A new measurement of the baryonic fraction using the sparse NGC 3258 group of galaxies

New X-ray observations of the sparse NGC 3258 group of galaxies made by the ASCA satellite with good spectral and spatial resolution has revealed that this group has a gravitational potential deep enough to prohibit significant mass removal from the system. The baryonic fraction within 240 kpc is found to be 0.065 +0.051/-0.020 for h_{50}=1, where h_{50}=H_0/50 km/s/Mpc, in good agreement with the universal value of 0.05 +/-0.01 predicted by standard Big Bang nucleosynthesis for a Universe with Omega_0=1 and h_{50}=1. Since the deep potential of the NGC 3258 group ensures that all pristine intragroup gas has been retained, the baryonic fraction of the NGC 3258 group is indicative of the universal value. Consequently it seems premature to rule out a critical Universe.Comment: 19 pages Latex, using aasms4.sty, paper also available at http://www.dsri.dk/~kristian To appear in Astrophysical Journal Letter

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

Telling the tale of the first stars

HE 0107-5240 is a star in more than once sense of the word. Chemically, it is the most primitive object yet discovered, and it is at the centre of debate about the origins of the first elements in the Universe.Comment: 3 pages, 0 figures, published in Nature "News and Views," Apr. 24, 200

Kinematics of Metal-Poor Stars in the Galaxy. II. Proper Motions for a Large Non-Kinematically Selected Sample

We present a revised catalog of 2106 Galactic stars, selected without kinematic bias, and with available radial velocities, distance estimates, and metal abundances in the range 0.0 <= [Fe/H] <= -4.0. This update of the Beers and Sommer-Larsen (1995) catalog includes newly-derived homogeneous photometric distance estimates, revised radial velocities for a number of stars with recently obtained high-resolution spectra, and refined metallicities for stars originally identified in the HK objective-prism survey (which account for nearly half of the catalog) based on a recent re-calibration. A subset of 1258 stars in this catalog have available proper motions, based on measurements obtained with the Hipparcos astrometry satellite, or taken from the updated Astrographic Catalogue (AC 2000; second epoch positions from either the Hubble Space Telescope Guide Star Catalog or the Tycho Catalogue), the Yale/San Juan Southern Proper Motion (SPM) Catalog 2.0, and the Lick Northern Proper Motion (NPM1) Catalog. Our present catalog includes 388 RR Lyrae variables (182 of which are newly added), 38 variables of other types, and 1680 non-variables, with distances in the range 0.1 to 40 kpc.Comment: 31 pages, including 8 figures, to appear in AJ (June 2000), full paper with all figures embedded available at http://pluto.mtk.nao.ac.jp/people/chiba/preprint/halo4

Electrical transport properties of bulk MgB2 materials synthesized by the electrolysis on fused mixtures of MgCl2, NaCl, KCl and MgB2O4

Electrolysis was carried out on fused mixtures of MgCl2, NaCl, KCl and MgB2O4 under an Ar flow at 600C. Electrical resistivity measurements for the grown deposits show an onset of superconducting transition at 37 K in the absence of applied magnetic field. The resistivity decreases down to zero below 32 K. From an applied-field dependence of resistivity, an upper critical field and a coherence length were calculated to be 9.7 T and 5.9 nm at 0 K, respectively

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

Unavoidable Selection Effects in the Analysis of Faint Galaxies in the Hubble Deep Field: Probing the Cosmology and Merger History of Galaxies

(Abridged) We present a detailed analysis of the number count and photometric redshift distribution of faint galaxies in the Hubble Deep Field (HDF), paying a special attention to the selection effects including the cosmological dimming of surface brightness of galaxies. We find a considerably different result from previous studies ignoring the selection effects, and these effects should therefore be taken into account in the analysis. We find that the model of pure luminosity evolution (PLE) of galaxies in the Einstein-de Sitter (EdS) universe predicts much smaller counts than those observed at faint magnitude limits by a factor of more than 10, so that a very strong number evolution of galaxies with \eta > 3-4 must be invoked to reproduce the I_{814} counts, when parametrized as \phi^* \propto (1+z)^\eta. However we show that such a strong number evolution under realistic merging processes of galaxies can not explain the steep slope of the B_{450} and V_{606} counts, and it is seriously inconsistent with their photometric redshift distribution. We find that these difficulties still persist in an open universe with \Omega_0 > 0.2, but are resolved only when we invoke a $\Lambda$-dominated flat universe, after examining various systematic uncertainties in modeling the formation and evolution of galaxies. The present analysis revitalizes the practice of using faint number counts as an important cosmological test, giving one of the arguments against the EdS universe and suggests acceleration of the cosmic expansion by vacuum energy density. While a modest number evolution of galaxies with \eta ~ 1 is still necessary even in a Lambda-dominated universe, a stronger number evolution with \eta > 1 is rejected from the HDF data, giving a strong constraint on the merger history of galaxies.Comment: 24 pages, 15 figures, final version matching publication in ApJ. Some references added. The complete ps file of Table 3 is available at http://th.nao.ac.jp/~totani/images/paper/ty2000-table3.p

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