arXiv subjects

Peter Schneider

Publications and source records attributed to Peter Schneider.

203 records · Page 12Linked to original sources

arXiv1994

The Density of 1-Jansky Sources Around Zwicky Clusters

In a recent paper by Rodrigues-Williams \& Hogan (RH94), a correlation between high-redshift, optically-selected QSOs and Zwicky clusters was reported at a very high significance level. Due to the fairly bright flux threshold of the cluster sample, these correlations cannot be interpreted as being due to an environmental effect of the clusters on the quasar activity. The most likely interpretation employed in RH94 was the effect of gravitational lensing by the foreground clusters, though the required magnification to explain the observed correlations has to be considerably higher than obtained from simple mass models for the clusters. We have repeated the analysis of RH94 using a different QSO sample, namely radio quasars and radio galaxies from the 1-Jy sample. In accordance with RH, we detect a statistically significant correlation between Zwicky clusters and 1-Jy sources with intermediate redshift ($z\approx 1$), but fail to detect significant effects for higher-redshift sources. In addition, we detect a highly significant underdensity of low-redshift radio sources around Zwicky clusters, for which an environmental interpretation seems to be most reasonable. Our result for the overdensity of $z\sim 1$ sources is in good agreement with previous results and can possibly be interpreted as a lensing effect, though we have not tried to quantitatively investigate a lensing model. Other interpretations, such as patchy dust obscuration, can not explain the observed effect and its tendency with redshift.

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arXiv1994

Steps towards Nonlinear Cluster Inversion Through Gravitational Distortions. II. Generalization of the Kaiser & Squires Method

The weak distortions of high-redshift galaxies caused by gravitational light deflection near clusters of galaxies can be used to reconstruct the projected (two-dimensional) surface mass density of intermediate redshift clusters. This technique, pioneered by Tyson, Valdes & Wenk, and Kaiser & Squires, is reconsidered in the present paper, where we generalize the inversion equation found by Kaiser & Squires (KS) in several respect. Adopting a different smoothing procedure for the discreetly sampled data (individual galaxy images), we effectively reduce the shot noise in the KS procedure. In particular we show that the best density reconstructions are obtained if the smoothing scale is adopted to the `strength of the signal', which yields a better resolution near the center of the cluster where the distortions are strongest. Furthermore, we point out the importance of boundary effects and demonstrate their disastrous impact on rectangular data fields (CCDs) with large side ratio. Most important, however, is the generalization of the KS method to critical clusters, i.e., to such clusters which are capable of producing multiple images and giant luminous arcs. The corresponding modifications of the inversion procedure are severe; in particular, the resulting inversion equation is much more difficult to solve. As we pointed out in a previous paper (Schneider & Seitz), there exists a local degeneracy if the cluster is critical. We have developed an iteration procedure to solve the inversion equation, which we demonstrate to yield a very accurate

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arXiv1994

Steps towards Nonlinear Cluster Inversion Through Gravitational Distortions. I. Basic Considerations and Circular Clusters

The distortion of images of faint, high-redshift galaxies by light deflection at foreground clusters of galaxies can be used to determine the (projected) mass distribution of the clusters. In the case of strong distortions, which lead to arcs in clusters, the position of the arc and/or its radius of curvature yields an estimate for the total mass inside a circle traced out by the arc. Weak distortions, which can be observed to much larger angular separations, can be used to determine the mass profile. In the case of weak distortions, an approximation which identifies the observed distortion with the shear produced by the lens can be made; this (linear) approximation breaks down, however, if one wants to probe the center of the cluster, i.e., approach the region within which giant arcs can be formed. The methods developed hitherto, the most advanced of which is due to Kaiser \& Squires, rely on the linear approximation and thus cannot yield reliable results for regions close to the center of the cluster. The purpose of this paper is to provide a theoretical basis to extend the Kaiser \& Squires method into the nonlinear regime, thereby making it more powerful, since the nonlinear distortions provide strong constraints on the mass profile, which in combination with the weak distortions should yield more reliable cluster inversions. We discuss the statistical properties of the observable image ellipticities and provide several methods to determine the local distortion by the lens from observed galaxy images, some of which rely

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arXiv1994

Diffuse X-Ray Emission Around High-Redshift, Radio-Loud Qsos

We announce the detection of correlations on angular scales of $\ga10'$ between optically bright, high-redshift, radio-loud QSOs with diffuse X-ray emission seen by ROSAT in the {\it All-Sky Survey}. These correlations reach significance levels of up to $99.8\%$. A comparison of the results with a sample of control fields, bootstrapping analyses, and Kolmogorov-Smirnov tests provide unambiguous support for the statistical significance of the correlations found. We argue that the detected enhanced diffuse X-ray emission is in the foreground of the QSOs, and that it is probably due to galaxy clusters which magnify the QSOs by their gravitational lensing effect, thereby giving rise to a magnification bias in the background source sample. A comparison of the results presented below with correlations previously found between the same QSO sample and either Lick or IRAS galaxies provides further evidence for this interpretation, and identifies positions in the sky where weak gravitational lensing may be detected by searching for coherent distortions of background galaxy images.

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arXiv1994

Light Propagation in Arbitrary Spacetimes and the Gravitational Lens Approximation

We reformulate the transport equation which determines the size, shape and orientation of infinitesimal light beams in arbitrary spacetimes. The behaviour of such light beams near vertices and conjugate points is investigated, with special attention to the singular behaviour of the optical scalars. We then specialize the general transport equation to the case of an approximate metric of an inhomogeneous universe, which is a Friedmann metric `on average' with superposed isolated weak matter inhomogeneities. In a series of well-defined approximations, the equations of gravitational lens theory are derived. Finally, we derive a relative optical focusing equation which describes the focusing of light beams relative to the case that the beam is unaffected by matter inhomogeneities in the universe, from which it follows immediately that no beam can be focused less than one which is unaffected by matter clumps, before it propagates through its first conjugate point.

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