Spin-Glass-like Transition and Hall Resistivity of Y2-xBixIr2O7

Various physical properties of the pyrochlore oxide Y2-xBixIr2O7 have been studied. The magnetizations M measured under the conditions of the zero-field-cooling(ZFC) and the field-cooling(FC) have different values below the temperature T=TG. The anomalous T-dependence of the electrical resistivities r and the thermoelectric powers S observed at around TG indicates that the behavior of the magnetization is due to the transition to the state with the spin freezing. In this spin-frozen state, the Hall resistivities rH measured with the ZFC and FC conditions are found to have different values, too, in the low temperature phase (T<TG). Possible mechanisms which induce such the hysteretic behavior are discussed.Comment: 9 pages, 7 figures, J. Phys. Soc. Jpn. 72 (2003) No.

Evolution of the Luminosity Density in the Universe: Implications for the Nonzero Cosmological Constant

We show that evolution of the luminosity density of galaxies in the universe provides a powerful test for the geometry of the universe. Using reasonable galaxy evolution models of population synthesis which reproduce the colors of local galaxies of various morphological types, we have calculated the luminosity density of galaxies as a function of redshift $z$. Comparison of the result with recent measurements by the Canada-France Redshift Survey in three wavebands of 2800{\AA}, 4400{\AA}, and 1 micron at z<1 indicates that the \Lambda-dominated flat universe with \lambda_0 \sim 0.8 is favored, and the lower limit on \lambda_0 yields 0.37 (99% C.L.) or 0.53 (95% C.L.) if \Omega_0+\lambda_0=1. The Einstein-de Sitter universe with (\Omega_0, \lambda_0)=(1, 0) and the low-density open universe with (0.2, 0) are however ruled out with 99.86% C.L. and 98.6% C.L., respectively. The confidence levels quoted apply unless the standard assumptions on galaxy evolution are drastically violated. We have also calculated a global star formation rate in the universe to be compared with the observed rate beyond z \sim 2. We find from this comparison that spiral galaxies are formed from material accretion over an extended period of a few Gyrs, while elliptical galaxies are formed from initial star burst at z >~ 5 supplying enough amount of metals and ionizing photons in the intergalactic medium.Comment: 11 pages including 3 figures, LaTeX, uses AASTeX. To Appear in ApJ Letter

The mass spectrum of metal-free Stars resulting from photodissociation feedback: A scenario for the formation of low-mass population III stars

The initial mass function (IMF) of metal-free stars that form in the initial starburst of massive (virial temperatures >10^4K) metal-free protogalaxies is studied. In particular, we focus on the effect of H2 photodissociation by pre-existing stars on the fragmentation mass scale, presumedly determined by the Jeans mass at the end of the initial free-fall phase, i.e., at the so-called ``loitering phase,'' characterized by the local temperature minimum. Photodissociation diminishes the Jeans mass at the loitering phase, thereby reducing the fragmentation mass scale of primordial clouds. Thus, in a given cloud, far ultraviolet (FUV) radiation from the first star, which is supposedly very massive (about 10^3Msun), reduces the mass scale for subsequent fragmentation. Through a series of similar processes the IMF for metal-free stars is established. If FUV radiation exceeds a threshold level, the star-forming clumps collapse solely through atomic cooling. Correspondingly, the fragmentation scale drops discontinuously from a few time 10Msun to sub-solar scales. In compact clouds (>1.6kpc for clouds of gas mass 10^8Msun), this level of radiation field is attained, and sub-solar mass stars are formed even in a metal-free environment. Consequently, the IMF becomes bi-modal, with peaks at a few tenths of Msun and a few times 10Msun. The high-mass portion of the IMF is found to be a very steep function of the stellar mass, xi_high(m) being proportinal to m^{-5}. Therefore, the typical mass scale of metal-free stars is significantly smaller than that of the very first stars. Also we study the thermal instability in collapsing primordial prestellar clumps, and discuss why the thermal instability occuring during the three-body H2 formation does not appear to manifest itself in causing further fragmentation of such clumps.Comment: 34 pages, 6 figures, ApJ accepte

New limits on a cosmological constant from statistics of gravitational lensing

We present new limits on cosmological parameters from the statistics of gravitational lensing, based on the recently revised knowledge of the luminosity function and internal dynamics of E/S0 galaxies that are essential in lensing high-redshift QSOs. We find that the lens models using updated Schechter parameters for such galaxies, derived from the recent redshift surveys combined with morphological classification, are found to give smaller lensing probabilities than earlier calculated. Inconsistent adoption of these parameters from a mixture of various galaxy surveys gives rise to systematic biases in the results. We also show that less compact dwarf-type galaxies which largely dominate the faint part of the Schechter-form luminosity function contribute little to lensing probabilities, so that earlier lens models overestimate incidents of small separation lenses. Applications of the lens models to the existing lens surveys indicate that reproduction of both the lensing probability of optical sources and the image separations of optical and radio lenses is significantly improved in the revised lens models. The likelihood analyses allow us to conclude that a flat universe with Omega=0.3(+0.2-0.1) and Omega+Lambda=1 is most preferable, and a matter-dominated flat universe with Lambda=0 is ruled out at 98 % confidence level. These new limits are unaffected by inclusion of uncertainties in the lens properties.Comment: 30 pages, 9 ps figures, AASTeX, ApJ in pres

Evidence for Carrier-Induced High-Tc Ferromagnetism in Mn-doped GaN film

A GaN film doped with 8.2 % Mn was grown by the molecular-beam-epitaxy technique. Magnetization measurements show that this highly Mn-doped GaN film exhibits ferromagnetism above room temperature. It is also revealed that the high-temperature ferromagnetic state is significantly suppressed below 10 K, accompanied by an increase of the electrical resistivity with decreasing temperature. This observation clearly demonstrates a close relation between the ferromagnetism with extremely high-Tc and the carrier transport in the Mn-doped GaN film.Comment: 9 pages, 3 figure

Age Dating of a High-Redshift QSO B1422+231 at Z=3.62 and its Cosmological Implications

The observed Fe II(UV+optical)/Mg II lambda lambda 2796,2804 flux ratio from a gravitationally lensed quasar B1422+231 at z=3.62 is interpreted in terms of detailed modeling of photoionization and chemical enrichment in the broad-line region (BLR) of the host galaxy. The delayed iron enrichment by Type Ia supernovae is used as a cosmic clock. Our standard model, which matches the Fe II/Mg II ratio, requires the age of 1.5 Gyr for B1422+231 with a lower bound of 1.3 Gyr, which exceeds the expansion age of the Einstein-de Sitter Omega_0=1 universe at a redshift of 3.62 for any value of the Hubble constant in the currently accepted range, H_0=60-80 km,s^{-1},Mpc^{-1}. This problem of an age discrepancy at z=3.62 can be unraveled in a low-density Omega_0<0.2 universe, either with or without a cosmological constant, depending on the allowable redshift range of galaxy formation. However, whether the cosmological constant is a required option in modern cosmology awaits a thorough understanding of line transfer processes in the BLRs.Comment: 7 pages including 3 figures, to appear in ApJ Letter

Hierarchical Formation of Galaxies with Dynamical Response to Supernova-Induced Gas removal

We reanalyze the formation and evolution of galaxies in the hierarchical clustering scenario. Using a semi-analytic model (SAM) of galaxy formation described in this paper, which we hereafter call the Mitaka model, we extensively investigate the observed scaling relations of galaxies among photometric, kinematic, structural and chemical characteristics. In such a scenario, spheroidal galaxies are assumed to be formed by major merger and subsequent starburst, in contrast to the traditional scenario of monolithic cloud collapse. As a new ingredient of SAMs, we introduce the effects of dynamical response to supernova-induced gas removal on size and velocity dispersion, which play an important role on dwarf galaxy formation. In previous theoretical studies of dwarf galaxies based on the monolithic cloud collapse given by Yoshii & Arimoto and Dekel & Silk, the dynamical response was treated in the extremes of a purely baryonic cloud and a baryonic cloud fully supported by surrounding dark matter. To improve this simple treatment, in our previous paper, we formulated the dynamical response in more realistic, intermediate situations between the above extremes. While the effects of dynamical response depend on the mass fraction of removed gas from a galaxy, how much amount of the gas remains just after major merger depends on the star formation history. A variety of star formation histories are generated through the Monte Carlo realization of merging histories of dark halos, and it is found that our SAM naturally makes a wide variety of dwarf galaxies and their dispersed characteristics as observed. (Abridged)Comment: 24 pages including 29 figures, using emulateapj.cls; accepted for publication in Ap