arXiv subjects

Katsuya Okoshi

Publications and source records attributed to Katsuya Okoshi.

15 recordsLinked to original sources

arXiv2025

Serendipitous Discovery of an Optically-Dark Ultra-Luminous Infrared Galaxy at $z$ = 3.4

Dusty, submillimeter-selected galaxies without optical counterparts contribute a non-negligible fraction of the star formation in the early universe. However, such a population is difficult to detect through classical optical/UV-based surveys. We report the serendipitous discovery of such an optically dark galaxy, behind the quadruply-lensed $z=2.56$ quasar, H1413+117, offset to the north by 6\arcsec. From $^{12}$CO $J=4$--3, $J=6$--5, and part of the $J=13$--12 transitions, which all spatially coincide with a compact submillimeter continuum emission, we determine an unambiguous spectroscopic redshift, $z=3.386\pm 0.005$. This galaxy has a molecular mass $M_{\rm mol} \sim 10^{11}$ M$_\odot$ and a black hole mass $M_{\rm BH} \sim 10^{8}$ M$_\odot$, estimated from $^{12}$CO $J=4$--3 and archival {\it Chandra} X-ray data ($L_{\rm 2-10,keV} \sim 4 \times 10^{44}$\,erg\,s$^{-1}$), respectively. We also estimate a total infrared luminosity of $L_{\rm FIR} = (2.8\pm{2.3}) \times 10^{12}$ L$_\odot$ and a stellar mass of $M_* \lesssim 10^{11}$ M$_{\odot}$, from spectral energy distribution fitting. According to these simple mass estimations, this gas-rich and X-ray bright galaxy might be in a transition phase from starburst to quasar offering a unique case for studying galaxy-black hole co-evolution under extremely dusty conditions.

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arXiv2022

The physical origin for spatially large scatter of IGM opacity at the end of reionization: the IGM Ly$α$ opacity-galaxy density relation

The large opacity fluctuations in the $z > 5.5$ Ly$α$ forest may indicate inhomogeneous progress of reionization. To explain the observed large scatter of the effective Ly$α$ optical depth ($τ_{\rm eff}$) of the intergalactic medium (IGM), fluctuation of UV background ($Γ$ model) or the IGM gas temperature ($T$ model) have been proposed, which predict opposite correlations between $τ_{\rm eff}$ and galaxy density. In order to address which model can explain the large scatter of $τ_{\rm eff}$, we search for Ly$α$ emitters (LAEs) around two (J1137+3549 and J1602+4228) quasar sightlines with $τ_{\rm eff}\sim3$ and J1630+4012 sightline with $τ_{\rm eff}\sim5.5$. Using a narrowband imaging with Subaru/Hyper Suprime-Cam, we draw LAE density maps to explore their spatial distributions. Overdensities are found within 20 $h^{-1}$Mpc of the quasar sightlines in the low $τ_{\rm eff}$ regions, while a deficit of LAEs is found in the high $τ_{\rm eff}$ region. Although the $τ_{\rm eff}$ of the three quasar sightlines are neither high nor low enough to clearly distinguish the two models, these observed $τ_{\rm eff}$-galaxy density relations all consistently support the $Γ$ model rather than the $T$ model in the three fields, along with the previous studies. The observed overdensities near the low $τ_{\rm eff}$ sightlines may suggest that the relic temperature fluctuation does not affect reionization that much. Otherwise, these overdensities could be attributed to other factors besides the reionization process, such as the nature of LAEs as poor tracers of underlying large-scale structures.

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arXiv2022

Exploratory Study of Transverse Proximity Effect around BAL Quasars

We aim to find out the reason why there exists an anisotropic HI absorption around quasars; i.e., the environments around quasars are highly biased toward producing strong HI absorption in transverse direction while there exists a significant deficit of HI absorption within a few Mpc of quasars along line-of-sight. The most plausible explanation for this opposite trend is that the transverse direction is shadowed from the quasar UV radiation due to dust torus. However, a critical weakness of this idea is that we have no information on inclination angle of our sightline relative to the torus. In this study, we examine environments of quasars with broad absorption troughs in their spectra (i.e., BAL quasars) because it is widely believed that BAL troughs are observed if the central continuum is viewed from the side through their powerful outflows near the dust torus. With closely separated 12 projected quasar pairs at different redshift with separation angle of $θ$$<$120$^{\prime\prime}$, we examine HI absorption at foreground BAL quasars in spectra of background quasars. We confirm there exist optically thick gas around two of 12 BAL quasars, and that the mean HI absorption strength is EW$_{\rm rest}$$\sim$1A. These are consistent to the past results around non-BAL quasars, although not statistically significant. However, the origins of optically thick HI absorbers around BAL and non-BAL quasars could be different since their column densities are different by $\sim$3 orders of magnitude. The larger sample would be required for narrowing down possible scenarios for the anisotropic HI absorption around quasars.

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arXiv2021

Multiple MgII Absorption Systems in the Lines of Sight to Quadruply Lensed Quasar H1413+1143

We find multiple Mg II absorption systems at redshift z=1.66, 2.069, and 2.097 in the spatially resolved spectra of the quadruply gravitationally lensed quasar H1413+1143 utilizing the Kyoto tridimensional spectrograph II (Kyoto 3DII) spectrograph on board the Subaru telescope. Here we present the first measurement of differences in Mg II absorption strength of the multiple intervening absorbers, which include ones identified as damped Lyman alpha (DLA) absorption systems. Our detection of the significant Mg II absorptions in the spatially resolved spectra reveals the inhomogeneous chemical enrichment on scales of about 12 kpc within the separation of the four sightlines. For the DLA system at z=1.66, the rest equivalent widths of the Mg II absorption lines between the four spatially resolved lines of sight change by factors of up to about 6, which trace the variations in the H I absorption strength. This suggests that inhomogeneous cold absorbers that give rise to the strong H I/Mg II absorptions dwell on a scale of about 6-12 kpc between the four lines of sight. We also investigate the degree of variation in the equivalent width of the absorption lines between the lines of sight. We find that the systems giving rise to strong absorptions in the spectra of the quadruply lensed quasars tend to have a high degree of variation in absorption strength between the lines of sight toward the lensed quasars.

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arXiv2020

Quantifying the effect of field variance on the H$α$ luminosity function with the New Numerical Galaxy Catalogue ($ν^2$GC)

We construct a model of H$α$ emitters (HAEs) based on a semi-analytic galaxy formation model, the New Numerical Galaxy Catalog ($ν^2$GC). In this paper, we report our estimate for the field variance of the HAE distribution. By calculating the H$α$ luminosity from the star-formation rate of galaxies, our model well reproduces the observed H$α$ luminosity function (LF) at $z=0.4$. The large volume of the $ν^2$GC makes it possible to examine the spatial distribution of HAEs over a region of (411.8 Mpc)$^3$ in the comoving scale. The surface number density of $z=0.4$ HAEs with $L_{\rm Hα} \geq 10^{40}$ erg s$^{-1}$ is 308.9 deg$^{-2}$. We have confirmed that the HAE is a useful tracer for the large-scale structure of the Universe because of their significant overdensity ($>$ 5$σ$) at clusters and the filamentary structures. The H$α$ LFs within a survey area of $\sim$2 deg$^2$ (typical for previous observational studies) show a significant field variance up to $\sim$1 dex. Based on our model, one can estimate the variance on the H$α$ LFs within given survey areas.

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arXiv2018

The New Numerical Galaxy Catalogue (ν^2 GC): Properties of Active Galactic Nuclei and Their Host Galaxies

We present the latest results of a semi-analytic model of galaxy formation, "New Numerical Galaxy Catalogue", which is combined with large cosmological N-body simulations. This model can reproduce statistical properties of galaxies at z < 6.0. We focus on the properties of active galactic nuclei (AGNs) and supermassive black holes, especially on the accretion timescale onto black holes. We find that the number density of AGNs at z < 1.5 and at hard X-ray luminosity 10^{ 44 }< erg/s is underestimated compared with recent observational estimates when we assume the exponentially decreasing accretion rate and the accretion timescale which is proportional to the dynamical time of the host halo or the bulge, as is often assumed in semi-analytic models. We show that to solve this discrepancy, the accretion timescale of such less luminous AGNs instead should be a function of the black hole mass and the accreted gas mass. This timescale can be obtained from a phenomenological modelling of the gas angular momentum loss in the circumnuclear torus and/or the accretion disc. Such models predict a longer accretion timescale for less luminous AGNs at z < 1.0 than bright QSOs whose accretion timescale would be 10^{ 7-8 } yr. With this newly introduced accretion timescale, our model can explain the observed luminosity functions of AGNs at z < 6.0.

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arXiv2017

Resolving the Internal Structure of Circum-Galactic Medium using Gravitationally Lensed Quasars

We study the internal structure of the Circum-Galactic Medium (CGM), using 29 spectra of 13 gravitationally lensed quasars with image separation angles of a few arcseconds, which correspond to 100 pc to 10 kpc in physical distances. After separating metal absorption lines detected in the spectra into high-ions with ionization parameter (IP) $>$ 40 eV and low-ions with IP $<$ 20 eV, we find that i) the fraction of absorption lines that are detected in only one of the lensed images is larger for low-ions ($\sim$16%) than high-ions ($\sim$2%), ii) the fractional difference of equivalent widths ($EW$s) between the lensed images is almost same (${\rm d}EW$ $\sim$ 0.2) for both groups although the low-ions have a slightly larger variation, and iii) weak low-ion absorbers tend to have larger ${\rm d}EW$ compared to weak high-ion absorbers. We construct simple models to reproduce these observed properties and investigate the distribution of physical quantities such as size and location of absorbers, using some free parameters. Our best models for absorbers with high-ions and low-ions suggest that i) an overall size of the CGM is at least $\sim$ 500 kpc, ii) a size of spherical clumpy cloud is $\sim$ 1 kpc or smaller, and iii) only high-ion absorbers can have diffusely distributed homogeneous component throughout the CGM. We infer that a high ionization absorber distributes almost homogeneously with a small-scale internal fluctuation, while a low ionization absorber consists of a large number of small-scale clouds in the diffusely distributed higher ionized region. This is the first result to investigate the internal small-scale structure of the CGM, based on the large number of gravitationally lensed quasar spectra.

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arXiv2016

The New Numerical Galaxy Catalog ($ν^2$GC): An Updated Semi-analytic Model of Galaxy and AGN with Large Cosmological N-body Simulation

We present a new cosmological galaxy formation model, $ν^2$GC, as an updated version of our previous model $ν$GC. We adopt the so-called "semi-analytic" approach, in which the formation history of dark matter halos is computed by ${\it N}$-body simulations, while the baryon physics such as gas cooling, star formation and supernova feedback are simply modeled by phenomenological equations. Major updates of the model are as follows: (1) the merger trees of dark matter halos are constructed in state-of-the-art ${\it N}$-body simulations, (2) we introduce the formation and evolution process of supermassive black holes and the suppression of gas cooling due to active galactic nucleus (AGN) activity, (3) we include heating of the intergalactic gas by the cosmic UV background, and (4) we tune some free parameters related to the astrophysical processes using a Markov chain Monte Carlo method. Our ${\it N}$-body simulations of dark matter halos have unprecedented box size and mass resolution (the largest simulation contains 550 billion particles in a 1.12 Gpc/h box), enabling the study of much smaller and rarer objects. The model was tuned to fit the luminosity functions of local galaxies and mass function of neutral hydrogen. Local observations, such as the Tully-Fisher relation, size-magnitude relation of spiral galaxies and scaling relation between the bulge mass and black hole mass were well reproduced by the model. Moreover, the model also well reproduced the cosmic star formation history and the redshift evolution of rest-frame ${\it K}$-band luminosity functions. The numerical catalog of the simulated galaxies and AGNs is publicly available on the web.

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arXiv2013

Extended Lyman-alpha emission from a high-z DLA at z=3.115

We searched for star formation activity associated with high-z Damped Lyman-alpha systems (DLAs) with Subaru telescope. We used a set of narrow-band (NB) filters whose central wavelengths correspond to the redshifted Lyman-alpha emission lines of targeted DLA absorbers at 3<z<4.5. We detected one apparent NB-excess object located 3.80 arcsec (~28kpc) away from the quasar SDSS J031036.84+005521.7. Follow-up spectroscopy revealed an asymmetric Lyman-alpha emission at z_em=3.115+/-0.003, which perfectly matches the sub-DLA trough at z_abs=3.1150 with logN(HI)/cm^-2=20.05. The Lyman-alpha luminosity is estimated to be L(LyA)=1.07x10^42 erg s^-1, which corresponds to a star formation rate of 0.97 M_\odot yr^-1. Interestingly, the detected Lyman-alpha emission is spatially extended with a sharp peak. The large extent of the Lyman-alpha emission is remarkably one-sided toward the quasar line-of-sight, and is redshifted. The observed spatially asymmetric surface brightness profile can be qualitatively explained by a model of a DLA host galaxy, assuming a galactic outflow and a clumpy distribution of HI clouds in the circumgalactic medium. This large Lyman-alpha extension, which is similar to those found in Rauch et al. (2008), could be the result of complicated anisotropic radiative transfer through the surrounding neutral gas embedded in the DLA.

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arXiv2012

Constraining stellar properties of intervening damped Lyα and MgII absorbing galaxies toward GRB 050730

We performed multi-band deep imaging of the field around GRB 050730 to identify the host galaxies of intervening absorbers, which consist of a damped Lyα absorption (DLA) system at zabs=3.564, a sub-DLA system at zabs=3.022, and strong MgII absorption systems at zabs=1.773 and 2.253. Our observations were performed after the gamma-ray burst afterglow had disappeared. Thus, our imaging survey has a higher sensitivity to the host galaxies of the intervening absorbers than the normal imaging surveys in the direction of QSOs, for which the QSO glare tends to hide the foreground galaxies. In this deep imaging survey, we could not detect any unambiguous candidates for the host galaxies of the intervening absorbers. Using the 3sigma upper limit of the flux in the optical to mid-infrared observing bands, which corresponds to the UV to optical bands in the rest-frame of the intervening absorbers, we constrained the star-formation rates and stellar masses of the hosts. We estimated the star-formation rates for the intervening absorbers as < 2.5 Msun/yr for z>3 DLAs and < 1.0 Msun/yr for z~2 MgII systems. Their stellar masses are estimated to be several times 10^9 Msun or smaller for all intervening galaxies. These properties are comparable to dwarf galaxies, rather than the massive star-forming galaxies commonly seen in the z>2 galaxy surveys based on emission-line selection or color selection.

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arXiv2012

Type-Ia Supernova Remnant Shell At $Z=3.5$ Seen In The Three Sightlines Toward The Gravitationally Lensed Qso B1422+231

Using the Subaru 8.2m Telescope with an IRCS Echelle spectrograph, we obtained high-resolution (R=10,000) near-infrared (1.01-1.38 μm) spectra of images A and B of the gravitationally lensed QSO B1422+231 (z=3.628) consisting of four known lensed images. We detected MgII absorption lines at z=3.54, which show a large variance of column densities (~ 0.3 dex) and velocities (~ 10 km/s) between the sightlines A and B with a projected separation of only 8.4h_{70}^{-1} pc at the redshift. This is the smallest spatial structure of the high-z gas clouds ever detected after Rauch et al. found a 20-pc scale structure for the same z=3.54 absorption system using optical spectra of images A and C. The observed systematic variances imply that the system is an expanding shell as originally suggested by Rauch et al. By combining the data for three sightlines, we managed to constrain the radius and expansion velocity of the shell (~ 50-100 pc, 130 km/s), concluding that the shell is truly a supernova remnant (SNR) rather than other types of shell objects, such as a giant HII region. We also detected strong FeII absorption lines for this system, but with much broader Doppler width than that of α-element lines. We suggest that this FeII absorption line originates in a localized FeII-rich gas cloud that is not completely mixed with plowed ambient interstellar gas clouds showing other α-element low-ion absorption lines. Along with the Fe richness, we conclude that the SNR is produced by an SNIa explosion.

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arXiv2010

HI-selected Galaxies as a probe of Quasar Absorption Systems

We investigate the properties of HI-rich galaxies detected in blind radio surveys within the hierarchical structure formation scenario using a semi-analytic model of galaxy formation. By drawing a detailed comparison between the properties of HI-selected galaxies and HI absorption systems, we argue a link between the local galaxy population and quasar absorption systems, particularly for Damped Ly-alpha absorption (DLA) systems and sub-DLA systems. First, we evaluate how many HI-selected galaxies exhibit HI column densities as high as those of DLA systems. We find that HI-selected galaxies with HI masses M(HI) > 10^8 solar masses have gaseous disks that produce HI column densities comparable to those of DLA systems. We conclude that DLA galaxies where the HI column densities are as high as those of DLA systems, contribute significantly to the population of HI-selected galaxies at M(HI) > 10^8 solar masses. Second, we find that star formation rates (SFRs) correlate tightly with HI masses rather than B- (and J-) band luminosities. In the low-mass range M(HI) < 10^8 solar masses, sub-DLA galaxies replace DLA galaxies as the dominant population. The number fraction of sub-DLA galaxies relative to galaxies reaches 40%-60% at HI masses 10^8 solar masses and 30%-80% at 10^7 solar masses. The HI-selected galaxies at 10^7 solar masses are a strong probe of sub-DLA systems that place stringent constraints on galaxy formation and evolution.

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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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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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arXiv1997

Evolution of Multiphase Hot Interstellar Medium in Elliptical Galaxies

We present the results of a variety of simulations concerning the evolution of multiphase (inhomogeneous) hot interstellar medium (ISM) in elliptical galaxies. We assume the gases ejected from stars do not mix globally with the circumferential gas. The ejected gas components evolve separately according to their birth time, position, and origin. We consider cases where supernova remnants (SNRs) mix with local ISM. The components with high metal abundance and/or high density cool and drop out of the hot ISM gas faster than the other components because of their high metal abundance and/or density. This makes the average metal abundance of the hot ISM low. Furthermore, since the metal abundance of mass-loss gas decreases with radius, gas inflow from outer region makes the average metal abundance of the hot ISM smaller than that of mass-loss gas in the inner region. As gas ejection rate of stellar system decreases, mass fraction of mass-loss gas ejected at outer region increases in a galaxy. If the mixing of SNRs is ineffective, our model predicts that observed [Si/Fe] and [Mg/Fe] should decrease towards the galactic center because of strong iron emission by SNRs. In the outer region, where the cooling of time of the ISM is long, the selective cooling is ineffective and most of gas components remain hot. Thus, the metal abundance of the ISM in this region directly reflects that of the gas ejected from stars. Our model shows that supernovae are not effective heating sources in the inner region of elliptical galaxies, because most of the energy released by them radiates. Therefore, cooling flow is established even if the supernova rate is high. Mixing of SNRs with ambient ISM makes the energy transfer between supernova explosion and ambient ISM more effective.

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