S=1/2 Kagome antiferromagnets Cs2_2Cu3MF_3MF_{12}$ with M=Zr and Hf

Magnetization and specific heat measurements have been carried out on Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$ single crystals, in which Cu$^{2+}$ ions with spin-1/2 form a regular Kagom\'{e} lattice. The antiferromagnetic exchange interaction between neighboring Cu$^{2+}$ spins is $J/k_{\rm B}\simeq 360$ K and 540 K for Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$, respectively. Structural phase transitions were observed at $T_{\rm t}\simeq 210$ K and 175 K for Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$, respectively. The specific heat shows a small bend anomaly indicative of magnetic ordering at $T_\mathrm{N}= 23.5$ K and 24.5 K in Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$, respectively. Weak ferromagnetic behavior was observed below $T_\mathrm{N}$. This weak ferromagnetism should be ascribed to the antisymmetric interaction of the Dzyaloshinsky-Moriya type that are generally allowed in the Kagom\'{e} lattice.Comment: 6 pages, 4 figure. Conference proceeding of Highly Frustrated Magnetism 200

Vlasov versus N-body: the H\'enon sphere

We perform a detailed comparison of the phase-space density traced by the particle distribution in Gadget simulations to the result obtained with a spherical Vlasov solver using the splitting algorithm. The systems considered are apodized H\'enon spheres with two values of the virial ratio, R ~ 0.1 and 0.5. After checking that spherical symmetry is well preserved by the N-body simulations, visual and quantitative comparisons are performed. In particular we introduce new statistics, correlators and entropic estimators, based on the likelihood of whether N-body simulations actually trace randomly the Vlasov phase-space density. When taking into account the limits of both the N-body and the Vlasov codes, namely collective effects due to the particle shot noise in the first case and diffusion and possible nonlinear instabilities due to finite resolution of the phase-space grid in the second case, we find a spectacular agreement between both methods, even in regions of phase-space where nontrivial physical instabilities develop. However, in the colder case, R=0.1, it was not possible to prove actual numerical convergence of the N-body results after a number of dynamical times, even with N=10$^8$ particles.Comment: 19 pages, 11 figures, MNRAS, in pres

Redshift-space Distortions of the Power Spectrum of Cosmological Objects on a Light Cone : Explicit Formulations and Theoretical Implications

We examine the effects of the linear and the cosmological redshift-space distortions on the power spectrum of cosmological objects on a light cone. We develop theoretical formulae for the power spectrum in linear theory of density perturbations in a rigorous manner starting from first principle corresponding to Fourier analysis. Approximate formulae, which are useful properly to incorporate the redshift-space distortion effects into the power spectrum are derived, and the validity is examined. Applying our formulae to galaxy and quasar samples which roughly match the SDSS survey, we will show how the redshift-space distortions distort the power spectrum on the light cone quantitatively.Comment: 30 pages, Accepted for publication in the Astrophysical Journal Supplement Serie

Probability Distribution of the Hubble Constant and the Age of the Universe Inferred from the Local Observation

We present a method to compute the probability distribution function of the (true) Hubble constant and the age of the universe, given the estimate of the Hubble constant in our nearby galaxy samples. Our method takes into account both the observational errors and the cosmic variance, and enables to quantitatively compute the constraints on the cosmological models. Based on the present local observation $H_0=80\pm17$ km/s/Mpc, the probability of $H_0<50$ km/s/Mpc is about 6\% for the Einstein -- de Sitter universe ($\Omega_0=1$) and 4\% for an open ($\Omega_0=0.2$) universe. These probabilities are reduced to 0.8\% and 0.03\%, respectively, if the accuracy of the observational uncertainty is improved within 10\%. Similar probabilistic constraints on $t_0$ are also discussed.Comment: 8 pages, 3 figures (uuencoded postscript

Strong Gravitational Lensing and Velocity Function as Tools to Probe Cosmological Parameters: Current Constraints and Future Predictions

Constraints on cosmological models from strong gravitational lensing statistics are investigated. We pay particular attention to the role of the velocity function in the calculation of the lensing probability. The velocity function derived from the observed galaxy luminosity function, which is used in most previous work, is unable to predict the large separation lensing events. In this paper, we also use the Press-Schechter theory to construct a velocity function theoretically. Model predictions are compared with the observed velocity function and the HST snapshot survey. Comparison with the latter observation shows that the predictions based on the theoretical velocity function are consistent with the observed large separation events in COBE normalized low-density models, especially with a non-vanishing cosmological constant. Adopting the COBE normalization, however, we could find no model which simultaneously satisfies both the observed velocity function and the HST snapshot survey. We systematically investigate various uncertainties in the gravitational lensing statistics including finite core radius, the distance formula, magnification bias, and dust obscuration. The results are very sensitive to these effects as well as theoretical models for the velocity function, implying that current limits on the cosmological parameters should be interpreted with caution. Predictions for future surveys are also presented.Comment: 27 pages, 8 figures, ptptex. Progress of Theoretical Physics, in pres

Infrared Spectroscopy of CO Ro-vibrational Absorption Lines toward the Obscured AGN IRAS 08572+3915

We present high-resolution spectroscopy of gaseous CO absorption in the fundamental ro-vibrational band toward the heavily obscured active galactic nucleus (AGN) IRAS 08572+3915. We have detected absorption lines up to highly excited rotational levels (J<=17). The velocity profiles reveal three distinct components, the strongest and broadest (delta_v > 200 km s-1) of which is due to blueshifted (-160 km s-1) gas at a temperature of ~ 270 K absorbing at velocities as high as -400 km s-1. A much weaker but even warmer (~ 700 K) component, which is highly redshifted (+100 km s-1), is also detected, in addition to a cold (~ 20 K) component centered at the systemic velocity of the galaxy. On the assumption of local thermodynamic equilibrium, the column density of CO in the 270 K component is NCO ~ 4.5 x 10^18 cm-2, which in fully molecular gas corresponds to a H2 column density of NH2 ~ 2.5 x 10^22 cm-2. The thermal excitation of CO up to the observed high rotational levels requires a density greater than nc(H2) > 2 x 10^7 cm-3, implying that the thickness of the warm absorbing layer is extremely small (delta_d < 4 x 10-2 pc) even if it is highly clumped. The large column densities and high radial velocities associated with these warm components, as well as their temperatures, indicate that they originate in molecular clouds near the central engine of the AGN.Comment: 13 pages, 7 figures, accepted for publication in PASJ (Vol.65 No.1 2013/02/25

Two-point correlation functions on the light cone: testing theoretical predictions against N-body simulations

We examine the light-cone effect on the two-point correlation functions using numerical simulations for the first time. Specifically, we generate several sets of dark matter particle distributions on the light-cone up to z=0.4 and z=2 over the field-of-view of \pi degree^2 from cosmological N-body simulations. Then we apply the selection function to the dark matter distribution according to the galaxy and QSO luminosity functions. Finally we compute the two-point correlation functions on the light-cone both in real and in redshift spaces using the pair-count estimator and compare with the theoretical predictions. We find that the previous theoretical modeling for nonlinear gravitational evolution, linear and nonlinear redshift-distortion, and the light-cone effect including the selection function is in good agreement with our numerical results, and thus is an accurate and reliable description of the clustering in the universe on the light-cone