Three-dimensional orbits of metal-poor halo stars and the formation of the Galaxy

We present the three-dimensional orbital motions of metal-poor stars in conjunction with their metal abundances, for the purpose of getting insight into the formation process of the Galaxy. Our sample stars, which include metal-deficient red giants and RR Lyrae variables observed by the Hipparcos satellite, are least affected by known systematics, stemmed from kinematic bias, metallicity calibration, and secondary metal contamination of stellar surface. We find, for the stars in the metallicity range of [Fe/H]<-1, that there is no evidence for the correlation between [Fe/H] and their orbital eccentricities e. Even for [Fe/H]<-1.6, about 16% of the stars have e less than 0.4. We show that the e distribution of orbits for [Fe/H]<-1.6 is independent of the height |z| away from the Galactic plane, whereas for [Fe/H]>-1.6 the stars at |z|>1 kpc are systematically devoid of low-e orbits with e<0.6. This indicates that low-e stars with [Fe/H]<-1.6 belong to the halo component, whereas the rapidly-rotating thick disk with a scale height about 1 kpc has a metal-weak tail in the range of -1.6<[Fe/H]<-1. The fraction of this metal-weak thick disk appears to be only less than 20%. The significance of these results for the early evolution of the Galaxy is briefly discussed.Comment: 11 pages, 3 figures, AASTeX, to appear in ApJ Letter

Calibration of AGN Reverberation Distance Measurements

In Yoshii et al. (2014), we described a new method for measuring extragalactic distances based on dust reverberation in active galactic nuclei (AGNs), and we validated our new method with Cepheid variable stars. In this paper, we validate our new method with Type Ia supernovae (SNe Ia) which occurred in two of the AGN host galaxies during our AGN monitoring program: SN 2004bd in NGC 3786 and SN 2008ec in NGC 7469. Their multicolor light curves were observed and analyzed using two widely accepted methods for measuring SN distances, and the distance moduli derived are $\mu=33.47\pm 0.15$ for SN 2004bd and $33.83\pm 0.07$ for SN 2008ec. These results are used to obtain independently the distance measurement calibration factor, $g$. The $g$ value obtained from the SN Ia discussed in this paper is $g_{\rm SN} = 10.61\pm 0.50$ which matches, within the range of 1$\sigma$ uncertainty, $g_{\rm DUST} = 10.60$, previously calculated ab initio in Yoshii et al. (2014). Having validated our new method for measuring extragalactic distances, we use our new method to calibrate reverberation distances derived from variations of H$\beta$ emission in the AGN broad line region (BLR), extending the Hubble diagram to $z\approx 0.3$ where distinguishing between cosmologies is becoming possible.Comment: Astrophysical Journal Letters accepte

Galaxy Number Counts in the Subaru Deep Field: Multi-band Analysis in a Hierarchical Galaxy Formation Model

Number counts of galaxies are re-analyzed using a semi-analytic model (SAM) of galaxy formation based on the hierarchical clustering scenario. Faint galaxies in the Subaru Deep Field (SDF) and the Hubble Deep Field (HDF) are compared with our model galaxies. We have determined the astrophysical parameters in the SAM that reproduce observations of nearby galaxies, and used them to predict the number counts and redshifts of faint galaxies for three cosmological models, the standard cold dark matter (CDM) universe, a flat lambda-CDM, and an open CDM. The novelty of our SAM analysis is the inclusion of selection effects arising from the cosmological dimming of surface brightness of high-z galaxies, and from the absorption of visible light by internal dust and intergalactic HI clouds. As was found in our previous work, in which the UV/optical HDF galaxies were compared with our model galaxies, we find that our SAM reproduces counts of near-IR SDF galaxies in low-density models, and that the standard CDM universe is not preferred, as suggested by other recent studies. Moreover, we find that simple prescriptions for (1) the timescale of star formation being proportional to the dynamical time scale of the formation of galactic disks, (2) the size of galactic disks being rotationally supported with the same specific angular momentum as that of surrounding dark halo, and (3) the dust optical depth being proportional to the metallicity of cold gas, cannot completely explain all of observed data. Improved prescriptions incorporating mild z-dependence for those are suggested from our SAM analysis.Comment: 16 pages, 13 figures, to appear in Ap

Galaxy number counts in the Hubble Deep Field as a strong constraint on a hierarchical galaxy formation model

Number counts of galaxies are re-analyzed using a semi-analytic model (SAM) of galaxy formation based on the hierarchical clustering scenario. We have determined the astrophysical parameters in the SAM that reproduce observations of nearby galaxies, and used them to predict the number counts and redshifts of faint galaxies for three cosmological models for (1) the standard cold dark matter (CDM) universe, (2) a low-density flat universe with nonzero cosmological constant, and (3) a low-density open universe with zero cosmological constant. The novelty of our SAM analysis is the inclusion of selection effects arising from the cosmological dimming of surface brightness of high-redshift galaxies, and also from the absorption of visible light by internal dust and intergalactic \ion{H}{1} clouds. Contrary to previous SAM analyses which do not take into account such selection effects, we find, from comparison with observed counts and redshifts of faint galaxies in the Hubble Deep Field (HDF), that the standard CDM universe is {\it not} preferred, and a low-density universe either with or without cosmological constant is favorable, as suggested by other recent studies. Moreover, we find that a simple prescription for the time scale of star formation (SF), being proportional to the dynamical time scale of the formation of the galactic disk, is unable to reproduce the observed number- redshift relation for HDF galaxies, and that the SF time scale should be nearly independent of redshift, as suggested by other SAM analyses for the formation of quasars and the evolution of damped Ly-$\alpha$ systems.Comment: 16 pages, 13 figures, LaTeX, using emulateapj5.st

Diffuse Extragalactic Background Light versus Deep Galaxy Counts in the Subaru Deep Field: Missing Light in the Universe?

Deep optical and near-infrared galaxy counts are utilized to estimate the extragalactic background light (EBL) coming from normal galactic light in the universe. Although the slope of number-magnitude relation of the faintest counts is flat enough for the count integration to converge, considerable fraction of EBL from galaxies could still have been missed in deep galaxy surveys because of various selection effects including the cosmological dimming of surface brightness of galaxies. Here we give an estimate of EBL from galaxy counts, in which these selection effects are quantitatively taken into account for the first time, based on reasonable models of galaxy evolution which are consistent with all available data of galaxy counts, size, and redshift distributions. We show that the EBL from galaxies is best resolved into discrete galaxies in the near-infrared bands (J, K) by using the latest data of the Subaru Deep Field; more than 80-90% of EBL from galaxies has been resolved in these bands. Our result indicates that the contribution by missing galaxies cannot account for the discrepancy between the count integration and recent tentative detections of diffuse EBL in the K-band (2.2 micron), and there may be a very diffuse component of EBL which has left no imprints in known galaxy populations.Comment: ApJ Letters in press. Two new reports on the diffuse EBL at 1.25 and 2.2 microns are added to the reference list and Table

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

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