arXiv subjects
Publications and source records attributed to Shin Sasaki.
The observed luminosity -- temperature relation of clusters is considerably steeper than that expected from a simple scaling relation. Although extra energy input is a likely solution, its source has not been identified. We propose intracluster supernova as a possible source, and also predict the supernova rate in intracluster space as a critical test.
We propose a new scenario to produce the superheavy dark matter based on the inflationary universe. In our scenario, the inflaton couples to both a boson and a stable fermion. Although the fermion is produced by the inflaton decay after inflation, almost energy density of the inflaton is transmitted into the radiation by parametric resonance which causes the explosively copious production of the boson. We show that the fermion produced by the inflaton decay can be the superheavy dark matter, whose abundance in the present universe coincides with the critical density. We also present two explicit models as examples in which our scenario can be realized. One is the softly broken supersymmetric theory. The other is the ``singlet majoron model'' with an assumed neutrino mass matrix. The latter example can simultaneously explain the neutrino oscillation data and the observed baryon asymmetry in the present universe through the leptogenesis scenario.
We describe a theoretical framework to compute the cluster gas distribution in hydrostatic equilibrium embedded in a class of spherical dark matter halo potentials. Unlike the conventional isothermal $β$-model, the present method provides a physical basis to directly probe the shape of dark matter halo from the observed X-ray surface brightness and temperature profiles of clusters of galaxies. Specifically, we examine the extent to which the resulting gas density and X-ray surface brightness profiles are sensitive to the inner slope of the dark matter halo density and other more realistic effects including the self-gravity of the gas and the polytropic equation of state. We also discuss a practical strategy to apply the present methodology to the actual cluster profiles from future X-ray observations.
Cosmological implications of clusters of galaxies are discussed with particular attention to their importance in probing the cosmological parameters. More specifically we compute the number counts of clusters of galaxies, Log $N$ -- Log $S$ relation, in X-ray and submm bands on the basis of the Press--Schechter theory. We pay particular attention to a set of theoretical models which well reproduce the {\it ROSAT} 0.5-2 keV band Log $N$ -- Log $S$, and explore possibilities to break the degeneracy among the viable cosmological models.
The X-ray cluster gas density distribution in hydrostatic equilibrium is computed from the universal density profile of the dark matter halo recently proposed by Navarro, Frenk and White (1996, 1997). If one assumes the isothermality, the resulting distribution is well approximated by the conventional $β$-model. We predict the core radius $r_{c}$, the $β$-parameter, and the X-ray luminosity of clusters as a function of the temperature $T_{X}$ of clusters in some representative cosmological models, and compare them with observations and results of numerical simulations. The predicted size of $r_{c}$ is a factor of ($3 \sim 10$) smaller than the average of observed values. If both the universal density profile and the hydrostatic equilibrium are reasonable approximation to the truth, then this suggests either that the previous X-ray observations systematically overestimate the core radius of gas densities in clusters of galaxies, or that some important physical mechanisms, which significantly increase the core radius, is still missing.
We compute the number counts of clusters of galaxies, the logN-logS relation, in several X-ray and submm bands on the basis of the Press-Schechter theory. We pay particular attention to a set of theoretical models which well reproduce the ROSAT 0.5-2 keV band logN-logS, and explore possibilities to further constrain the models from future observations with ASCA and/or at submm bands. The latter is closely related to the European PLANCK mission and the Japanese Large Millimeter and Submillimeter Array (LMSA) project. We exhibit that one can break the degeneracy in an acceptable parameter region on the $Ω_0 - σ_8$ plane by combining the ROSAT logN-logS and the submm number counts. Models which reproduce the ROSAT band logN-logS will have $N(>S) \sim (150-300) (S/10^{-12} erg cm^{-2} s^{-1})^{-1.3}$ str$^{-1}$ at $S > 10^{-12} erg cm^{-2} s^{-1}$ in the ASCA 2-10 keV band, and $N(>S_ν) \sim (10^2-10^4) (S_ν/100 mJy)^{-1.5} str^{-1}$ at $S_ν> 100 mJy$ in the submm (0.85mm) band. The amplitude of the logN-logS is very sensitive to the model parameters in the submm band. We also compute the redshift evolution of the cluster number counts and compare with that of the X-ray brightest Abell-type clusters. The results, although still preliminary, point to low density ($Ω_0\sim 0.3$) universes. The contribution of clusters to the X-ray and submm background radiations is shown to be insignificant in any model compatible with the ROSAT logN-logS.
We propose a new method to estimate cosmological parameters using the baryon fraction of clusters of galaxies for a range of redshifts. The basic assumption is that the baryon fraction of clusters is constant, which is a reasonable assumption when it is averaged within a Mpc scale. We find that the baryon fraction vs. redshift diagram can estimate the cosmological parameters, since the derived value of the baryon fraction from observations depends on the adopted value of cosmological parameters. We also discuss some points in comparing theoretical calculations with observations.
We performed a systematic and comprehensive estimate of the Hubble constant $H_0$ via the Sunyaev-Zel'dovich temperature decrement and the X-ray measurements for clusters up to the redshift $z=0.541$, with particular attention to its dependence on the density parameter $Ω_0$, and the cosmological constant $λ_0$. The resulting values of $H_0$ are largely consistent with that derived from the Cepheid distance for nearby galaxies. We discuss how the angular diameter distance $d_A$ vs. $z$ diagram from the accurate SZ measurements of high-$z$ clusters can distinguish the values of $Ω_0$ and $λ_0$.
A simple model of the Majorana neutrino with the see-saw mechanism is studied, assuming that two light neutrinos are the hot dark matters with equal mass of 2.4 eV in the cold plus hot dark matter model of cosmology. We find that the heavy neutrino, which is the see-saw partner with the remaining one light neutrino, can be the cold dark matter, if the light neutrino is exactly massless. This cold dark matter neutrino is allowed to have the mass of the wide range from 5.9 x 10^2 eV to 2.2 x 10^7 eV.
We compute the angular two-point correlation functions of the gamma-ray bursts at cosmological distances. Since the gamma-ray burst emission mechanism is not yet established, we simply assume that the gamma-ray burst sources are associated with high-redshift galaxies in some way. Then on the basis of several simple models for the evolution of galaxy spatial correlations, we calculate the amplitude of angular two-point correlation functions on scales appropriate for the Compton Gamma Ray Observatory data. we find that in most cases the predicted correlations are difficult to detect with the current data rate and the angular resolution, but models in which the bursts preferentially occur at relatively low redshift ($z \simlt 0.5$) predict correlation amplitudes on $θ\sim 5^\circ$ which will be marginally detectable with the Gamma Ray Observatory data in several years. If future observations detect a signal of angular correlations, it will imply useful information on the correlation of galaxies at high redshifts provided that the gamma-ray bursts are cosmological.
Recently, Jakobsen et al. reported that the HeII Gunn-Peterson optical depth is greater than 1.7 at $ z=3.3$. When we consider this observation combined with the HI Gunn-Peterson optical depths and the intensity of the UV background at high redshifts, we can put constraints on the spectrum of the UV background and the mass fraction of intergalactic medium. We find that the spectrum must be steep ($α\gsim 3; J_ν \propto ν^{-α} $) to be consistent with observations, and that the mass fraction of the intergalactic medium must be greater than 0.3 (0.1) in the case $ α=3 (4)$ for baryon density consistent with the primordial nucleosynthesis. We also find that HeII fraction is as low as $10^{-3} \sim 10^{-1} $ even for this steep spectrum.