arXiv subjects

Masamune Oguri

Publications and source records attributed to Masamune Oguri.

348 records · Page 20Linked to original sources

arXiv2002

Gravitational lens time delays for distant supernovae: break the degeneracy between radial mass profiles and the Hubble constant

An attempt to measure the Hubble constant with gravitational lens time delays is often limited by the strong degeneracy between radial mass profiles of lens galaxies and the Hubble constant. We show that strong gravitational lensing of type Ia supernovae breaks this degeneracy; the standard candle nature of type Ia supernova luminosity function allows one to measure the magnification factor directly, and this information is essential to constrain radial mass profiles and the Hubble constant separately. Our numerical simulation demonstrates that the Hubble constant can be determined with \sim 5% accuracy from only several lens events if magnification factors are used as constraints. Therefore, distant supernova survey is a promising way to measure the global Hubble constant independently with the local estimates.

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arXiv2002

Gravitational Lensing Magnification and Time Delay Statistics for Distant Supernovae

Strong gravitational lensing of distant supernovae (SNe), particularly Type Ia's, has some exploitable properties not available when other sorts of cosmologically distant sources are lensed. One such property is that the ``standard candle'' nature of the SN at peak brightness allows a direct determination of the lensing magnification factor for each well observed image. Another is that the duration of an SN event is of the same order as the differential time delays between the various lens images. The comparable time scales of the photometric event and the time delay invite a variety of applications, including high precision measurements of the delay and the targeting of especially interesting phases of the explosion for intensive observation when they appear in trailing images. As an initial exploration of these possibilities we present calculations of SN lensing statistics in a ``concordance cosmology''. We emphasize magnification and time delay effects. Plausible SN surveys, such as the proposed SNAP space mission, would discover several to some tens of strongly lensed SNe Ia per year, and at least a few of these will be at redshifts well beyond those that would be accessible via unlensed events. The total number of such anomalously high redshift SNe Ia will be a useful test of high redshift star formation models. SN surveys of finite duration will, of course, miss the appearance of some images; we quantify this selection bias. Finally, we investigate how well the appearance of trailing images can be predicted based on various amounts of available information on the lensing event. Knowledge of the magnification factor for the leading image makes it possible to predict the appearance of a trailing image relatively accurately if the lens redshift is also known. (Abridged)

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arXiv2002

Constraints on the Baryonic Compression and Implications for the Fraction of Dark Halo Lenses

We predict the fraction of dark halo lenses, that is, the fraction of lens systems produced by the gravitational potential of dark halos, on the basis of a simple parametric model of baryonic compression. The fraction of dark halo lenses primarily contains information on the effect of baryonic compression and the density profile of dark halos, and is expected to be insensitive to cosmological parameters and source population. The model we adopt comprises the galaxy formation probability p_g(M) which describes the global efficiency of baryonic compression and the ratio of circular velocities of galaxies to virial velocities of dark halos gamma_v=v_c/v_{vir} which means how the inner structure of dark halos is modified due to baryonic compression. The model parameters are constrained from the velocity function of galaxies and the distribution of image separations in gravitational lensing, although the degeneracy between model parameters still remains. We show that the fraction of dark halo lenses depends strongly on gamma_v and the density profile of dark halos such as inner slope alpha. This means that the observation of the fraction of dark halos can break the degeneracy between model parameters if the density profile of dark halo lenses is fully settled. On the other hand, by restricting gamma_v to physically plausible range we can predict the lower limit of the fraction of dark halo lenses on the basis of our model. Our result indicates that steeper inner cusps of dark halos (alpha >~ 1.5) or too centrally concentrated dark halos are inconsistent with the lack of dark halo lenses in observations.

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arXiv2002

Systematic Effects on Tangential and Radial Arc Statistics: The Finite Source Size and Ellipticities of the Lens and Source

It has been recognized that the arc statistics of gravitational lensing is a useful probe of the density profile of clusters of galaxies. We examine several systematic effects which are important in predicting the number of arcs, with particular attention to the difference between tangential and radial arcs. First we derive an analytic expression of the cross section for radial arcs taking account of the source size and find that the moderate source size enhances the cross section for radial arcs while larger source size (\gtrsim 1'' in our example) suppresses the number of radial arcs. On the other hand, tangential arcs are much less sensitive to the source size. Next we numerically calculate the cross section for arcs considering the lens and source ellipticities. We find that the numbers of both tangential and radial arcs are highly enhanced by both ellipticities, by one or two orders of magnitude. The number ratio of radial to tangential arcs is, however, not so affected if the threshold axis ratio of arcs is large (\gtrsim 7). The number ratio therefore still remains good statistics which probe the density profile of the lens objects, if the source size effect is correctly taken into account.

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arXiv2001

Strong Gravitational Lensing Time Delay Statistics and the Density Profile of Dark Halos

The distribution of differential time delays Δt between images produced by strong gravitational lensing contains information on the mass distributions in the lensing objects as well as on cosmological parameters such as H_0. We derive an explicit expression for the conditional probability distribution function of time delays P(Δt|θ), given an image separation between multiple images θ, and related statistics. We consider lensing halos described by the singular isothermal sphere (SIS) approximation and by its generalization as proposed by Navarro, Frenk, & White (NFW) which has a density profile ρ\propto r^{-α} in the innermost region. The time delay distribution is very sensitive to these profiles; steeper inner slopes tend to produce larger time delays. For example, if H_0=70 km/s/Mpc, a Λ-dominated cosmology and a source redshift z_S=1.27 are assumed, lenses with θ=5'' produce a time delay of Δt[yr]=1.5^{+1.7}_{-0.9}, 0.39^{+0.37}_{-0.22}, 0.15^{+0.11}_{-0.09}, and 0.071^{+0.054}_{-0.038} (50% confidence interval), for SIS, generalized NFW with α=1.5, α=1.0, and α=0.5, respectively. At a fixed image separation, the time delay is determined by the difference in the lensing potential between the position of the two images, which typically occur at different impact parameters. Although the values of Δt are proportional to the inverse of H_0, P(Δt|θ) is rather insensitive to all other cosmological model parameters, source redshifts, magnification biases and so on. A knowledge of P(Δt|θ) will also be useful in designing the observing program of future large scale synoptic variability surveys and for evaluating possible selection biases operating against large splitting lens systems.

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arXiv2001

Probing the Core Structure of Dark Halos with Tangential and Radial Arc Statistics

We study the arc statistics of gravitational lensing generated by dark matter halos in order to probe their density profile. We characterize the halo profile by two parameters, the inner slope of the central cusp $α$, and the median amplitude of the concentration parameter, $c_{\rm norm}$, for a halo of mass $10^{14}h^{-1}M_\odot$ at $z=0$, and compute the numbers of tangential and radial arcs produced by gravitational lensing of galaxy clusters. We find that the number of arcs divided by the number of halos is a good statistic which is sensitive to both $c_{\rm norm}$ and $α$ with very weak dependence on the cosmological parameters. If the arc samples with well-defined selection criteria for the clusters become available, one can strongly constrain both $c_{\rm norm}$ and $α$. While our tentative comparison with the existing observational data indicates that the inner density profile of dark halos is indeed as steep as predicted by recent simulations ($α\sim 1.5$), the homogeneous samples of tangential and radial arcs are required for more quantitative discussions.

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