arXiv subjects

Masamune Oguri

Publications and source records attributed to Masamune Oguri.

At least 343 records · Page 19Linked to original sources

arXiv2005

Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields

We study generation of magnetic fields by the Biermann mechanism in the pair-instability supernovae explosions of first stars. The Biermann mechanism produces magnetic fields in the shocked region between the bubble and interstellar medium (ISM), even if magnetic fields are absent initially. We perform a series of two-dimensional magnetohydrodynamic simulations with the Biermann term and estimate the amplitude and total energy of the produced magnetic fields. We find that magnetic fields with amplitude $10^{-14}-10^{-17}$ G are generated inside the bubble, though the amount of magnetic fields generated depend on specific values of initial conditions. This corresponds to magnetic fields of $10^{28}-10^{31}$ erg per each supernova remnant, which is strong enough to be the seed magnetic field for galactic and/or interstellar dynamo.

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arXiv2005

SDSS J0246-0825: A New Gravitationally Lensed Quasar from the Sloan Digital Sky Survey

We report the discovery of a new two-image gravitationally lensed quasar, SDSS J024634.11-082536.2 (SDSS J0246-0825). This object was selected as a lensed quasar candidate from the Sloan Digital Sky Survey (SDSS) by the same algorithm that was used to discover other SDSS lensed quasars (e.g., SDSS J0924+0219). Multicolor imaging with the Magellan Consortium's Walter Baade 6.5-m telescope and the spectroscopic observations using the W. M. Keck Observatory's Keck II telescope confirm that SDSS J0246-0825 consists of two lensed images ($Δθ=$1\farcs04) of a source quasar at z=1.68. Imaging observations with the Keck telescope and the Hubble Space Telescope reveal an extended object between the two quasar components, which is likely to be a lensing galaxy of this system. From the absorption lines in the spectra of quasar components and the apparent magnitude of the galaxy, combined with the expected absolute magnitude from the Faber-Jackson relation, we estimate the redshift of the lensing galaxy to be z=0.724. A highly distorted ring is visible in the Hubble Space Telescope images, which is likely to be the lensed host galaxy of the source quasar. Simple mass modeling predicts the possibility that there is a small (faint) lensing object near the primary lensing galaxy.

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arXiv2005

Lens Galaxy Environments and Anomalous Flux Ratios in Gravitational Lenses

The fraction of substructures required to account for anomalous flux ratios in gravitational lens systems appears to be higher than that predicted in the standard cold dark matter cosmology. We present a possible alternative route to anomalous flux ratios from lens galaxy environments. We consider the compound lens system such that a lens galaxy lie in a group or cluster, and estimate the contribution of substructures in the group/cluster to the fraction using an analytic model of substructures. We find that the contribution becomes dominant when the impact parameter of the lens is less than ~30% of the virial radius of the group/cluster. This indicates that the environmental effect can partly explain the high incidence of anomalous flux ratios.

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arXiv2005

Binary Quasars in the Sloan Digital Sky Survey: Evidence for Excess Clustering on Small Scales

We present a sample of 218 new quasar pairs with proper transverse separations R_prop < 1 Mpc/h over the redshift range 0.5 < z < 3.0, discovered from an extensive follow up campaign to find companions around the Sloan Digital Sky Survey and 2dF Quasar Redshift Survey quasars. This sample includes 26 new binary quasars with separations R_prop < 50 kpc/h (theta < 10 arcseconds), more than doubling the number of such systems known. We define a statistical sample of binaries selected with homogeneous criteria and compute its selection function, taking into account sources of incompleteness. The first measurement of the quasar correlation function on scales 10 kpc/h < R_prop < 400 kpc/h is presented. For R_prop < 40 kpc/h, we detect an order of magnitude excess clustering over the expectation from the large scale R_prop > 3 Mpc/h quasar correlation function, extrapolated down as a power law to the separations probed by our binaries. The excess grows to ~ 30 at R_prop ~ 10 kpc/h, and provides compelling evidence that the quasar autocorrelation function gets progressively steeper on sub-Mpc scales. This small scale excess can likely be attributed to dissipative interaction events which trigger quasar activity in rich environments. Recent small scale measurements of galaxy clustering and quasar-galaxy clustering are reviewed and discussed in relation to our measurement of small scale quasar clustering.

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arXiv2005

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

We report the discovery of a fifth image in the large separation lensed quasar system SDSS J1004+4112. A faint point source located 0.2'' from the center of the brightest galaxy in the lensing cluster is detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves and thereby the mass distribution of the lensing cluster.

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arXiv2004

Discovery of Two Gravitationally Lensed Quasars with Image Separations of 3 Arcseconds from the Sloan Digital Sky Survey

We report the discovery of two doubly-imaged quasars, SDSS J100128.61+502756.9 and SDSS J120629.65+433217.6, at redshifts of 1.838 and 1.789 and with image separations of 2.86'' and 2.90'', respectively. The objects were selected as lens candidates from the Sloan Digital Sky Survey (SDSS). Based on the identical nature of the spectra of the two quasars in each pair and the identification of the lens galaxies, we conclude that the objects are gravitational lenses. The lenses are complicated; in both systems there are several galaxies in the fields very close to the quasars, in addition to the lens galaxies themselves. The lens modeling implies that these nearby galaxies contribute significantly to the lens potentials. On larger scales, we have detected an enhancement in the galaxy density near SDSS J100128.61+502756.9. The number of lenses with image separation of ~3'' in the SDSS already exceeds the prediction of simple theoretical models based on the standard Lambda-dominated cosmology and observed velocity function of galaxies.

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arXiv2004

An Analytic Model of Superclusters

We construct an analytic model for the mass distributions of superclusters and clusters in each supercluster. Our model is a modification of the Press-Schechter theory, and defines superclusters as the regions that have some overdensity smaller than those for usual virialized objects. We compare the mass functions with a catalog of superclusters in the Sloan Digital Sky Survey Early Data Release and found that they are in reasonable agreement with each other.

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arXiv2004

Strong Gravitational Lenses in a Cold Dark Matter Universe

We present theoretical and observational studies of strong gravitational lenses produced by clusters of galaxies. Our purpose is to test the Cold Dark Matter (CDM) model at small and highly non-linear scales where it has been claimed that the CDM model may confront several difficulties. We concentrate our attention on the statistics of strong gravitational lenses. We use two complementary statistics, lensed arcs and quasars, to probe the mass distributions. First, we construct a triaxial lens model, and develop a new method to include triaxiality of dark halos in the lens statistics. We find that the effect of triaxiality is significant; it enhances lensing probabilities by a factors of a few to ten, assuming the degree of triaxiality predicted in the CDM model. In particular, we argue that both central concentration and large triaxiality of dark halos are required to reproduce the observed number of arcs in clusters; thus the result can be interpreted as a strong evidence for the cold and collisionless dark matter. One of the most notable advantages of the triaxial modeling is that the triaxial modeling allows us to predict image multiplicities. We find that the CDM halos predict significant fraction (more than 20%) of naked cusp lenses. In addition, we point out the image multiplicities depend strongly on the central concentration of dark halos. Therefore we propose image multiplicities as a new powerful test of the CDM model. Finally we searched for large-separation lensed quasars from the data of the Sloan Digital Sky Survey, and succeeded in discovering the first large-separation lensed quasar SDSS J1004+4112. We find that the discovery of the large-separation quadruple lens SDSS J1004+4112 is quite consistent with the theoretical predictions based on the CDM model. (Abridged)

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arXiv2004

A Realistic Model for Spatial and Mass Distributions of Dark Halo Substructures: An Analytic Approach

We construct a realistic model for the dark halo substructures, and derive analytically their spatial, mass, and velocity distributions. Basically, our model is a modification of the Press-Schechter theory to account for the dominant dynamical processes that mark the evolution of dark halo substructures such as the tidal stripping and dynamical friction. Our analytic model successfully reproduces all the well known behaviors of the substructure distributions that have been found in recent numerical simulations: the weak dependence of the mass distributions on the host halo mass; the anti-bias of the spatial distribution relative to the dark matter particle components; the nearly power-law shapes of the mass and velocity distributions. We compare our analytic results with recent high-resolution N-body simulation data and find that they are in excellent agreement with each other.

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arXiv2004

WMAP Constraints on Decaying Cold Dark Matter

We re-formulate cosmological perturbations in the decaying cold dark matter model, and calculate cosmological microwave background (CMB) anisotropies. By comparing our theoretical predictions with recent observational data from the Wilkinson Microwave Anisotropy Probe(WMAP), we derive a new bound on the abundance and lifetime of decaying dark matter particles. We show that the data of WMAP alone do not prefer the decaying cold dark matter model: the lifetime is constrained to Γ^{-1}> 123 Gyr at 68% C.L. (52 Gyr at 95.4% C.L.) when cold dark matter consists only of such decaying particles. We also consider a more general case in which cold dark matter consists of both stable and decaying particles, and show that the constraint generalizes down to Ω_{DDM}h^2 < -0.5 (Gamma^{-1}/1 Gyr)^{-1}+0.12 for Γ^{-1} > 5 Gyr at 95.4% C.L.. These bounds are robust and widely applicable, because they are derived only from gravitational effects and therefore do not rely on the details of decay channels or decay products.

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arXiv2004

Effects of Triaxiality on the Statistics of Large-Separation Gravitational Lenses

We study the statistics of large-separation gravitational lens systems produced by non-spherical halos in the Cold Dark Matter (CDM) model. Specifically, we examine how the triaxiality of CDM halos affects the overall lensing probabilities and the relative numbers of different image configurations (double, quadruple, and naked cusp lenses). We find that triaxiality significantly enhances lensing probabilities by a factor of ~2-4, so it cannot be ignored. If CDM halos have central density slopes alpha<~1.5, we predict that a significant fraction (>~20%) of large-separation lenses should have naked cusp image configurations; this contrasts with lensing by isothermal (α~2) galaxies where naked cusp configurations are rare. The image multiplicities depend strongly on the inner density slope α: for α=1, the naked cusp fraction is >~60%; while for α=1.5, quadruple lenses are actually the most probable. Thus, the image multiplicities in large-separation lenses offer a simple new probe of the inner density profiles of dark matter halos. We also compute the expected probabilities and image multiplicities for lensed quasars in the Sloan Digital Sky Survey, and argue that the recent discovery of the large-separation quadruple lens SDSS J1004+4112 is consistent with expectations for CDM.

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arXiv2004

Galaxy Structures and External Perturbations in Gravitational Lenses

In modeling strong gravitational lens systems, one often adopts simple models, such as singular isothermal elliptical density plus lowest-order external perturbation. However, such simple models may mislead us if the real mass distribution is more complicated than that in the assumed models. In particular, assumptions on mass models are crucial in studying flux ratio anomalies that have been suggested as evidence for a cold dark-matter substructure. We reinvestigated four quadruple lens systems using power-law Fourier models, which have advantages of clear physical meanings and the applicability of a linear method, as well as a simple singular isothermal elliptical density model. We also investigated the effect of external perturbations, including a singular isothermal sphere, lowest-order expansion, and next-order expansion. We have found that the $\cos3θ$ terms of the primary galaxy and/or of external perturbation significantly reduce the χ^2 in PG 1115+080 and B1422+231. In particular, we could reproduce the flux ratios of B1422+231 with next-order external perturbation assuming 5% flux uncertainties, suggesting that external perturbation cannot be described by a simple singular isothermal sphere approximation. On the other hand, we could not fit B0712+472 and B2045+265 very well even with our models, although the χ^2 were reduced compared with the case of using simple models. Our results clearly demonstrate that both the primary lens galaxy and the external perturbation are often more complicated than we usually assume.

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arXiv2004

Microlensing of the Broad Emission Line Region in the Quadruple Lens SDSS J1004+4112

We present seven epochs of spectroscopy on the quadruply imaged quasar SDSS J1004+4112, spanning observed-frame time delays from 1 to 322 days. The spectra reveal differences in the emission lines between the lensed images. Specifically, component A showed a strong enhancement in the blue wings of several high-ionization lines relative to component B, which lasted at least 28 days (observed frame) then faded. Since the predicted time delay between A and B is <30 days, our time coverage suggests that the event was not intrinsic to the quasar. We attribute these variations to microlensing of part of the broad emission line region of the quasar, apparently resolving structure in the source plane on a scale of ~10^{16} cm at z=1.734. In addition, we observed smaller differences in the emission line profiles between components A and B that persisted throughout the time span, which may also be due to microlensing or millilensing. Further spectroscopic monitoring of this system holds considerable promise for resolving the structure of the broad emission line region in quasars.

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arXiv2004

Arc Statistics in Triaxial Dark Matter Halos: Testing the Collisionless Cold Dark Matter Paradigm

Statistics of lensed arcs in clusters of galaxies serve as a powerful probe of both the non-sphericity and the inner slope of dark matter halos. We develop a semi-analytic method to compute the number of arcs in triaxial dark matter halos. This combines the lensing cross section from the Monte Carlo ray-tracing simulations, and the probability distribution function (PDF) of the axis ratios evaluated from cosmological N-body simulations. This approach enables one to incorporate both asymmetries in the projected mass density and elongations along the line-of-sight analytically, for the first time in cosmological lensed arc statistics. As expected, triaxial dark matter halos significantly increase the number of arcs relative to spherical models; the difference amounts to more than one order of magnitude while the value of enhancement depends on the specific properties of density profiles. Then we compare our theoretical predictions with the observed number of arcs from 38 X-ray selected clusters. In contrast to the previous claims, our triaxial dark matter halos with inner density profile ρ\propto r^{-1.5} in a Lambda-dominated cold dark matter (CDM) universe reproduces well the observation. Since both the central mass concentration and triaxial axis ratios (minor to major axis ratio ~0.5) required to account for the observed data are consistent with cosmological N-body simulations, our result may be interpreted to lend strong support for the collisionless CDM paradigm at the mass scale of clusters.

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arXiv2004

SDSS J1335+0118: A New Two-Image Gravitational Lens

We report the discovery of the two-image gravitationally lensed quasar SDSS J1335+0118. The object was selected as a lens candidate from the Sloan Digital Sky Survey. The imaging and spectroscopic follow-up observations confirm that the system exhibits two gravitationally lensed images of a quasar at z=1.57. The image separation is 1.56''. We also detect an extended component between the two quasar images, likely the lensing galaxy. Preliminary mass modeling predicts the differential time delay Δt ~ 30 h^{-1} day assuming the redshift of the lens galaxy is 0.5.

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arXiv2003

Sunyaev-Zel'dovich power spectrum with decaying cold dark matter

Recent studies of structures of galaxies and clusters imply that dark matter might be unstable and decay with lifetime $Γ^{-1}$ about the age of universe. We study the effects of the decay of cold dark matter on the Sunyaev-Zel'dovich (SZ) power spectrum. We analytically calculate the SZ power spectrum taking finite lifetime of cold dark matter into account. We find the finite lifetime of dark matter decreases the power at large scale ($l < 4000$) and increases at small scale ($l > 4000$). This is in marked contrast with the dependence of other cosmological parameters such as the amplitude of mass fluctuations $σ_{8}$ and the cosmological constant $Ω_λ$ (under the assumption of a flat universe) which mainly change the normalization of the angular power spectrum. This difference allows one to determine the lifetime and other cosmological parameters rather separately. We also investigate sensitivity of a future SZ survey to the cosmological parameters including the life time, assuming a fiducial model $Γ^{-1} = 10 h^{-1} {\rm Gyr}$, $σ_{8} = 1.0$, and $Ω_λ = 0.7$. We show that future SZ surveys such as ACT, AMIBA, and BOLOCAM can determine the lifetime within factor of two even if $σ_{8}$ and $Ω_λ$ are marginalized.

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arXiv2003

Decaying Cold Dark Matter and the Evolution of the Cluster Abundance

The cluster abundance and its redshift evolution are known to be a powerful tool to constrain the amplitude of mass fluctuations σ_8 and the mass density parameter Ω_m0. We study the impact of the finite decay rate of cold dark matter particles on the cluster abundances. On the basis of spherical model in decaying cold dark matter universe, we calculate the mass function of clusters and compare it with observed cluster abundance. We find the decay of cold dark matter particles significantly changes the evolution of the cluster abundance. In particular, we point out that the lifetime of dark matter particles comparable to the age of the universe lowers the ratio of the local cluster abundance to the high-redshift cluster abundance and can account for the observed evolution of the cluster abundance quite well. The strong dependence of the cluster abundance on the decay rate of dark matter suggests that distant cluster surveys may offer clues to the nature of dark matter.

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arXiv2003

How many arcminute-separation lenses are expected in the 2dF QSO survey?

Wide separation lensing statistics offer information about the density profile and abundance of dark halos. Recently a possible discovery of six quasar pairs, which may be lensed multiple images, was reported by Miller et al. (2003, astro-ph/0210644). These pairs are selected from a catalog of the 2dF quasar (QSO) survey comprising 22163 quasars. We calculate expected numbers of lensed quasars taking account of the redshift and magnitude distributions of the quasar catalog. Given some of the six pairs are genuine lensed systems, we put interesting constraints on the inner slope of dark halos, Ω_0, and σ_8. We show that the detection of even one lens with separation >30'' is marginally consistent with models that have cuspy inner density profile, ρ\propto r^{-1.5}, and very large σ_8, σ_8>~1.2 for Ω_0=0.3. To reconcile with constraints from X-ray clusters or cosmic shear, much lower Ω_0 and much higher σ_8 are needed, although such high σ_8 seems too extreme. The shallower inner density profile ρ\propto r^{-1} is hardly acceptable. In particular, the expected number of lenses with separation >200'' is too small to explain the discovery of such anomalously wide separation lens systems. These results imply that we miss some important systematic effects, there is a problem in the cold dark matter scenario, or none of these six quasar pairs is likely to be lensed images.

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