2024

JASMINE image simulator for high-precision astrometry and photometry

JASMINE is a Japanese planned space mission that aims to reveal the formation history of our Galaxy and discover habitable exoEarths. For these objectives, the JASMINE satellite performs high-precision astrometric observations of the Galactic bulge and high-precision transit monitoring of M-dwarfs in the near-infrared (1.0-1.6 microns in wavelength). For feasibility studies, we develop an image simulation software named JASMINE-imagesim, which produces realistic observation images. This software takes into account various factors such as the optical point spread function (PSF), telescope jitter caused by the satellite's attitude control error (ACE), detector flat patterns, exposure timing differences between detector pixels, and various noise factors. As an example, we report a simulation for the feasibility study of astrometric observations using JASMINE-imagesim. The simulation confirms that the required position measurement accuracy of 4 mas for a single exposure of 12.5-mag objects is achievable if the telescope pointing jitter uniformly dilutes the PSF across all stars in the field of view. On the other hand, the simulation also demonstrates that the combination of realistic pointing jitter and exposure timing differences in the detector can significantly degrade accuracy and prevent achieving the requirement. This means that certain countermeasures against this issue must be developed. This result implies that this kind of simulation is important for mission planning and advanced developments to realize more realistic simulations help us to identify critical issues and also devise effective solutions.

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arXiv2023

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $\mu$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $\mu$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

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arXiv2019

Metallicity dependence of the Hercules stream in Gaia/RAVE data -- explanation by non-closed orbits

The origin of the Hercules stream, the most prominent velocity substructure in the Solar neighbour disc stars, is still under debate. Recent accurate measurements of position, velocity, and metallicity provided by Tycho Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiments (RAVE) have revealed that the Hercules stream is most clearly seen in the metal-rich region ([Fe/H] > 0), while it is not clearly seen in lower metallicity region ([Fe/H] < -0.25). By using a large number of chemo-dynamical 2D test-particle simulations with a rotating bar and/or spiral arms, we find that the observed [Fe/H] dependence of the Hercules stream is a natural consequence of the inside-out formation of the stellar disc and the existence of highly non-closed orbits in the rotating frame of the bar or spiral arms. Our successful models that reproduce the observed properties of the Hercules stream include not only fast-bar-only and fast-bar+spiral models, but also slow-bar+spiral models. This indicates that it is very difficult to estimate the pattern speed of the bar or spiral arms based only on the observations of the Hercules stream in the Solar neighbourhood. As a by-product of our simulations, we make some predictions about the locations across the Galactic plane where we can observe velocity bimodality that is not associated with the Hercules stream. These predictions can be tested by the Gaia Data Release 2, and such a test will improve our understanding of the evolution of the Milky Way stellar disc.

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arXiv2016

The validation of made-to-measure method for reconstruction of phase space distribution functions

We investigate how accurately phase space distribution functions (DFs) in galactic models can be reconstructed by a made-to-measure (M2M) method, which constructs $N$-particle models of stellar systems from photometric and various kinematic data. The advantage of the M2M method is that this method can be applied to various galactic models without assumption of the spatial symmetries of gravitational potentials adopted in galactic models, and furthermore, numerical calculations of the orbits of the stars cannot be severely constrained by the capacities of computer memories. The M2M method has been applied to various galactic models. However, the degree of accuracy for the recovery of DFs derived by the M2M method in galactic models has never been investigated carefully. Therefore, we show the degree of accuracy for the recovery of the DFs for the anisotropic Plummer model and the axisymmetric Stäckel model, which have analytic solutions of the DFs. Furthermore, this study provides the dependence of the degree of accuracy for the recovery of the DFs on various parameters and a procedure adopted in this paper. As a result, we find that the degree of accuracy for the recovery of the DFs derived by the M2M method for the spherical target model is a few percent, and more than ten percent for the axisymmetric target model.

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arXiv2015

Early Cosmic Merger of Multiple Black Holes

We perform numerical simulations on the merger of multiple black holes (BHs) in primordial gas at early cosmic epochs. We consider two cases of BH mass: $M_{BH} = 30 M_{\odot}$ and $M_{BH} = 10^4 M_{\odot}$. Attention is concentrated on the effect of the dynamical friction by gas in a host object. The simulations incorporate such general relativistic effects as the pericentre shift and gravitational wave emission. As a result, we find that multiple BHs are able to merge into one BH within 100 Myr in a wide range of BH density. The merger mechanism is revealed to be categorized into three types: gas-drag-driven merger (type A), interplay-driven merger (type B), and three-body-driven merger (type C). We find the relation between the merger mechanism and the ratio of the gas mass within the initial BH orbit ($M_{gas}$) to the total BH mass ($ΣM_{BH}$). Type A merger occurs if $M_{gas} \gtrsim 10^5 ΣM_{BH}$, type B if $M_{gas} \lesssim 10^5 ΣM_{BH}$, and type C if $M_{gas} \ll 10^5 ΣM_{BH}$. Supposing the gas and BH density based on the recent numerical simulations on first stars, all the BH remnants from first stars are likely to merge into one BH through the type B or C mechanism. Also, we find that multiple massive BHs ($M_{BH} = 10^4 M_{\odot}$) distributed over several parsec can merge into one BH through the type B mechanism, if the gas density is higher than $5\times 10^6$ cm$^{-3}$. The present results imply that the BH merger may contribute significantly to the formation of supermassive BHs at high redshift epochs.

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arXiv2014

Torus-fitting method for obtaining action variables in two-dimensional Galactic potentials

A phase-space distribution function of the steady state in galaxy models that admits regular orbits overall in the phase-space can be represented by a function of three action variables. This type of distribution function in Galactic models is often constructed theoretically for comparison of the Galactic models with observational data as a test of the models. On the other hand, observations give Cartesian phase-space coordinates of stars. Therefore it is necessary to relate action variables and Cartesian coordinates in investigating whether the distribution function constructed in galaxy models can explain observational data. Generating functions are very useful in practice for this purpose, because calculations of relations between action variables and Cartesian coordinates by generating functions do not require a lot of computational time or computer memory in comparison with direct numerical integration calculations of stellar orbits. Here, we propose a new method called a torus-fitting method, by which a generating function is derived numerically for models of the Galactic potential in which almost all orbits are regular. We confirmed the torus-fitting method can be applied to major orbit families (box and loop orbits) in some two-dimensional potentials. Furthermore, the torus-fitting method is still applicable to resonant orbit families, besides major orbit families. Hence the torus-fitting method is useful for analyzing real Galactic systems in which a lot of resonant orbit families might exist.

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arXiv2005

The relation between the two-point and the three-point correlation functions in the non-linear gravitational clustering regime

The connection between the two-point and the three-point correlation functions in the non-linear gravitational clustering regime is studied. Under a scaling hypothesis, we find that the three-point correlation function, $ζ$, obeys the scaling law $ζ\propto ξ^{\frac{3m+4w-2ε}{2m+2w}}$ in the nonlinear regime, where $ξ$, $m$, $w$, and $ε$ are the two-point correlation function, the power index of the power spectrum in the nonlinear regime, the number of spatial dimensions, and the power index of the phase correlations, respectively. The new formula reveals the origin of the power index of the three-point correlation function. We also obtain the theoretical condition for which the ``hierarchical form'' $ζ\proptoξ^2$ is reproduced.

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arXiv2003

Universality in the distribution of caustics in the expanding Universe

We numerically investigate the long--time evolution of density perturbations after the first appearance of caustics in an expanding cosmological model with one--dimensional `single--wave' initial conditions. Focussing on the time--intervals of caustic appearances and the spatial distribution of caustics at subsequent times, we find that the time--intervals of caustic appearances approach a constant, i.e., their time--subsequent ratio converges to 1; it is also found that the spatial distribution of caustics at a given time features some universality rules, e.g., the ratio between the position of the nearest caustic from the center and that of the second nearest caustic from the center approaches a constant. Furthermore we find some rules for the mass distribution for each caustic. Using these universality constants we are in the position to predict the spatial distribution of caustics at an arbitrary time in order to give an estimate for the power spectral index in the fully--developed non--dissipative turbulent (`virialized') regime.

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arXiv2002

Analytical Approach to the Mass Distribution Function of Subhalos and Cold Fronts in Galaxy Clusters

We construct an analytical model of the mass distribution function of subhalos in a galaxy cluster in the context of the cold dark matter (CDM) theory. Our model takes account of two important effects; the high density of the precluster region and the spatial correlation of initial density fluctuations. For subhalos with small masses, the mass distribution function that our model predicts is intermediate between the Press-Schechter (PS) mass distribution function and a conditional mass distribution function as an extension of the PS formalism. We compare the results of our model with those of numerical simulations. We find that our model predictions are consistent with the mass and velocity distribution functions of subhalos in a cluster obtained by numerical simulations. We estimate the probability of finding large X-ray subhalos that often have ``cold fronts''. The observed large X-ray subhalos and cold fronts may not always be the result of cluster mergers, but instead may be internal structures in clusters.

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arXiv2000

Stellar Kinematics of the Spiral Structure in the Solar Neighborhood

We present a method, based on the kinematic analysis of the Galactic disk stars, to clarify whether the internal motions of the stellar system in spiral arms follow those expected in the density wave theory. The method relies on the comparison with the linear relation between the phases of spatial positions and epicyclic motions of stars, as drawn from the theory. The application of the method to the 78 Galactic Cepheids near the Sun, for which accurate proper motions are available from the {\it Hipparcos} Catalogue, has revealed that these Cepheids hold no correlation between both phases, thereby implying that their motions are in contradiction with the theoretical predictions. Possible reasons for this discrepancy are discussed and future prospects are outlined.

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arXiv1999

A Universal Profile of the Dark Matter Halo and the Two-point Correlation Function

We have investigated the relation between the two-point spatial correlation function and the density profile of the dark matter halo in the strongly non-linear regime. It is well known that when the density fluctuation grows into dark matter halo whose density profile is $ρ\propto r^{-ε}$(${3/2}<ε<3$) on almost all mass scales, the two-point spatial correlation function obeys a power law with the power index $γ= 2ε-3$ in the strongly non-linear regime. We find the form of the two-point spatial correlation function, which does not obey the power law when the power index $ε$ is smaller than 3/2, such as the density profile $ρ\propto r^{-1}$ around the center of the halo which is proposed by Navarro, Frenk & White (1996,1997). By using the BBGKY equation in the strongly non-linear regime, it is also found that velocity parameter $h \equiv - < v > / \dot{a}x$ is not a constant even in the strongly non-linear regime ($\tilde{x} \equiv x/x_{nl} \to 0$) although it is a constant when $ε> 3/2$ and then the two-point spatial correlation function can be regarded as the power law. The velocity parameter $h$ becomes 0 at the non-linear limit of $\tilde{x} \to 0$, that is, the stable clustering hypothesis cannot be satisfied when $ε< 3/2$.

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arXiv1998

Evolution of the Power Spectrum and the Self-Similarity in the Expanding One-Dimensional Universe

We have investigated time evolutions of power spectra of density fluctuations for long time after the first appearance of caustics in the expanding one-dimensional universe. It is found that when an initial power spectrum is sale-free with a power index $n$, a self-similarity of the time evolution of the power spectrum is achieved. We find that the power spectrum can be separated roughly into three regimes according to the shape of the power spectrum: the linear regime ($k < k_{nl}$ : the regime {\cal 1}),the single-caustic regime($k_{nl} < k < k_{snl}$ : the regime 2), and the multi-caustics regime($k > k_{snl}$ : the regime 3). The power index of the power spectrum in each regime has the values of $n,-1$, and $μ$ which depends on $n$, respectively. Even in the case of an initial power-law spectrum with a cutoff scale, there might be the possibility of the self-similar evolution of the power spectrum after the appearance of the caustics. It is found, however, the self-similarity is not achieved in this case. The shape of the power spectrum on scales smaller than the cutoff scale can be separated roughly in two regimes: the virialized regime ($k_{cut}< k < k_{cs}$ : the regime 4), and the smallest-single-caustic regime ($ k > k_{cs}$ : the regime 5). The power index of the power spectrum is $ν$ which may be determined by the distribution of singular points in the regime 4. In the regime 5, the value of the power index is -1. Moreover we show the general property about the shape of a power spectrum with a general initial condition.

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arXiv1997

Stability of Scale-Invariant Cosmological Correlation Functions in the Strongly Non-Linear Clustering Regime

We investigate stability of the scale-invariant solutions of the BBGKY equations for two-point spatial correlation functions of the density fluctuations in the strongly non-linear regime. In the case that the background skewness of the velocity field is equal to 0, we found that there is no local instability in the strongly non-linear regime. The perturbation does not grow nor does it decay. It has an only marginal stable mode. This result means that no special value of the power index of the two-point spatial correlation function are favored in terms of the stability of the solutions. In other words, the argument about the stability does not determine the power index of the two-point spatial correlation functions in the strongly non-linear regime.

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arXiv1996

Scale-Invariant Correlation Functions of Cosmological Density Fluctuations in the Strong Clustering Regime

We have investigated the scale-invariant solutions of the BBGKY equations for spatial correlation functions of cosmological density fluctuations and the mean relative peculiar velocity in the strongly nonlinear regime. It is found that the solutions for the mean relative physical velocity depend on the three-point spatial correlation function and the skewness of the velocity fields. We find that the stable condition in which the mean relative physical velocity vanishes on the virialized regions is satisfied only under the assumptions which Davis \& Peebles took in his paper. It is found, however, that their assumptions may not be general in real. The power index of the two-point correlation function in the strongly nonlinear regime depends on the mean relative peculiar velocity, the three-point correlation function and the skewness. If the self-similar solutions exist, the power index in the strongly nonlinear regime is related to the power index of the initial power spectrum and its relation depends on the three-point correlation function and the skewness through the mean relative peculiar velocity. Furthermore it is found that the mean relative physical velocity should have the values between 0 and the Hubble expanding velocity. When the mean relative physical velocity equals to the Hubble expanding velocity, there might exist self-similar solutions in which the power index of the two-point correlation function in the strongly nonlinear regime is independent of the initial power index $n$.

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