arXiv subjects

Masakazu A. R. Kobayashi

Publications and source records attributed to Masakazu A. R. Kobayashi.

At least 37 records · Page 2Linked to original sources

arXiv2016

Morphological Properties of Lyman Alpha Emitters at Redshift 4.86 in the COSMOS Field: Clumpy Star Formation or Merger?

We investigate morphological properties of 61 Lyman-alpha emitters (LAEs) at z = 4.86 identified in the COSMOS field, based on Hubble Space Telescope Advanced Camera for Surveys (ACS) imaging data in the F814W-band. Out of the 61 LAEs, we find the ACS counterparts for the 54 LAEs. Eight LAEs show double-component structures with a mean projected separation of 0."63 (~ 4.0 kpc at z = 4.86). Considering the faintness of these ACS sources, we carefully evaluate their morphological properties, that is, size and ellipticity. While some of them are compact and indistinguishable from the PSF half-light radius of 0."07 (~ 0.45 kpc), the others are clearly larger than the PSF size and spatially extended up to 0."3 (~ 1.9 kpc). We find that the ACS sources show a positive correlation between ellipticity and size and that the ACS sources with large size and round shape are absent. Our Monte Carlo simulation suggests that the correlation can be explained by (1) the deformation effects via PSF broadening and shot noise or (2) the source blending in which two or more sources with small separation are blended in our ACS image and detected as a single elongated source. Therefore, the 46 single-component LAEs could contain the sources which consist of double (or multiple) components with small spatial separation (i.e., < 0."3 or 1.9 kpc). Further observation with high angular resolution at longer wavelengths (e.g., rest-frame wavelengths of > 4000 A) is inevitable to decipher which interpretation is adequate for our LAE sample.

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arXiv2015

Quasar clustering in a galaxy and quasar formation model based on ultra high-resolution N-body simulations

We investigate clustering properties of quasars using a new version of our semi-analytic model of galaxy and quasar formation with state-of-the-art cosmological N-body simulations. In this study, we assume that a major merger of galaxies triggers cold gas accretion on to a supermassive black hole and quasar activity. Our model can reproduce the downsizing trend of the evolution of quasars. We find that the median mass of quasar host dark matter haloes increases with cosmic time by an order of magnitude from z=4 (a few 1e+11 Msun) to z=1 (a few 1e+12 Msun), and depends only weakly on the quasar luminosity. Deriving the quasar bias through the quasar--galaxy cross-correlation function in the model, we find that the quasar bias does not depend on the quasar luminosity, similar to observed trends. This result reflects the fact that quasars with a fixed luminosity have various Eddington ratios and thus have various host halo masses that primarily determine the quasar bias. We also show that the quasar bias increases with redshift, which is in qualitative agreement with observations. Our bias value is lower than the observed values at high redshifts, implying that we need some mechanisms that make quasars inactive in low-mass haloes and/or that make them more active in high-mass haloes.

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arXiv2015

Discovery of Massive, Mostly Star-formation Quenched Galaxies with Extremely Large Lyman-alpha Equivalent Widths at z ~ 3

We report a discovery of 6 massive galaxies with both extremely large Lya equivalent width and evolved stellar population at z ~ 3. These MAssive Extremely STrong Lya emitting Objects (MAESTLOs) have been discovered in our large-volume systematic survey for strong Lya emitters (LAEs) with twelve optical intermediate-band data taken with Subaru/Suprime-Cam in the COSMOS field. Based on the SED fitting analysis for these LAEs, it is found that these MAESTLOs have (1) large rest-frame equivalent width of EW_0(Lya) ~ 100--300 A, (2) M_star ~ 10^10.5--10^11.1 M_sun, and (3) relatively low specific star formation rates of SFR/M_star ~ 0.03--1 Gyr^-1. Three of the 6 MAESTLOs have extended Ly$α$ emission with a radius of several kpc although they show very compact morphology in the HST/ACS images, which correspond to the rest-frame UV continuum. Since the MAESTLOs do not show any evidence for AGNs, the observed extended Lya emission is likely to be caused by star formation process including the superwind activity. We suggest that this new class of LAEs, MAESTLOs, provides a missing link from star-forming to passively evolving galaxies at the peak era of the cosmic star-formation history.

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arXiv2015

The Impact of Dust in Host Galaxies on Quasar Luminosity Functions

We have investigated effects of dust attenuation on quasar luminosity functions using a semi-analytic galaxy formation model combined with a large cosmological N-body simulation. We estimate the dust attenuation of quasars self-consistently with that of galaxies by considering the dust in their host bulges. We find that the luminosity of the bright quasars is strongly dimmed by the dust attenuation, about 2 mag in the B-band. Assuming the empirical bolometric corrections for active galactic nuclei (AGNs) by Marconi et al., we find that this dust attenuation is too strong to explain the B-band and X-ray quasar luminosity functions simultaneously. We consider two possible mechanisms that weaken the dust attenuation. As such a mechanism, we introduce a time delay for AGN activity, that is, gas fueling to a central black hole starts some time after the beginning of the starburst induced by a major merger. The other is the anisotropy in the dust distribution. We find that in order to make the dust attenuation of the quasars negligible, either the gas accretion into the black holes has to be delayed at least three times the dynamical timescale of their host bulges or the dust covering factor is as small as 0.1.

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arXiv2014

A New Chemical Evolution Model for Dwarf Spheroidal Galaxies based on Observed Long Star Formation Histories

We present a new chemical evolution model for dwarf spheroidal galaxies (dSphs) in the Local Universe. Our main aim is to explain both their observed star formation histories and metallicity distribution functions simultaneously. Applying our new model for the four local dSphs, that is, Fornax, Sculptor, Leo II, and Sextans, we find that our new model reproduces the observed chemical properties of the dSphs consistently. Our results show that the dSphs have evolved with both a low star formation efficiency and a large gas outflow efficiency compared with the Milky Way, as suggested by previous works. Comparing the observed [alpha/Fe]--[Fe/H] relation of the dSphs with the model predictions, we find that our model favors a longer onset time of Type Ia supernovae (i.e., 0.5 Gyr) than that suggested in previous studies (i.e., 0.1 Gyr). We discuss the origin of this discrepancy in detail.

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arXiv2014

Anti-hierarchical evolution of the Active Galactic Nucleus space density in a hierarchical universe

Recent observations show that the space density of luminous active galactic nuclei (AGNs) peaks at higher redshifts than that of faint AGNs. This downsizing trend in the AGN evolution seems to be contradictory to the hierarchical structure formation scenario. In this study, we present the AGN space density evolution predicted by a semi-analytic model of galaxy and AGN formation based on the hierarchical structure formation scenario. We demonstrate that our model can reproduce the downsizing trend of the AGN space density evolution. The reason for the downsizing trend in our model is a combination of the cold gas depletion as a consequence of star formation, the gas cooling suppression in massive halos and the AGN lifetime scaling with the dynamical timescale. We assume that a major merger of galaxies causes a starburst, spheroid formation, and cold gas accretion onto a supermassive black hole (SMBH). We also assume that this cold gas accretion triggers AGN activity. Since the cold gas is mainly depleted by star formation and gas cooling is suppressed in massive dark halos, the amount of cold gas accreted onto SMBHs decreases with cosmic time. Moreover, AGN lifetime increases with cosmic time. Thus, at low redshifts, major mergers do not always lead to luminous AGNs. Because the luminosity of AGNs is correlated with the mass of accreted gas onto SMBHs, the space density of luminous AGNs decreases more quickly than that of faint AGNs. We conclude that the anti-hierarchical evolution of the AGN space density is not contradictory to the hierarchical structure formation scenario.

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arXiv2014

Galaxy luminosity function and its cosmological evolution: Testing a new feedback model depending on galaxy-scale dust opacity

We present a new version of a semi-analytic model of cosmological galaxy formation, incorporating a star formation law with a feedback depending on the galaxy-scale mean dust opacity and metallicity, motivated by recent observations of star formation in nearby galaxies and theoretical considerations. This new model is used to investigate the effect of such a feedback on shaping the galaxy luminosity function and its evolution. Star formation activity is significantly suppressed in dwarf galaxies by the new feedback effect, and the faint-end slope of local luminosity functions can be reproduced with a reasonable strength of supernova feedback, which is in contrast to the previous models that require a rather extreme strength of supernova feedback. Our model can also reproduce the early appearance of massive galaxies manifested in the bright-end of high redshift K-band luminosity functions. Though some of the previous models also succeeded in reproducing this, they assumed a star formation law depending on the galaxy-scale dynamical time, which is not supported by observations. We argue that the feedback depending on dust opacity (or metal column density) is essential, rather than that simply depending on gas column density, to get these results.

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arXiv2013

Extragalactic Background Light from Hierarchical Galaxy Formation: Gamma-ray Attenuation up to the Epoch of Cosmic Reionization and the First Stars

We present a new model of the extragalactic background light (EBL) and corresponding gamma-gamma opacity for intergalactic gamma-ray absorption from z = 0 up to z = 10, based on a semi-analytical model of hierarchical galaxy formation that reproduces key observed properties of galaxies at various redshifts. Including the potential contribution from Population III stars and following the cosmic reionization history in a simplified way, the model is also broadly consistent with available data concerning reionization, particularly the Thomson scattering optical depth constraints from WMAP. In comparison with previous EBL studies up to z ~ 3-5, our predicted gamma-gamma opacity is in general agreement for observed gamma-ray energy below 400/(1 + z) GeV, whereas it is a factor of ~ 2 lower above this energy because of a correspondingly lower cosmic star formation rate, even though the observed UV luminosity is well reproduced by virtue of our improved treatment of dust obscuration and direct estimation of star formation rate. The horizon energy at which the gamma-ray opacity is unity does not evolve strongly beyond z ~ 4 and approaches ~ 20 GeV. The contribution of Population III stars is a minor fraction of the EBL at z = 0, and is also difficult to distinguish through gamma-ray absorption in high-z objects, even at the highest levels allowed by the WMAP constraints. Nevertheless, the attenuation due to Population II stars should be observable in high-z gamma-ray sources by telescopes such as Fermi or CTA and provide a valuable probe of the evolving EBL in the rest-frame UV. The detailed results of our model are publicly available in numerical form at the URL http://www.slac.stanford.edu/~yinoue/Download.html.

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arXiv2013

Revisiting the Cosmic Star Formation History: Caution on the Uncertainties in Dust Correction and Star Formation Rate Conversion

The cosmic star formation rate density (CSFRD) has been observationally investigated out to redshift z~10. However, most of theoretical models for galaxy formation underpredict the CSFRD at z>1. Since the theoretical models reproduce the observed luminosity functions (LFs), luminosity densities (LDs), and stellar mass density at each redshift, this inconsistency does not simply imply that theoretical models should incorporate some missing unknown physical processes in galaxy formation. Here, we examine the cause of this inconsistency in UV wavelengths by using a mock catalog of galaxies generated by a semi-analytic model of galaxy formation. We find that this inconsistency is due to two observational uncertainties: dust obscuration correction and conversion from UV luminosity to star formation rate (SFR). The methods for correction of obscuration and SFR conversion used in observational studies result in the overestimation of CSFRD by ~ 0.1-0.3 dex and ~ 0.1-0.2 dex, respectively, compared to the results obtained directly from our mock catalog. We present new empirical calibrations for dust attenuation and conversion from observed UV LFs and LDs into CSFRD.

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arXiv2012

Infrared Spectral Energy Distribution of Galaxies in the AKARI All Sky Survey: Correlations with Galaxy Properties, and Their Physical Origin

We have studied the properties of more than 1600 low-redshift galaxies by utilizing high-quality infrared flux measurements of the AKARI All-Sky Survey and physical quantities based on optical and 21-cm observations. Our goal is to understand the physics determining the infrared spectral energy distribution (SED). The ratio of the total infrared luminosity L_TIR, to the star-formation rate (SFR) is tightly correlated by a power-law to specific SFR (SSFR), and L_TIR is a good SFR indicator only for galaxies with the largest SSFR. We discovered a tight linear correlation for normal galaxies between the radiation field strength of dust heating, estimated by infrared SED fits (U_h), and that of galactic-scale infrared emission (U_TIR ~ L_TIR/R^2), where R is the optical size of a galaxy. The dispersion of U_h along this relation is 0.3 dex, corresponding to 13% dispersion in the dust temperature. This scaling and the U_h/U_TIR ratio can be explained physically by a thin layer of heating sources embedded in a thicker, optically-thick dust screen. The data also indicate that the heated fraction of the total dust mass is anti-correlated to the dust column density, supporting this interpretation. In the large U_TIR limit, the data of circumnuclear starbursts indicate the existence of an upper limit on U_h, corresponding to the maximum SFR per gas mass of ~ 10 Gyr^{-1}. We find that the number of galaxies sharply drops when they become optically thin against dust-heating radiation, suggesting that a feedback process to galaxy formation (likely by the photoelectric heating) is working when dust-heating radiation is not self-shielded on a galactic scale. Implications are discussed for the M_HI-size relation, the Kennicutt-Schmidt relation, and galaxy formation in the cosmological context.

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arXiv2010

Contribution from Star-Forming Galaxies to the Cosmic Gamma-Ray Background Radiation

We present a new theoretical calculation of the contribution to the extragalactic gamma-ray background radiation (EGRB) from star-forming galaxies, based on a state-of-the-art model of hierarchical galaxy formation that is in quantitative agreement with a variety of observations of local and high-redshift galaxies. Gamma-ray luminosity ($L_γ$) and spectrum of galaxies are related to star formation rate ($ψ$), gas mass ($M_gas$), and star formation mode (quiescent or starburst) of model galaxies using latest observed data of nearby galaxies. We try the two limiting cases about gamma-ray production: the escape limit ($L_γ\propto ψM_gas$) and the calorimetric limit ($L_γ\propto ψ$), and our standard model predicts 7 and 4% contribution from star-forming galaxies to the total EGRB flux (including bright resolved sources) recently reported by the Fermi Gamma-Ray Space Telescope. Systematic uncertainties do not allow us to determine the EGRB flux better than by a factor of ~2. The predicted number of nearby galaxies detectable by Fermi is consistent with the observation. Intergalactic absorption by pair-production attenuates the EGRB flux only by a modest factor of ~1.3 at the highest Fermi energy band, and the reprocessed cascade emission does not significantly alter EGRB at lower photon energies. The sum of the known contributions from AGNs and star-forming galaxies can explain a large part of EGRB, with a remarkable agreement between the predicted model spectrum and observation.

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arXiv2010

Luminosity Distribution of Gamma-Ray Burst Host Galaxies at redshift z=1 in Cosmological Smoothed Particle Hydrodynamic Simulations: Implications for the Metallicity Dependence of GRBs

We study the relationship between the metallicity of gamma-ray burst (GRB) progenitors and the probability distribution function (PDF) of GRB host galaxies as a function of luminosity using cosmological hydrodynamic simulations of galaxy formation. We impose a maximum limit to the gas metallicity in which GRBs can occur, and examine how the predicted luminosity PDF of GRB host galaxies changes in the simulation. We perform the Kolmogorov-Smirnov test, and show that the result from our simulation agrees with the observed luminosity PDF of core-collapse supernovae (SNe) host galaxies when we assume that the core-collapse SNe trace star formation. When we assume that GRBs occur only in a low-metallicity environment with $Z\lesssim 0.1 \Zsun$, GRBs occur in lower luminosity galaxies, and the simulated luminosity PDF becomes quantitatively consistent with the observed luminosity PDF. The observational bias against the host galaxies of optically dark GRBs owing to dust extinction may be another reason for the lower luminosities of GRB host galaxies, but the observed luminosity PDF of GRB host galaxies cannot be reproduced solely by the dust bias in our simulation.

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arXiv2010

Searching for the most distant blazars with the Fermi Gamma-ray Space Telescope

We investigate the prospects for discovering blazars at very high-redshifts (z>3-6) with the Fermi Gamma-Ray Space Telescope (Fermi), employing a model for the evolving gamma-ray luminosity function (GLF) of the blazar population. Our previous GLF model is used as a basis, which features luminosity-dependent density evolution implied from X-ray data on active galactic nuclei, as well as the blazar sequence paradigm for their spectral energy distribution, and which is consistent with EGRET and current Fermi observations of blazars.Here we augment the high-redshift evolution of this model by utilizing the luminosity function of quasars from the Sloan Digital Sky Survey (SDSS), which is well-constrained up to z~5. We find that Fermi may discover a few blazars up to z~6 in the entire sky during its 5-year survey. We further discuss how such high-redshift blazar candidates may be efficiently selected in future Fermi data.

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arXiv2010

Lyman Alpha Emitters at z=7 in the Subaru/XMM-Newton Deep Survey Field: Photometric Candidates and Luminosity Function

We conducted a deep narrowband NB973 (FWHM = 200 A centered at 9755 A) survey of z=7 Lyman alpha emitters (LAEs) in the Subaru/XMM-Newton Deep Survey Field, using the fully depleted CCDs newly installed on the Subaru Telescope Suprime-Cam, which is twice more sensitive to z=7 Lyman alpha at ~ 1 micron than the previous CCDs. Reaching the depth 0.5 magnitude deeper than our previous survey in the Subaru Deep Field that led to the discovery of a z=6.96 LAE, we detected three probable z=7 LAE candidates. Even if all the candidates are real, the Lyman alpha luminosity function (LF) at z=7 shows a significant deficit from the LF at z=5.7 determined by previous surveys. The LAE number and Lyman alpha luminosity densities at z=7 is ~ 7.7-54% and ~5.5-39% of those at z=5.7 to the Lyman alpha line luminosity limit of L(Ly-alpha) >~ 9.2 x 10^{42} erg s^{-1}. This could be due to evolution of the LAE population at these epochs as a recent galaxy evolution model predicts that the LAE modestly evolves from z=5.7 to 7. However, even after correcting for this effect of galaxy evolution on the decrease in LAE number density, the z=7 Lyman alpha LF still shows a deficit from z=5.7 LF. This might reflect the attenuation of Lyman alpha emission by neutral hydrogen remaining at the epoch of reionization and suggests that reionization of the universe might not be complete yet at z=7. If we attribute the density deficit to reionization, the intergalactic medium (IGM) transmission for Lyman alpha photons at z=7 would be 0.4 <= T_{Ly-alpha}^{IGM} <= 1, supporting the possible higher neutral fraction at the earlier epochs at z > 6 suggested by the previous surveys of z=5.7-7 LAEs, z ~ 6 quasars and z > 6 gamma-ray bursts.

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arXiv2009

The Cosmological Evolution of Blazars and the Extragalactic Gamma-Ray Background in the Fermi Era

The latest determination of the extragalactic gamma-ray background (EGRB) radiation by Fermi is compared with the theoretical prediction of the blazar component by Inoue & Totani (2009; hereafter IT09). The Fermi EGRB spectrum is in excellent agreement with IT09, indicating that blazars are the dominant component of the EGRB, and contributions from any other sources (e.g., dark matter annihilations) are minor. It also indicates that the blazar SED (spectral energy distribution) sequence taken into account in IT09 is a valid description of mean blazar SEDs. The possible contribution of MeV blazars to the EGRB in the MeV band is also discussed. In five total years of observations, we predict that Fermi will detect ~1200 blazars all sky down to the corresponding sensitivity limit. We also address the detectability of the highest-redshift blazars. Updating our model with regard to high-redshift evolution based on SDSS quasar data, we show that Fermi may find some blazars up to z~6 during the five-year survey. Such blazars could provide a new probe of early star and galaxy formation through GeV spectral attenuation signatures induced by high-redshift UV background radiation.

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arXiv2009

Lyman Alpha Emitters in Hierarchical Galaxy Formation II. UV Continuum Luminosity Function and Equivalent Width Distribution

We present theoretical predictions of UV continuum luminosity function (UV LF) and Lya equivalent width (EW) distribution of Lyman alpha emitters (LAEs) in the framework of the hierarchical clustering model of galaxy formation. The model parameters about LAEs were determined by fitting to the observed Lya LF at z=5.7 in our previous study, and the fit indicates that extinction of Lya photons by dust is significantly less effective than that of UV continuum photons, implying clumpy dust distribution in interstellar medium. We then compare the predictions about UV LFs and EW distributions with a variety of observations at z ~ 3-6, allowing no more free parameters and paying careful attention to the selection conditions of LAEs in each survey. We find that the predicted UV LFs and EW distributions are in nice agreement with observed data, and especially, our model naturally reproduces the existence of large EW LAEs (> 240 A) without introducing Pop III stars or top-heavy initial mass function. We show that both the stellar population (young age and low metallicity) and extinction by clumpy dust are the keys to reproduce large EW LAEs. The evidence of EW enhancement by clumpy dust is further strengthened by the quantitative agreement between our model and recent observations about a positive correlation between EW and extinction. The observed trend that brighter LAEs in UV continuum tend to have smaller mean EW is also reproduced, and the clumpy dust is playing an important role again for this trend. We suggested in our previous study that the transmission of intergalactic medium for Lya emission rapidly decreases from z ~ 6 to 7 by the fitting to Lya LFs, and this evidence is quantitatively strengthened by the comparison with the UV LF and EW distribution at z ~ 6.6.

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arXiv2009

Probing the Long Gamma-Ray Burst Progenitor by Lyman-alpha Emission of Host Galaxies

Long gamma-ray bursts (GRBs) have been suggested to occur preferentially in low metallicity environment. We discuss the possibility and theoretical aspects of using Lyman alpha emission properties of long GRB host galaxies as a metallicity indicator of high redshift GRB environments, where direct metallicity measurements are not easy. We propose to use the fraction of Ly-alpha emitters (LAEs) in long GRB host galaxies as a function of UV luminosity, which can be compared with star-formation-rate weighted LAE fraction of Lyman-break galaxies as the standard in the case of no metallicity dependence. There are two important effects of metallicity dependence of long GRB rate to change the LAE fraction of host galaxies. One is the enhancement of intrinsic Ly-alpha equivalent width (EW) by stronger ionizing UV luminosity of low metallicity stellar population, and the other is extinction by interstellar dust to change the observable EW. Based on a latest theoretical model of LAEs that reproduce observations, we argue that the latter is likely to work in the opposite direction to the former, i.e., to decrease LAE fraction if GRBs preferentially occur in low-metallicity environments, because of the clumpy inter-stellar medium effect. The high LAE fraction of GRB host galaxies indicated by observations is quantitatively be explained by the LAE model if GRBs occur when Z < 0.1Z_sun, although this result is still indicative because of the limited statistics and theoretical uncertainties. This result demonstrates that the LAE statistics of GRB hosts may give us useful information in the future.

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arXiv2008

The Reionization and Galaxy Evolution Probed by z=7 Lyman Alpha Emitters

We made a narrowband NB973 (bandwidth of 200A at 9755A) imaging of the Subaru Deep Field (SDF) and found two z=7 Lyman alpha emitter (LAE) candidates down to NB973=24.9. Carrying out deep follow-up spectroscopy, we identified one of them as a real z=6.96 LAE. This has shown that galaxy formation was in progress just 750 Myr after the Big Bang. Meanwhile, the Lyman alpha line luminosity function of LAE is known to decline from z=5.7 to 6.6 in the SDF. L* at z=6.6 is 40-60% of that at z=5.7. We also confirm that the number density of z=7 LAE is only 17% of the density at z=6.6 comparing the latest SDF LAE samples. This series of significant decreases in LAE density with increasing redshift can be the result of galaxy evolution during these epochs. However, using the UV continuum luminosity functions of LAEs, those of Lyman break galaxies, and a LAE evolution model based on the hierarchical clustering, we find that galaxy evolution alone cannot explain all the decrease in density. This extra density deficit can be interpreted as the attenuation of the Lyman alpha photons from LAEs due to a rapid evolution of neutral hydrogen fraction during the ongoing cosmic reionization at z~6.6-7.

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