arXiv subjects

Yun Wang

Publications and source records attributed to Yun Wang.

At least 379 records · Page 21Linked to original sources

arXiv1999

Analytical Modeling of the Weak Lensing of Standard Candles I. Empirical Fitting of Numerical Simulation Results

Weak lensing leads to the non-Gaussian magnification distribution of standard candles at given redshift $z$, $p(μ|z)$. In this paper, we give accurate and simple empirical fitting formulae of the weak lensing numerical simulation results with the generalized Dyer-Roeder prescription. The smoothness parameter $\tildeα$ essentially represents the amount of matter that can cause magnification of a given source. Since matter distribution in our universe is inhomogeneous, we can think of our universe as a mosaic of cones centered on the observer, each with a different value of $\tildeα$. We define the {\it direction dependent} smoothness parameter $\tildeα$ via the Dyer-Roeder equation; there is a unique mapping between $\tildeα$ and the magnification of a source. We find that the distribution of $\tildeα$ at given $z$, $p(\tildeα|z)$, is well described by a modified Gaussian distribution. For the same matter distribution, i.e., the same $p(\tildeα|z)$, different values of $Ω_m$ and $Ω_Λ$ can lead to very different magnification distributions. Our formulae can be conveniently used to calculate the weak lensing effects for observed Type Ia supernovae at arbitrary redshifts.

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arXiv1999

A Model-Independent Photometric Redshift Estimator

We derive a simple empirical photometric redshift estimator using a training set of galaxies with multiband photometry and measured redshifts in the Hubble Deep Field (HDF). This estimator is model-independent; it does not use spectral templates. The dispersion between the estimated redshifts and the spectroscopically measured ones is small; the dispersions range from $σ_z\simeq 0.03$ to 0.1 for $z\la 2$ galaxies, and from $σ_z\simeq 0.14$ to 0.25 for $z\ga 2$ galaxies. The predictions provided by our empirical redshift estimator agree well with recently measured galaxy redshifts. We illustrate how our empirical redshift estimator can be modified to include flat spectrum galaxies with $1.4\la z \la 2$.

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arXiv1998

Model-Independent Measurement of the Primordial Power Spectrum

In inflationary models with minimal amount of gravity waves, the primordial power spectrum of density fluctuations, $A_S^2(k)$, together with the basic cosmological parameters, completely specify the predictions for the cosmic microwave background (CMB) anisotropy and large scale structure. Here we show how we can strongly constrain both $A_S^2(k)$ and the cosmological parameters by combining the data from the Microwave Anisotropy Probe (MAP) and the galaxy redshift survey from the Sloan Digital Sky Survey (SDSS). We allow $A_S^2(k)$ to be a free function, and thus probe features in the primordial power spectrum on all scales. MAP and SDSS have scale-dependent measurement errors that decrease in opposite directions on astrophysically interesting scales; they complement each other and allow the measurement of the primordial power spectrum independent of inflationary models, giving us valuable information on physics in the early Universe, and providing clues to the correct inflationary model.

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arXiv1998

A Catalog of Color-based Redshift Estimates for z <~ 4 Galaxies in the Hubble Deep Field

We derive simple empirical color-redshift relations for $z\la 4$ galaxies in the Hubble Deep Field (HDF) using a linear function of three photometric colors ($U-B$, $B-V$, $V-I$). The dispersion between the estimated redshifts and the spectroscopically observed ones is small for relations derived in several separate color regimes; the dispersions range from $σ_z\simeq 0.03$ to 0.1 for $z\la 2$ galaxies, and from $σ_z\simeq 0.14$ to 0.25 for $z\ga 2 $ galaxies. We apply the color-redshift relations to the HDF photometric catalog and obtain estimated redshifts that are consistent with those derived from spectral template fitting methods. The advantage of these color-redshift relations is that they are simple and easy to use and do not depend on the assumption of any particular spectral templates; they provide model independent redshift estimates for $z\la 4$ galaxies using only multi-band photometry, and they apply to about 90% of all galaxies. We provide a color-based estimated redshift catalog of HDF galaxies to $z\la 4$. We use the estimated redshifts to investigate the redshift distribution of galaxies in the HDF; we find peaks in the redshift distribution that suggest large-scale clustering of galaxies to at least $z\sim 1$ and that are consistent with those identified in spectroscopic probes of the HDF.

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arXiv1998

Cosmology in the Next Millennium: Combining MAP and SDSS Data to Constrain Inflationary Models

The basic cosmological parameters and the primordial power spectrum together completely specify predictions for the cosmic microwave background radiation anisotropy and large scale structure. Here we show how we can strongly constrain both $A_S^2(k)$ and the cosmological parameters by combining the data from the Microwave Anisotropy Probe (MAP) and the galaxy redshift survey from the Sloan Digital Sky Survey (SDSS). We allow $A_S^2(k)$ to be a free function, and thus probe features in the primordial power spectrum on all scales. The primordial power spectrum in 20 steps in $\log k$ to $k\leq 0.5h$Mpc$^{-1}$ can be determined to $\sim 16%$ accuracy for $k\sim 0.01h$Mpc$^{-1}$, and to $\sim 1%$ accuracy for $k\sim 0.1h$Mpc$^{-1}$. The uncertainty in the primordial power spectrum increases by a factor up to 3 on small scales if we solve simultaneously for the dimensionless Hubble constant $h$, the cosmological constant $Λ$, the baryon fraction $Ω_b$, the reionization optical depth $τ_{ri}$, and the effective bias between the matter density field and the redshift space galaxy density field $b_{\it eff}$. Alternately, if we restrict $A_S^2(k)$ to be a power law, we find that inclusion of the SDSS data breaks the degeneracy between the amplitude of the power spectrum and the optical depth inherent in the MAP data, significantly reduces the uncertainties in the determination of the matter density and the cosmological constant, and allows a determination of the galaxy bias parameter. Thus, combining the MAP and SDSS data allows the independent measurement of important cosmological parameters, and a measurement of the primordial power spectrum independent of inflationary models, giving us valuable information on physics in the early Universe, and providing clues to the correct inflationary model.

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arXiv1997

Implications of Cosmic Microwave Background Anisotropies for Large Scale Variations in Hubble's Constant

Low amplitude (linear regime) cosmic density fluctuations lead to spatial variations in the locally measurable value of $H_0$ (denoted as $H_L$), $δ_H \equiv (H_L-H_0)/H_0$, which are of order 3-6% (95% confidence interval) in a sphere of 200$ h^{-1}$Mpc in diameter, and of order 1-2% in a sphere of 400$ h^{-1}$Mpc in diameter, for three currently viable structure formation models (tilted CDM, $Λ$CDM, and open CDM) as normalized by the 4 year COBE DMR data. However, the true matter distribution power spectrum may differ from any of the currently viable models. For example, it may contain sharp features which have escaped detection so far. The measured CMB dipole velocity (the Galaxy's peculiar velocity with respect to the CMB rest frame) provides additional constraints on the probability distribution of $δ_H$ that supplement our limited knowledge of the power spectrum. For a matter power spectrum which consists of the smooth power spectrum of a viable cosmological model plus a delta-function bump, we find that given the CMB dipole velocity, the 95% CL upper limit of $|δ_H|$ increases approximately by a factor of two, but the probability distribution of $δ_H$ is non-Gaussian, with increased probability at small $δ_H$ compared to Gaussian. Abandoning model power spectra entirely, we find that the observed CMB dipole velocity alone provides a very robust limit, $\sqrt{< δ_H^2 >_R} <10.5 h^{-1}{Mpc}/R$ at 95% CL, in a sphere of radius $R$, for an arbitrary power spectrum. Thus, variations between currently available local measures of $H_0$ and its true global value of a few to several percent are to be expected and differences as large as 10% are possible based on our current knowledge of the CMB anisotropies.

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arXiv1997

Astrophysical Effects of Extreme Gravitational Lensing Events

Every astrophysical object (dark or not) is a gravitational lens, as well as a receiver/observer of the light from sources lensed by other objects in its neighborhood. For a given pair of source and lens, there is a thin on-axis tubelike volume behind the lens in which the radiation flux from the source is greatly increased due to gravitational lensing. Any objects which pass through such a thin tube or beam will experience strong bursts of radiation, i.e., Extreme Gravitational Lensing Events (EGLEs). We have studied the physics and statistics of EGLEs. EGLEs may have interesting astrophysical effects, such as the destruction of dust grains, ignition of masers, etc. Here we illustrate the possible astrophysical effects of EGLEs with one specific example, the destruction of dust grains in globular clusters.

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arXiv1997

Caustics, Critical Curves and Cross Sections for Gravitational Lensing by Disk Galaxies

We study strong gravitational lensing by spiral galaxies, modeling them as infinitely thin uniform disks embedded in singular isothermal spheres. We derive general properties of the critical curves and caustics analytically. The multiple-image cross section is a sensitive function of the inclination angle of the disk relative to the observer. We compute the inclination averaged cross section for several sets of lensing parameters. For realistic disk mass and size parameters, we find that the cross section for multiple imaging is increased by only a modest factor and no dramatic increase in the optical depth for strong lensing of QSOs would be expected. However, the cross section for high magnifications is significantly increased due to the inclusion of a disk, especially for nearly edge-on configurations; due to the strong observational selection effects favoring high magnifications, there might be significant consequences for lensing statistics.

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arXiv1997

The Cosmological Constant and Advanced Gravitational Wave Detectors

Interferometric gravitational wave detectors could measure the frequency sweep of a binary inspiral [characterized by its chirp mass] to high accuracy. The observed chirp mass is the intrinsic chirp mass of the binary source multiplied by $(1+z)$, where $z$ is the redshift of the source. Assuming a non-zero cosmological constant, we compute the expected redshift distribution of observed events for an advanced LIGO detector. We find that the redshift distribution has a robust and sizable dependence on the cosmological constant; the data from advanced LIGO detectors could provide an independent measurement of the cosmological constant.

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arXiv1997

A Robust Determination of the Time Delay in 0957+561A,B and a Measurement of the Global Value of Hubble's Constant

Photometric monitoring of the gravitational lens system 0957+561A,B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g band event in the trailing (B) image light curve at the precise time predicted from the observation of an event during 1995 in the leading (A) image with a delay of 415 days. This success confirms the "short delay," and the lack of any feature at a delay near 540 days rejects the "long delay" for this system, resolving a long-standing controversy. A series of statistical analyses of our light curve data yield a best fit delay of 417 +/- 3 days (95% confidence interval). Recent improvements in the modeling of the lens system (consisting of a galaxy and cluster) allow us to derive a value of the global (at z = 0.36) value of Hubble's constant H_0 using Refsdal's method, a simple and direct distance determination based on securely understood physics and geometry. The result is H_0 = 63 +/- 12 km/s/Mpc (for Omega = 1) where this 95% confidence interval is dominated by remaining lens model uncertainties.

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arXiv1996

The secondary infall model of galactic halo formation and the spectrum of cold dark matter particles on Earth

The spectrum of cold dark matter particles on Earth is expected to have peaks in velocity space associated with particles which are falling onto the Galaxy for the first time and with particles which have fallen in and out of the Galaxy only a small number of times in the past. We obtain estimates for the velocity magnitudes and the local densities of the particles in these peaks. To this end we use the secondary infall model of galactic halo formation which we have generalized to take account of the angular momentum of the dark matter particles. The new model is still spherically symmetric and it admits self-similar solutions. In the absence of angular momentum, the model produces flat rotation curves for a large range of values of a parameter $ε$ which is related to the spectrum of primordial density perturbations. We find that the presence of angular momentum produces an effective core radius, i.e. it makes the contribution of the halo to the rotation curve go to zero at zero radius. The model provides a detailed description of the large scale properties of galactic halos including their density profiles, their extent and total mass. We obtain predictions for the kinetic energies of the particles in the velocity peaks and estimates for their local densities as functions of the amount of angular momentum, the age of the universe and $ε$.

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arXiv1996

Statistics of Extreme Gravitational Lensing Events. II.The Small Shear Case

We consider an astrophysical system with a population of sources and a population of lenses. For each pair of source and lens, there is a thin on-axis tube-like volume behind the lens in which the radiation flux from the source is greatly increased due to gravitational lensing. Any objects (such as dust grains) which pass through such a thin tube will experience strong bursts of radiation, i.e., Extreme Gravitational Lensing Events (EGLE). We study the physics and statistics of EGLE for the case in which the shear is larger or comparable to the finite source size. We find that the presence of shear has only a small effect on the EGLE statistics.

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arXiv1996

Statistics of Extreme Gravitational Lensing Events. I.The Zero Shear Case

For a given source and lens pair, there is a thin on-axis tube-like volume behind the lens in which the radiation flux from the source is greatly increased due to gravitational lensing. Any objects (such as dust grains) which pass through such a thin tube will experience strong bursts of radiation, i.e., Extreme Gravitational Lensing Events (EGLE). We study the physics and statistics of EGLE for the case in which finite source size is more important than shear. One of the several possible significant astrophysical effects is investigated with an illustrative calculation.

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arXiv1996

Gravitational Lensing of Gravitational Waves from Merging Neutron Star Binaries

We discuss the gravitational lensing of gravitational waves from merging neutron star binaries, in the context of advanced LIGO type gravitational wave detectors. We consider properties of the expected observational data with cut on the signal-to-noise ratio $ρ$, i.e., $ρ>ρ_0$. An advanced LIGO should see unlensed inspiral events with a redshift distribution with cut-off at a redshift $z_{\rm max} < 1$ for $h \leq 0.8$. Any inspiral events detected at $z>z_{\rm max}$ should be lensed. We compute the expected total number of events which are present due to gravitational lensing and their redshift distribution for an advanced LIGO in a flat Universe. If the matter fraction in compact lenses is close to 10\%, an advanced LIGO should see a few strongly lensed events per year with $ρ>5$.

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arXiv1995

A Fast and Accurate Algorithm for Computing Tensor CBR Anisotropy

Inflation gives rise to a nearly scale-invariant spectrum of tensor perturbations (gravitational waves), their contribution to the Cosmic Background Radiation (CBR) anisotropy depends upon the present cosmological parameters as well as inflationary parameters. The analysis of a sampling-variance-limited CBR map offers the most promising means of detecting tensor perturbations, but will require evaluation of the predicted multipole spectrum for a very large number of cosmological parameter sets. We present accurate polynomial formulae for computing the predicted variance of the multipole moments in terms of the cosmological parameters $Ω_Λ$, $Ω_0h^2$, $Ω_B h^2$, $N_ν$, and the power-law index $n_T$ which are accurate to about 1\% for $l\le 50$ and to better than 3\% for $50< l \le 100$ (as compared to the numerical results of a Boltzmann code).

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arXiv1995

The Statistics of Nearly On-Axis Gravitational Lensing Events

A small volume of space, nearly on-axis behind a gravitational lens with respect to a given source, will receive a greatly increased radiation flux. In the idealized case of a point mass lens acting on a point source in complete isolation, the volume will approach zero only as the flux tends to infinity; in fact, the volume weighted rms flux is divergent. In realistic cases, finite source size and the effects of other gravitational deflections (i.e., non-zero shear) limit the maximum flux and considerably complicate the physics, but very large fluxes are still produced in small volumes. We consider the physics and statistics of these Extreme Gravitational Lensing Events (EGLE) and present an initial examination of their possible astrophysical effects for various known and putative populations of lensing objects and sources, with particular attention to the case in which finite source size is important but shear is not.

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arXiv1995

Simple Analytical Methods for Computing the Gravity-Wave Contribution to the Cosmic Background Radiation Anisotropy

We present two simple analytical methods for computing the gravity wave contribution to the cosmic background radiation (CBR) anisotropy in inflationary models; one method uses a time-dependent transfer function, the other method uses an approximate gravity-wave mode function which is a simple combination of the lowest order spherical Bessel functions. We compare the CBR anisotropy tensor multipole spectrum computed using our methods with the previous result of the highly accurate numerical method, the ``Boltzmann'' method. Our time-dependent transfer function is more accurate than the time-independent transfer function found by Turner, White, and Lidsey; however, we find that the transfer function method is only good for $l \la 120$. Using our approximate gravity-wave mode function, we obtain much better accuracy; the tensor multipole spectrum we find differs by less than 2\% for $l \la 50$, less than 10\% for $l \la 120$, and less than 20\% for $l \leq 300$ from the ``Boltzmann'' result. Our approximate graviton mode function should be quite useful in studying tensor perturbations from inflationary models.

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arXiv1995

The velocity peaks in the cold dark matter spectrum on Earth

The cold dark matter spectrum on earth is expected to have peaks in velocity space. We obtain estimates for the sizes and locations of these peaks. To this end we have generalized the secondary infall model of galactic halo formation to include angular momentum of the dark matter particles. This new model is still spherically symmetric and it has self-similar solutions. Our results are relevant to direct dark matter search experiments.

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