arXiv subjects

Tomonori Totani

Publications and source records attributed to Tomonori Totani.

At least 73 records · Page 4Linked to original sources

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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arXiv2010

Prospects for Very High Energy Blazar Survey by the Next Generation Cherenkov Telescopes

The prospects for future blazar surveys by next-generation very high energy (VHE) gamma-ray telescopes, such as Advanced Gamma-ray Imaging System (AGIS) and Cherenkov Telescope Array (CTA), are investigated using the latest model of blazar luminosity function and its evolution which is in good agreement with the flux and redshift distribution of observed blazars as well as the extragalactic gamma-ray background. We extend and improve the template of spectral energy distributions (SEDs) based on the blazar SED sequence paradigm, to make it reliable also in the VHE bands (above 100 GeV) by comparing with the existing VHE blazar data. Assuming the planned CTA sensitivities, a blind survey using a total survey time of ~100 hrs could detect ~3 VHE blazars, with larger expected numbers for wider/shallower surveys. We also discuss following-up of Fermi blazars. Detectability of VHE blazars in the plane of Fermi flux and redshift is presented, which would be useful for future survey planning. Prospects and strategies are discussed to constrain the extragalactic background light (EBL) by using the absorption feature of brightest blazar spectra, as well as cut-offs in the redshift distribution. We will be able to get useful constraints on EBL by VHE blazars at different redshifts ranging 0.3-1 TeV corresponding to z=0.10-0.36.

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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

Size Evolution of Early-Type Galaxies and Massive Compact Objects as the Dark Matter

The dramatic size evolution of early-type galaxies from z ~ 2 to 0 poses a new challenge in the theory of galaxy formation, which may not be explained by the standard picture. It is shown here that the size evolution can be explained if the non-baryonic cold dark matter is composed of compact objects having a mass scale of ~10^5 M_sun. This form of dark matter is consistent with or only weakly constrained by the currently available observations. The kinetic energy of the dark compact objects is transferred to stars by dynamical friction, and stars around the effective radius are pushed out to larger radii, resulting in a pure size evolution. This scenario has several good properties to explain the observations, including the ubiquitous nature of size evolution and faster disappearance of higher density galaxies.

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arXiv2009

Subaru FOCAS Spectroscopic Observations of High-Redshift Supernovae

We present spectra of high-redshift supernovae (SNe) that were taken with the Subaru low resolution optical spectrograph, FOCAS. These SNe were found in SN surveys with Suprime-Cam on Subaru, the CFH12k camera on the Canada-France-Hawaii Telescope (CFHT), and the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST). These SN surveys specifically targeted z>1 Type Ia supernovae (SNe Ia). From the spectra of 39 candidates, we obtain redshifts for 32 candidates and spectroscopically identify 7 active candidates as probable SNe Ia, including one at z=1.35, which is the most distant SN Ia to be spectroscopically confirmed with a ground-based telescope. An additional 4 candidates are identified as likely SNe Ia from the spectrophotometric properties of their host galaxies. Seven candidates are not SNe Ia, either being SNe of another type or active galactic nuclei. When SNe Ia are observed within a week of maximum light, we find that we can spectroscopically identify most of them up to z=1.1. Beyond this redshift, very few candidates were spectroscopically identified as SNe Ia. The current generation of super red-sensitive, fringe-free CCDs will push this redshift limit higher.

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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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arXiv2009

The Blazar Sequence and the Cosmic Gamma-Ray Background Radiation in the Fermi Era

(Abridged) We present a new model of the blazar gamma-ray luminosity function (GLF) and the spectrum of the extragalactic gamma-ray background (EGRB), which is consistent with the observed distributions of EGRET blazars. The unified sequence of blazar spectral energy distribution (SED) is taken into account to make a non-trivial prediction for the EGRB spectrum and more realistic comparison with the data than previous studies. We then try to explain the EGRB data by the two AGN populations: one is blazars, and the other is non-blazar AGNs that are responsible for the EGRB in the MeV band. We find that ~80% of the EGRB photon flux at > 100 MeV can be explained by the sum of the two populations, while ~45% can be accounted for only by blazars. The predicted EGRB spectrum is in agreement with a wide range of the observed data from X-ray to GeV, within the systematic uncertainties in the EGRB determination by EGRET. These results indicate that AGNs including blazars are the primary source of EGRB. Blazars are dominant in EGRB at higher energy bands of >100 MeV, while non-blazar AGNs dominate at <100 MeV. Almost all of the EGRB flux from blazars will be resolved into discrete sources by the Fermi Gamma-ray Space Telescope, while that from non-blazar AGNs will largely remain unresolved. Therefore, comparison between the integrated source counts and diffuse EGRB flux as a function of photon energy will give a simple and clear test of our model. Various quantitative predictions for Fermi observations are also made. Especially, our model predicts 600-1200 blazars in all sky down to 2e-9 photons/cm^-2/s (>100 MeV), which is considerably smaller than most of previous studies.

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arXiv2009

Photometric H alpha and [O II] Luminosity Function of SDF and SXDF Galaxies: Implications for Future Baryon Oscillation Surveys

Efficient selection of emission line galaxies at z > 1 by photometric information in wide field surveys is one of the keys for future spectroscopic surveys to constrain dark energy using the baryon acoustic oscillation (BAO) signature. Here we estimate the H alpha and [O II] line luminosity functions of galaxies at z = 0.5-1.7 using a novel approach where multi-wavelength imaging data is used to jointly estimate both photometric redshifts and star-formation rates. These photometric estimates of line luminosities at high-redshift use the large data sets of the Subaru Deep Field and Subaru XMM-Newton Deep Field (covering \sim 1 deg^2) and are calibrated with the spectroscopic data of the local Sloan Digital Sky Survey galaxies. The derived luminosity functions (especially H alpha) are in reasonable agreement with the past estimates based on spectroscopic or narrow-band-filter surveys. This dataset is useful for examining the photometric selection of target galaxies for BAO surveys because of the large cosmological volume covered and the large number of galaxies with detailed photometric information. We use the sample to derive the photometric and physical properties of emission line galaxies to assist planning for future spectroscopic BAO surveys. We also show some examples of photometric selection procedures which can efficiently select these emission line galaxies.

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arXiv2008

No Evidence for Variability of Intervening Absorption Lines toward GRB 060206: Implications for the Mg II Incidence Problem

We examine variability of absorption line strength of intervening systems along the line of sight to GRB 060206 at $z = 4.05$, by the low-resolution optical spectra obtained by the Subaru telescope from six to ten hours after the burst. Strong variabilities of Fe\emissiontype{II} and Mg\emissiontype{II} lines at $z=1.48$ during $t=$5--8 hours have been reported for this GRB \citep{Hao07}, and this has been used to support the idea of clumpy Mg\emissiontype{II} cloudlets that was originally proposed to explain the anomalously high incidence of Mg\emissiontype{II} absorbers in GRB spectra compared with quasars . However, our spectra with higher signal-to-noise ratio do not show any evidence for variability in $t=$6-10 hours. There is a clear discrepancy between our data and Hao et al. data in the overlapping time interval. Furthermore, the line strengths in our data are in good agreement with those observed at $t \sim$ 2 hours by \citet{Tho08}. We also detected Fe\emissiontype{II} and Mg\emissiontype{II} absorption lines for a system at $z = 2.26$, and these lines do not show evidence for variability either. Therefore we conclude that there is no strong evidence for variability of intervening absorption lines toward GRB 060206, significantly weakening the support to the Mg\emissiontype{II} cloudlet hypothe sis by the GRB 060206 data.

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arXiv2008

Delay Time Distribution Measurement of Type Ia Supernovae by the Subaru/XMM-Newton Deep Survey and Implications for the Progenitor

The delay time distribution (DTD) of type Ia supernovae (SNe Ia) from star formation is an important clue to reveal the still unknown progenitor system of SNe Ia. Here we report on a measurement of the SN Ia DTD in a delay time range of t_Ia = 0.1-8.0 Gyr by using the faint variable objects detected in the Subaru/XMM-Newton Deep Survey (SXDS) down to i' ~ 25.5. We select 65 SN candidates showing significant spatial offset from nuclei of the host galaxies having old stellar population at z ~ 0.4-1.2, out of more than 1,000 SXDS variable objects. Although spectroscopic type classification is not available for these, we quantitatively demonstrate that more than ~80% of these should be SNe Ia. The DTD is derived using the stellar age estimates of the old galaxies based on 9 band photometries from optical to mid-infrared wavelength. Combined with the observed SN Ia rate in elliptical galaxies at the local universe, the DTD in t_Ia ~ 0.1-10 Gyr is well described by a featureless power-law as f_D(t_Ia) \propto t_Ia^{-1}. The derived DTD is in excellent agreement with the generic prediction of the double-degenerate scenario, giving a strong support to this scenario. In the single-degenerate (SD) scenario, although predictions by simple analytic formulations have broad DTD shapes that are similar to the observation, DTD shapes calculated by more detailed binary population synthesis tend to have strong peaks at characteristic time scales, which do not fit the observation. This result thus indicates either that the SD channel is not the major contributor to SNe Ia in old stellar population, or that improvement of binary population synthesis theory is required. Various sources of systematic uncertainties are examined and tested, but our main conclusions are not affected significantly.

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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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arXiv2008

Intracluster Short Gamma-Ray Bursts by Compact Binary Mergers

One of the possible origins of short gamma-ray bursts (SGRBs) is merging of compact binaries, and the effect of large kick velocity is a signature that can be used as an observational test for this hypothesis. Intracluster SGRBs that escaped from a host galaxy in a galaxy cluster are interesting in this context, since they would escape more easily by cluster tidal force, and would have brighter afterglow luminosity by dense intracluster gas, than those in general field galaxies. Here we calculate the escape fraction of compact binaries from their host galaxies in a galaxy cluster, and discuss some observational implications. We found that the escape fraction strongly depends on the nature of dark matter subhalos associated with member galaxies. If the amount of dark matter around member galaxies is not large and the gravitational potential for an escaping binary is determined mostly by stellar mass, most of SGRBs should escape and be observed as hostless, which is a much higher fraction than those in the field. Hence, statistics of intracluster SGRBs could give important information about the dark matter distribution in galaxy clusters, as well as hints for the origin of SGRBs.

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arXiv2007

The Subaru/XMM-Newton Deep Survey (SXDS) - V. Optically Faint Variable Object Survey

We present our survey for optically faint variable objects using multi-epoch (8-10 epochs over 2-4 years) $i'$-band imaging data obtained with Subaru Suprime-Cam over 0.918 deg$^2$ in the Subaru/XMM-Newton Deep Field (SXDF). We found 1040 optically variable objects by image subtraction for all the combinations of images at different epochs. This is the first statistical sample of variable objects at depths achieved with 8-10m class telescopes or HST. The detection limit for variable components is $i'_{\rm{vari}}\sim25.5$ mag. These variable objects were classified into variable stars, supernovae (SNe), and active galactic nuclei (AGN), based on the optical morphologies, magnitudes, colors, and optical-mid-infrared colors of the host objects, spatial offsets of variable components from the host objects, and light curves. Detection completeness was examined by simulating light curves for periodic and irregular variability. We detected optical variability for $36\pm2%$ ($51\pm3%$ for a bright sample with $i'<24.4$ mag) of X-ray sources in the field. Number densities of variable obejcts as functions of time intervals $Δ{t}$ and variable component magnitudes $i'_{\rm{vari}}$ are obtained. Number densities of variable stars, SNe, and AGN are 120, 489, and 579 objects deg$^{-2}$, respectively. Bimodal distributions of variable stars in the color-magnitude diagrams indicate that the variable star sample consists of bright ($V\sim22$ mag) blue variable stars of the halo population and faint ($V\sim23.5$ mag) red variable stars of the disk population. There are a few candidates of RR Lyrae providing a possible number density of $\sim10^{-2}$ kpc$^{-3}$ at a distance of $>150$ kpc from the Galactic center.

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arXiv2007

The Subaru/XMM-Newton Deep Survey (SXDS) - VI. Properties of Active Galactic Nuclei Selected by Optical Variability

We present the properties of active galactic nuclei (AGN) selected by optical variability in the Subaru/XMM-Newton Deep Field (SXDF). Based on the locations of variable components and light curves, 211 optically variable AGN were reliably selected. We made three AGN samples; X-ray detected optically non-variable AGN (XA), X-ray detected optically variable AGN (XVA), and X-ray undetected optically variable AGN (VA). In the VA sample, we found a bimodal distribution of the ratio between the variable component flux and the host flux. One of these two components in the distribution, a class of AGN with a faint variable component $i'_{\rm{vari}}\sim25$ mag in bright host galaxies $i'\sim21$ mag, is not seen in the XVA sample. These AGN are expected to have low Eddington ratios if we naively consider a correlation between bulge luminosity and black hole mass. These galaxies have photometric redshifts $z_{\rm{photo}}\sim0.5$ and we infer that they are low-luminosity AGN with radiatively inefficient accretion flows (RIAFs). The properties of the XVA and VA objects and the differences from those of the XA objects can be explained within the unified scheme for AGN. Optical variability selection for AGN is an independent method and could provide a complementary AGN sample which even deep X-ray surveys have not found.

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arXiv2007

The Cosmic MeV Gamma-ray Background and Hard X-ray Spectra of Active Galactic Nuclei: Implications for the Origin of Hot AGN Coronae

The origin of the extragalactic gamma-ray background radiation at 1-10 MeV is still unknown. Although the cosmic X-ray background (CXB) up to a few hundreds keV can be accounted for by the sum of Active Galactic Nuclei (AGNs), current models of AGN spectra cannot explain the background spectrum beyond ~1 MeV, because of the thermal exponential cutoff of electron energy distribution assumed in the models. Here we construct a new spectral model by calculating the Comptonization process including nonthermal electrons, which are expected to exist in an AGN hot corona if it is heated by magnetic reconnections. We show that the MeV background spectrum can nicely be explained by our model, when coronal electrons have a nonthermal power-law component whose total energy is a few percent of the thermal component and whose spectral index is d(ln N_e)/d(ln E_e) ~ -4. Although the MeV gamma-ray flux from such a component in nearby AGN spectra is below the detection limit of past observations, it could be detected by planned future MeV detectors. We point out that the amount of the nonthermal component and its electron index are similar to those found for electrons accelerated by magnetic reconnections in solar flares and the Earth magnetosphere, giving a support to the reconnection hypothesis for the origin of hot AGN coronae.

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