arXiv subjects

Xiang-Ping Wu

Publications and source records attributed to Xiang-Ping Wu.

84 records · Page 5Linked to original sources

arXiv1996

Environmental Effect on the Associations of Background Quasars with Foreground Objects: I. Analytic Investigation

The associations of the angular positions of background quasars with foreground galaxies, clusters of galaxies and quasars are often attributed to the statistical lensing by gravitational potentials of the matter along the lines of sight, although it has been known that none of the individual objects (galaxies, clusters or quasars) are able to fully explain the reported amplitudes of the quasar number enhancements. This probably arises from the fact that the gravitational lensing effect by the environmental matter surrounding these objects has been ignored. In this paper we conduct an extensive study of the influence of the environmental matter on the prediction of quasar enhancement factor by employing the spatial two-point correlation function. Assuming a singular isothermal sphere for mass density profile in galaxy and cluster of galaxies, we estimate the average surface mass density $\overlineΣ$ around galaxies, clusters and quasars from the galaxy-galaxy, cluster-cluster, cluster-galaxy and quasar-galaxy correlations. Our results show that the $\overlineΣ$ induced quasar number enhancement in the scenario of gravitational magnification depends critically on the mass density parameters of galaxies ($Ω_g$) and clusters of galaxies ($Ω_c$) in the universe. For a flat cosmological model of $Ω_0=1$ the environmental matter can indeed play an important role in the lensing origin of the quasar-quasar and quasar-galaxy associations if $Ω_g\simΩ_c\simΩ_0$, while it is unlikely that $\overlineΣ$ is sufficient to account for the reported quasar overdensity behind quasars/galaxies if galaxies and clusters of galaxies contribute no more than $25\%$ to the matter of the universe. Nonetheless, the

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arXiv1996

On the Possible Variations of the Hubble Constant with Distance

Current measurements of the Hubble constant $H_0$ on scale less than $\sim100$ Mpc appear to be controversial, while the observations made at high redshift seem to provide a relatively low value. On the other hand, the Hubble expansion is driven by the matter content of the universe. The dynamical analysis on scale of a few $\sim10$ Mpc indicates that the matter density $Ω_0$ is only $\sim0.2$--$0.3$, which is significantly smaller than $Ω_0=1$ predicted in the standard inflation model. This might support the tendency of a decreasing Hubble constant towards distance. In this paper, we discuss the influence of a possible variant Hubble constant on two fundamental relations in astronomy: the magnitude-redshift ($m$--$z$) and the number-magnitude relations. Using a distant type Ia supernova at $z=0.458$, we show that the deceleration parameter $q_0$ or $Ω_0$ cannot be determined from the $m$--$z$ relation at moderate/high redshift unless the variation of the Hubble constant is {\it a priori} measured. It is further demonstrated that the number density of distant sources would be underestimated when their local calibration is employed, which may partially account for the number excess of the faint blue galaxies observed at moderate/high redshift.

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arXiv1996

Quasar-Cluster Associations and Gravitational Lensing by Large-Scale Matter Clumps

Motivated by the significant overdensity of background bright quasars recently detected behind the foreground clusters of galaxies on scale of $10$ arcminutes, we have investigated the possibility of attributing the quasar-cluster associations to gravitational lensing by large-scale matter inhomogeneities. Based on the conventional lensing models, we have shown that the reported quasar overdensity is unlikely to be generated by cluster matter alone. The situation does not change even if all the clusters of galaxies which follow their spatial two-point correlation function are taken into account, while matter clumps on scale of $>20$ Mpc are also found to be unable to provide the required mass surface density since their density contrast is strictly limited by the anisotropy measurements of the cosmic background radiation. Moreover, we have pointed out that the influence of a nonzero cosmological constant on the quasar-cluster associations is very minor. We conclude that either the observed quasar number counts have been seriously contaminated by the magnification bias of matter inhomogeneities of the universe or there should exists some intercluster matter on scale of less than $\sim20$ Mpc, e.g. from cluster-galaxy correlation, whose mean cosmic density is about an order of magnitude higher than that of clusters of galaxies.

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arXiv1995

Gravitational Lensing in the Universe

This work reviews the basic theoretical aspects, the main observational evidences and the recent applications of gravitational lensing in the Universe. The article is aimed particularly at providing the readers who don't work on gravitational lensing a relatively easy introduction to this active research field in today's astrophysics.

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arXiv1995

Cosmological Constant and Statistical Lensing of Giant Arcs

Using a singular isothermal sphere model for the matter distribution of foreground clusters of galaxies, we study the statistics of giant arcs in flat cosmologies with and without a cosmological constant. We find that the relative number of arcs predicted within $z=1$ in a universe with $Ω_0=0.3$ and $λ_0 \equiv Λ/(3H_0^2)=0.7$ is a factor of $\sim2$ larger than the one in the Einstein-de Sitter universe ($Ω_0=1, λ_0=0$). For a luminosity-dependent evolution model of the number density of background galaxies that accounts for the over-density of faint blue galaxies at $z_s\approx0.4$, the Einstein- de Sitter cosmological model predicts that about $5\%$ of clusters of galaxies with an X-ray luminosity $L_x > 2 \times 10^{44}$ erg s$^{-1}$ should have giant arcs with length-to-width ratio larger than 10. This is a factor of $\sim4$ lower than the observed fraction in the gravitational lensing survey of distant X-ray selected EMSS clusters of galaxies, indicating that the matter distribution of clusters of galaxies deviates significantly from simple isothermal spheres or/and the presence of a significant cosmological constant. It is profitable to further study the constraint on the cosmological constant from giant arcs using more realistic cluster models.

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arXiv1994

On the measurement of the Hubble constant in a local low-density universe

Astrophysical observations indicate that the ``Local Universe" has a relatively lower matter density ($Ω_0$) than the predictions of the standard inflation cosmology and the large-scale motions of galaxies which provide a mean mass density to be very close to unity. In such a local underdense region the Hubble expansion may not be representative of the global behaviour. Utilizing an underdense sphere embedded in a flat universe as the model of our ``Local Universe", we show that the local Hubble constant would be 1.2 -- 1.4 times larger than the global value on scale of $\sim80$ Mpc, depending on the variation of $Ω_0$. This may account for the recent measurements of the unpleasantly large Hubble constant of $\sim$80 km/s/Mpc using the Cepheid variables in the Virgo cluster and the relative distance between Virgo and Coma cluster and removes the resulted apparent paradox of the age of our universe.

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arXiv1994

A search for the associations of distant radio-bright quasars with Abell clusters: an effect of gravitational amplification bias ?

The theory predicts that the effect of gravitational lensing by the matter associated with clusters of galaxies can magnify background sources, leading to an enhancement of source number density around foreground clusters of galaxies. We conduct a search for the associations of distant radio-bright quasars with Abell clusters using the 1-Jy and 2-Jy all-sky catalogs. Statistics turns to be very poor on the basis of the 1-Jy sample, which shows no correlations between the distant radio quasars with the foreground Abell clusters above $1σ$ level. However, an apparent association ($>1σ$) of the 2-Jy radio sources with the foreground Abell clusters has been detected on scale of $\sim20$ arcminutes. We point out that this enhancement is unlikely to be produced by the statistical gravitational lensing hypothesis utilizing the matter associated with a population of isolated clusters unless (1)their velocity dispersion is a few times larger than the presently adopted value and/or (2)the intrinsic counts of the radio-bright sources with flux have a steeper slope than the presently observed ones. This indicates that (1)the observed associations, if real, can be the integrated result of all the matter along the line of sight to the distant quasars, namely, the weak lensing effect of clusters of galaxies that trace large-scale structures of the universe, and/or (2)the number counts of the radio-bright sources have been seriously contaminated by lensing.

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arXiv1994

Determining Gravitational Masses of Galaxy Clusters With (1)Virial Equilibrium And (2)Arc-like Images

The mass derived from gravitational lensing reflects the total mass contained in the lensing system, independent of the specific matter contents and states. A comparison of the dynamical masses from hydrostatic equilibrium with the gravitational masses from arc-like images of background galaxies is made for four clusters of galaxies at intermediate redshits. It is found that virial analysis has underestimated the total cluster masses (from lensing) by a factor of $3\sim6$ within a radius of $\sim0.3$ Mpc $h_{50}^{-1}$ around the cluster centers, indicating that clusters of galaxies might not be regarded as the well relaxed virialized systems. The increase of the total cluster masses obtained from lensing leads to the decrease of the baryon fractions of clusters of galaxies, which provides a crue for solving the ``$Ω_0$ disprepancy puzzle" in cosmology.

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arXiv1993

A further discussion on quasar-galaxy associations from gravitational lensing

Quasar-galaxy associations, if they result from the effect of gravitational lensing by foreground galaxies, depend sensitively on the shape of the quasar number counts. Two kinds of quasar number-magnitude relations are predicted to produce quite different properties in quasar-galaxy associations: the counts of Boyle, Shanks and Peterson (1988; BSP) provide both positive and ``negative" associations between distant quasars and foreground galaxies, relating closely with the knee ($B\approx19.15$) in these counts. However, Hawkins and Véron (1993; HV) quasar data lead to only a positive magnitude-independent quasar-galaxy association. The current observational evidence on quasar-galaxy associations, either positive or null, is shown to be the natural result of gravitational lensing if quasars follow the BSP number-magnitude relation. On the other hand, the HV counts are unable to produce the reported associations by the mechanism of gravitational lensing. It is emphasized that special attention should be paid to the limiting magnitudes in the selected quasar samples when one works on quasar-galaxy associations.

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arXiv1993

Galaxy-cluster associations from gravitational lensing

We investigate the associations between background galaxies and foreground clusters of galaxies due to the effect of gravitational lensing by clusters of galaxies. Similar to the well-known quasar-galaxy ones, these associations depend sensitively on the shape of galaxy number-magnitude or number-flux relation, and both positive and ``negative" associations are found to be possible, depending on the limiting magnitude and/or the flux threshold in the surveys. We calculate the enhancement factors assuming a singular isothermal sphere model for clusters of galaxies and a pointlike model for background sources selected in three different wavelengths, $B$, $K$ and radio. Our results show that $K-$ selected galaxies might constitute the best sample to test the ``negative" associations while it is unlikely that one can actually observe any association for blue galaxies. We also point out that bright radio sources ($S>1$ Jy) can provide strong positive associations, which may have been already detected in 3CR sample.

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arXiv1993

Anisotropies of Cosmic Background Radiation from a Local Collapse

We present an exact solution of the anisotropies of cosmic background radiation (CBR) from a local collapse described by a spherical over-dense region embedded in a flat universe, with the emphasis on the relationship between the dipole $(Δ{\sf T}/{\sf T})_d$ and the quadrupole $(Δ{\sf T}/{\sf T})_q$ anisotropy. This result has been used to examine the kinematic quadrupole correction $(Δ{\sf T}/{\sf T})_q=(Δ{\sf T}/{\sf T})_d^2/2$, which is usually applied to remove the contamination of the quadrupole produced by local density inhomogeneities when finding the cosmic amplitude of the quadrupole at the surface of last scattering. We have found that the quadrupole of local collapse origin cannot always be approximately described by the kinematic quadrupole. Our numerical result shows that the difference between the kinematic and local quadrupoles depends on the size and matter density in the peculiar field, and the position of the observer. For a given dipole, the local quadrupole can be different from the kinematic quadrupole by a factor as large as 3. Therefore, the kinematic quadrupole correction remains an uncertain factor in the determination of the amplitude of a cosmic quadrupole. Nevertheless, a preliminary analysis shows that this uncertainty might not dramatically change the cosmological origin of the COBE-DMR's quadrupole, unless a huge peculiar gravitational field is assumed.

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arXiv1993

Gravitational Microlensing by the MACHOs of the LMC

The expected microlensing events of the LMC by the MACHOs of the LMC itself are calculated and compared with analogue events by objects in the Galactic halo. The LMC matter distribution is modelled by a spherical halo and an exponential disk while a face-on exponential disk is used for the stellar distribution of the LMC. Among the microlensing events discovered by the MACHOs and EROS projects, a fraction of $22\%$ could be caused by the lenses near the center of the LMC or $13\%$ from lenses at $5^o$ from the LMC center. Therefore, any statistical study of these microlensing events must take the LMC lenses into account.

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