arXiv subjects

Masahiro Nagashima

Publications and source records attributed to Masahiro Nagashima.

At least 55 records · Page 3Linked to original sources

arXiv2005

Gamma-Ray Background from Neutralino Annihilation in the First Cosmological Objects

The paradigm of the neutralino dark matter predicts that the first gravitationally bound objects are earth-mass sized microhaloes, which would emit annihilation gamma-rays. Here we show that, though the flux from individual nearest microhaloes is extremely difficult to detect, meaningful constraints on their survival probability and internal density profile can be set by requiring that the galactic and extragalactic gamma-ray background flux from the microhaloes does not exceed the existing EGRET background data. Possible disruption of microhaloes by stellar encounters does not significantly reduce the background flux. If the probability for microhaloes to survive the hierarchical clustering process of dark matter is as large as indicated by a recent simulation, they could be a significant component of the observed background flux in some photon energy range, even with the standard annihilation cross section and conservative internal density profile of microhaloes. The integrated gamma-ray flux from microhaloes in the halo of the Andromeda galaxy may also be detectable by observations in the near future.

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arXiv2005

Numerical Galaxy Catalog -I. A Semi-analytic Model of Galaxy Formation with N-body simulations

We construct the Numerical Galaxy Catalog ($ν$GC), based on a semi-analytic model of galaxy formation combined with high-resolution N-body simulations in a $Λ$-dominated flat cold dark matter ($Λ$CDM) cosmological model. The model includes several essential ingredients for galaxy formation, such as merging histories of dark halos directly taken from N-body simulations, radiative gas cooling, star formation, heating by supernova explosions (supernova feedback), mergers of galaxies, population synthesis, and extinction by internal dust and intervening HI clouds. As the first paper in a series using this model, we focus on basic photometric, structural and kinematical properties of galaxies at present and high redshifts. Two sets of model parameters are examined, strong and weak supernova feedback models, which are in good agreement with observational luminosity functions of local galaxies in a range of observational uncertainty. Both models agree well with many observations such as cold gas mass-to-stellar luminosity ratios of spiral galaxies, HI mass functions, galaxy sizes, faint galaxy number counts and photometric redshift distributions in optical pass-bands, isophotal angular sizes, and cosmic star formation rates. In particular, the strong supernova feedback model is in much better agreement with near-infrared (K'-band) faint galaxy number counts and redshift distribution than the weak feedback model and our previous semi-analytic models based on the extended Press-Schechter formalism. (Abridged)

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arXiv2005

The metal enrichment of elliptical galaxies in hierarchical galaxy formation models

We investigate the metal enrichment of elliptical galaxies in the framework of hierarchical models of galaxy formation. The semi-analytical model we use in this Letter, which has been used to study the metal enrichment of the intracluster medium (ICM) by Nagashima et al., includes the effects of flows of gas and metals both into and out of galaxies and the processes of metal enrichment due to both type Ia and type II supernovae. We adopt a solar neighbourhood IMF for star formation in discs, but consider models in which starbursts have either a solar neighbourhood IMF or a top-heavy IMF. We find that the alpha-element abundance in ellipticals is consistent with observed values only if the top-heavy IMF is used. This result is consistent with our previous study on the metal enrichment of the ICM. We also discuss the abundance ratio of alpha elements to iron as a function of velocity dispersion and metallicity. We find that models with a top-heavy IMF match the alpha/Fe ratios observed in typical L_* ellipticals, but none of the models reproduce the observed increase of alpha/Fe with velocity dispersion.

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arXiv2005

Evaporation and Condensation of HI clouds in thermally bistable interstellar media: semi-analytic description of isobaric dynamics of curved interfaces

We analyze the evaporation and condensation of spherical and cylindrical HI clouds of the cold neutral medium surrounded by the warm neutral medium. Because the interstellar medium including those two phases is well described as a thermally bistable fluid, it is useful to apply pattern formation theories to the dynamics of the interface between the two phases. Assuming isobaric evolution of fluids and a simple cubic form of the heat-loss function, we show the curvature effects of the interface. We find that approximate solutions for spherical clouds are in good agreement with numerically obtained solutions. We extend our analysis to general curved fronts taking into account the curvature effects explicitly. We find that the curvature effects always stabilise curved interfaces under assumptions such as isobaric evolution we adopt in this Letter.

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arXiv2005

Gravitational Radiation from Coalescing Supermassive Black Hole Binaries in a Hierarchical Galaxy Formation Model

We investigate the expected gravitational wave emission from coalescing supermassive black hole (SMBH) binaries resulting from mergers of their host galaxies. We employ a semi-analytic model of galaxy and quasar formation based on the hierarchical clustering scenario to estimate the amplitude of the expected stochastic gravitational wave background owing to inspiraling SMBH binaries and bursts rates owing to the SMBH binary coalescence events. We find that the characteristic strain amplitude of the background radiation is $h_c(f) \sim 10^{-16} (f/1 μ{\rm Hz})^{-2/3}$ for $f \lesssim 1 μ{\rm Hz}$. The main contribution to the total strain amplitude of the background radiation comes from SMBH coalescence events at $0<z<1$. We also find that a future space-based gravitational wave interferometer such as the planned \textit{Laser Interferometer Space Antenna} ({\sl LISA}) might detect intense gravitational wave bursts associated with coalescence of SMBH binaries with total mass $M_{\rm tot} < 10^7 M_{\odot}$ at $z \gtrsim 2$ at a rate $ \sim 1.0 {\rm yr}^{-1}$. Our model predicts that burst signals with a larger amplitude $h_{\rm burst} \sim 10^{-15}$ correspond to coalescence events of massive SMBH binary with total mass $M_{\rm tot} \sim 10^8 M_{\odot}$ at low redshift $ z \lesssim 1$ at a rate $ \sim 0.1 {\rm yr}^{-1}$ whereas those with a smaller amplitude $h_{\rm burst} \sim 10^{-17}$ correspond to coalescence events of less massive SMBH binary with total mass $M_{\rm tot} \sim 10^6 M_{\odot}$ at high redshift $ z \gtrsim 3$.

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arXiv2005

Gamma-Rays from Large Scale Structure Formation and the Warm-Hot Intergalactic Medium: Cosmic Baryometry with Gamma-Rays

It is shown that inverse Compton gamma-rays from electrons accelerated in large scale structure formation shocks can be crucially affected by non-gravitational effects such as radiative cooling and galaxy formation, with corresponding uncertainties by an order of magnitude in either the gamma-ray source counts or the extragalactic background contribution. However, this also implies that such gamma-rays may in the near future provide us with valuable information about the fraction of cosmic baryons in different forms, particularly the warm-hot intergalactic medium where the majority of the baryons in the universe are believed to reside. We address this problem in a simple way through semi-analytic modeling of structure formation shocks which self-consistently treats merger and accretion shocks.

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arXiv2004

The metal enrichment of the intracluster medium in hierarchical galaxy formation models

We investigate the metal enrichment of the intracluster medium (ICM) in the framework of hierarchical models of galaxy formation. We calculate the formation and evolution of galaxies and clusters using a semi-analytical model which includes the effects of flows of gas and metals both into and out of galaxies. For the first time in a semi-analytical model, we calculate the production of both alpha and iron-peak elements based on theoretical models for the lifetimes and ejecta of type Ia and type II supernovae (SNe Ia and SNe II). It is essential to include the long lifetimes of the SNIa progenitors in order to correctly model the evolution of the iron-peak elements. We find that if all stars form with an IMF similar to that found in the solar neighbourhood, then the metallicities of O, Mg, Si and Fe in the ICM are predicted to be 2-3 times lower than observed values. In contrast, a model (also favoured on other grounds) in which stars formed in bursts triggered by galaxy mergers have a top-heavy IMF reproduces the observed ICM abundances of O, Mg, Si and Fe. The same model predicts ratios of ICM mass to total stellar luminosity in clusters which agree well with observations. According to our model, the bulk of the metals in clusters are produced by L* and brighter galaxies. [abridged]

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arXiv2004

Damped Lyman alpha Absorbing Galaxies At Low Redshifts z<1 From Hierarchical Galaxy Formation Models

We investigate Damped Ly-alpha absorbing galaxies (DLA galaxies) at low redshifts z<1 in the hierarchical structure formation scenario to clarify the nature of DLA galaxies because observational data of such galaxies mainly at low redshifts are currently available. We find that our model well reproduces distributions of fundamental properties of DLA galaxies such as luminosities, column densities, impact parameters obtained by optical and near-infrared imagings. Our results suggest that DLA systems primarily consist of low luminosity galaxies with small impact parameters (typical radius about 3 kpc, surface brightness from 22 to 27 mag arcsec^{-2}) similar to low surface brightness (LSB) galaxies. In addition, we investigate selection biases arising from the faintness and from the masking effect which prevents us from identifying a DLA galaxy hidden or contaminated by a point spread function of a background quasar. We find that the latter affects the distributions of DLA properties more seriously rather than the former, and that the observational data are well reproduced only when taking into account the masking effect. The missing rate of DLA galaxies by the masking effect attains 60-90 % in the sample at redshift 0<z<1 when an angular size limit is as small as 1 arcsec. Furthermore we find a tight correlation between HI mass and cross section of DLA galaxies, and also find that HI-rich galaxies with M(HI) \sim 10^{9} M_sun dominate DLA systems. These features are entirely consistent with those from the Arecibo Dual-Beam Survey which is a blind 21 cm survey. Finally we discuss star formation rates, and find that they are typically about 10^{-2} M_sun yr^{-1} as low as those in LSB galaxies.

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arXiv2004

Gravitational Waves from Supermassive Black Hole Coalescence in a Hierarchical Galaxy Formation Model

We investigate the expected gravitational wave emission from coalescing supermassive black hole (SMBH) binaries resulting from mergers of their host galaxies. When galaxies merge, the SMBHs in the host galaxies sink to the center of the new merged galaxy and form a binary system. We employ a semi-analytic model of galaxy and quasar formation based on the hierarchical clustering scenario to estimate the amplitude of the expected stochastic gravitational wave background owing to inspiraling SMBH binaries and bursts owing to the SMBH binary coalescence events. We find that the characteristic strain amplitude of the background radiation is $h_c(f) \sim 10^{-16} (f/1 μ{\rm Hz})^{-2/3}$ for $f \lesssim 1 μ{\rm Hz}$ just below the detection limit from measurements of the pulsar timing provided that SMBHs coalesce simultaneously when host galaxies merge. The main contribution to the total strain amplitude of the background radiation comes from SMBH coalescence events at $0<z<1$. We also find that a future space-based gravitational wave interferometer such as the planned \textit{Laser Interferometer Space Antenna} ({\sl LISA}) might detect intense gravitational wave bursts associated with coalescence of SMBH binaries with total mass $M_{\rm tot} < 10^7 M_{\odot}$ at $z \gtrsim 2$ at a rate $ \sim 1.0 {\rm yr}^{-1}$. Our model predicts that burst signals with a larger amplitude $h_{\rm burst} \sim 10^{-15}$ correspond to coalescence events of massive SMBH binary with total mass $M_{\rm tot} \sim 10^8 M_{\odot}$ at low redshift $ z \lesssim 1$ at a rate $ \sim 0.1 {\rm yr}^{-1}$ whereas those with a smaller amplitude $h_{\rm burst} \sim 10^{-17}$ correspond to coalescence events of less massive SMBH binary with total mass $M_{\rm tot} \sim 10^6 M_{\odot}$ at high redshift $ z \gtrsim 3$.

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arXiv2004

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)

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arXiv2004

The roles of ram-pressure stripping and minor mergers in evolution of galaxies

We investigate environmental effects on evolution of bright cluster galaxies in a Lambda-dominated cold dark matter universe using a combination of dissipationless N-body simulations and a semi-analytic galaxy formation model. We incorporate effects of ram-pressure stripping (RPS) and minor merger-induced small starburst (minor burst) into our model. By considering minor burst, observed morphology-radius relation is successfully reproduced. When we do not consider minor burst, the RPS hardly increases the intermediate B/T population. In addition, the RPS and minor burst are not important for colours or star formation rates of galaxies in the cluster core if star formation time-scale is properly chosen, because the star formation is sufficiently suppressed by consumption of the cold gas. We also find that SF in bulge-dominated galaxies is mainly terminated by starburst induced by major mergers in all environments.

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arXiv2004

Mass Function of Low Mass Dark Halos

The mass function of dark halos in a Lambda-dominated cold dark matter (LambdaCDM) universe is investigated. 529 output files from five runs of N-body simulations are analyzed using the friends-of-friends cluster finding algorithm. All the runs use 512^3 particles in the box size of 35 h^{-1}Mpc to 140 h^{-1}Mpc. Mass of particles for 35 h^{-1} Mpc runs is 2.67 times 10^7 h^{-1} M_{solar}. Because of the high mass resolution of our simulations, the multiplicity function in the low-mass range, where the mass is well below the characteristic mass and $ν= δ_c / σ\lesssim 1.0$, is evaluated in the present work, and is well fitted by the functional form proposed by Sheth & Tormen (ST). However, the maximum value of the multiplicity function from our simulations at $ν\sim 1$ is smaller, and its low mass tail is shallower when compared with the ST multiplicity function.

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arXiv2003

Chemical Enrichment in Damped Lyman Alpha Systems From Hierarchical Galaxy Formation Models

We investigate chemical enrichment in Damped Lyman alpha (DLA) systems in the hierarchical structure formation scenario using a semi-analytic model of galaxy formation. The model developed by Nagashima, Totani, Gouda and Yoshii takes into account various selection effects on high-redshift galaxies and can show fundamental observational properties of galaxies, such as luminosity functions and number-magnitude/redshift relations. DLA systems offer the possibilities of measuring metal abundance more accurately than faint galaxies. For example, recent measurements of zinc abundance can provide good evidence for understanding the processes of metal pollution and star formation in DLA systems because zinc is virtually unaffected by dust depletion. Here we focus on this advantage for observation in order to explore the metallicity evolution in DLA systems at high redshifts. We can consistently show the metallicity evolution for reasonable models which also reproduce fundamental properties of local galaxy population. This result suggests that the chemical evolution of DLA systems can be consistently reconciled with the observational features of typical galaxies. We also investigate other properties of DLA systems (column density distribution and mass density of cold gas), and find that star formation in massive galaxies should be more active than that in low-mass ones. This is consistent with the results by Nagashima et al. and Cole et al. in which the star formation timescale is set by reproducing cold gas mass fraction in local spiral galaxies. Finally we discuss host galaxies associated with DLA systems. We conclude that they primarily consist of sub-L* and/or dwarf galaxies from the observations.

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arXiv2003

Searching for a Long Cosmic String Through the Gravitational Lensing Effect

It has been suggested that cosmic strings produced at a phase transition in the early universe can be the origin of the extremely high energy cosmic rays (EHCR) observed by AGASA above 10^20 eV. Superheavy cosmic strings with linear mass density of 10^22 g/cm can be indirectly observed through the gravitational lensing effect the distant galaxies. The lensing effect by a long straight object can be characterized by a line of double galaxies or quasars with angular separation of about 5 arcsec. We have searched for aligned double objects from the archived data taken by the Subaru Prime Focus Camera (Suprime-Cam). The Suprime-Cam has a great advantage in observing the wide field of view (30x30 arcmin^2) with high sensitivity (R<26 400s exposure), so it is suitable for this research. In this paper, we describe the result of simulation study for developing the method of searching the objects lensed by cosmic strings, and present the observational result obtained by this method.

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arXiv2003

Environmental Effects on Evolution of Cluster Galaxies in a LCDM Universe

We investigate environmental effects on evolution of bright cluster galaxies in a $Λ$ dominated cold dark matter universe using a combination of dissipationless N-body simulations and a semi-analytic galaxy formation model. The N-body simulations enable us to calculate orbits of galaxies in simulated clusters. Therefore we can incorporate stripping of cold gas from galactic disks by ram pressure from intracluster medium into our model. In this paper we study how ram pressure stripping (RPS) and small starburst induced by a minor merger affect colors, star formation rates, and morphologies of cluster galaxies. We find that the RPS is not important for colors and SFRs of galaxies in the cluster core if star formation time-scale is properly chosen, because the star formation is sufficiently suppressed by consumption of the cold gas in the disks. Then observed color and SFR gradients can be reproduced without the RPS. The small starburst triggered by a minor merger hardly affects the SFRs and colors of the galaxies as well. We also examine whether these two processes can resolve the known problem that the major merger-driven bulge formation scenario predict too few galaxies of intermediate bulge-to-total luminosity ratio (B/T) in clusters. When the minor burst is taken into account, the intermediate B/T population is increased and the observed morphology gradients in clusters are successfully reproduced. When the minor burst is considered, the RPS also plays an important role in formation of the intermediate B/T galaxies. We present redshift evolution of morphological fractions predicted by our models. The predicted number ratios of the intermediate B/T galaxies to the bulge-dominated galaxies show nearly flat or slightly increasing trends with increasing redshift.

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arXiv2002

Dynamical response to supernova-induced gas removal in two-component spherical galaxies

We investigate dynamical response on size and velocity dispersion to mass loss by supernovae in formation of two-component spherical galaxies composed of baryon and dark matter. Three-dimensional deprojected de Vaucouleurs-like and exponential-like profiles for baryon, embedded in truncated singular isothermal and homogeneous profiles for dark matter, are considered. As a more realistic case, we also consider a dark matter profile proposed by Navarro, Frenk & White. For simplicity we assume that dark matter distribution is not affected by mass loss and that the change of baryonic matter distribution is homologous. We found that the degree of the response depends on the fraction of dark matter in the region where baryon is distributed, so that dwarf spheroidal galaxies would be affected even in a dark halo if they are formed by galaxy mergers in the envelope of the dark halo. Our results suggest that this scenario, combined with dynamical response, would make not only the observed trends but the dispersed characteristics of dwarf spheroidals.

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arXiv2002

Relations Between Environments of Quasars and Galaxy Formation

We investigate the environments of quasars such as number distribution of galaxies using a semi-analytic model which includes both galaxy and quasar formations based on the hierarchical clustering scenario. We assume that a supermassive black hole is fueled by accretion of cold gas and that it is a source of quasar activity during a major merger of the quasar host galaxy with another galaxy. This major merger causes spheroid formation of the host galaxy. Our model can reproduce not only general form of the galaxy luminosity functions in the local Universe but also the observed relation between a supermassive black hole mass and a spheroid luminosity, the present black hole mass function and the quasar luminosity functions at different redshifts. Using this model, we predict the mean number of quasars per halo, bias parameter of quasars and the probability distribution of the number of galaxies around quasars. In our model, analysis of the mean number of quasars per halo shows that the spatial distribution of galaxies is different from that of quasars. Furthermore, we found from calculation of the probability distribution of galaxy numbers that at $0.2 \lesssim z \lesssim 0.5$, most quasars are likely to reside in galaxy groups. On the other hand, at $1 \lesssim z \lesssim 2$ most quasars seem to reside in more varied environments than at a lower redshift; quasars reside in environments ranging from small groups of galaxies to clusters of galaxies. Comparing these predictions with observations in future will enable us to constrain our quasar formation model.

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arXiv2002

Subaru Deep Survey III. Evolution of Rest-Frame Luminosity Functions Based on the Photometric Redshifts for A K'-band Selected Galaxy Sample

We have constructed a very deep K'-selected multicolor BVRIz'JK' sample of 439 field galaxies. Photometric redshifts for sample galaxies were estimated. The overall redshift distribution N(z) for the K'<21.0 sample is consistent with previous observations, and for the first time we derive the N(z) down to K'=24.0. After taking account of the dust extinction and selection effects of the sample, the observed N(z) distribution is well described with the PLE model, while the hierarchical galaxy formation model shows an apparent deficiency of galaxies especially at z>2. The photometric redshift and the best-fit SED model evaluations allow the derivation of the rest-frame K', B, and UV(2000A) luminosity functions (LFs) and their evolutions. The rest-K' LF shows almost no evolution up to z=3, while the rest B LF shows mild luminosity evolution, and the rest UV LF shows strong luminosity evolution. This trend corresponds to the evolution of the rest UV-K' color, which gets bluer with increasing redshift. We also found that more massive galaxies are redder in this rest-frame color in every epoch. The rest-UV LF of our K'-selected galaxies shows a much shallower faint end slope at z=3 than that of previous estimations for rest-UV selected Lyman break galaxies. As a consequence, the contribution to the global star formation rate of our K'-selected galaxies is about 42% of that derived from the integration of LF of Lyman break galaxies at z=3. This result suggests that a large fraction of the star formation rate density at z>1.5 comes from the contribution from the faint (M_2000A>-20) blue galaxy population at high redshift universe that have not yet obviously been identified. (abridged)

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