arXiv subjects

Tom Broadhurst

Publications and source records attributed to Tom Broadhurst.

185 records · Page 11Linked to original sources

arXiv1995

Gravitational `Convergence' and Cluster Masses

Two colour photometry of the cluster A1689 reveals a `relative magnification-bias' between lensed blue and red background galaxies, arising from a dependence of the faint galaxy count-slope on colour. The colour distribution is skewed blueward of the far field, allowing us to measure the cluster magnification and to understand the notorious blueness of large arcs. We show that the magnification information can be combined with the usual image distortion measurements to isolate the local `convergence' component of lensing and hence derive the projected mass. This is achieved through a simple local relation between the convergence and the observables, which can be applied generally over the surface a cluster. In the weak lensing limit, the convergence reduces to a dependence on the magnification alone, so that in the outskirts of clusters the surface-density of matter is obtained directly from the surface-density of background galaxies. Hence, useful lensing work requires colour information but not necessarily good seeing. Interestingly, convergence varies slowly at high redshift, saturating at a level depending on the Horizon distance, allowing a useful model-independent measurement of the Global Geometry.

astro-ph↗

arXiv1995

Mass Distributions of Clusters Using Gravitational Magnification

Lensing in the context of rich clusters is normally quantified from small image distortions, yielding a relative mass distribution in the limit of weak lensing. Here we show the magnification effect of lensing can also be mapped over a cluster, resulting in absolute mass determinations for the weak limit. Furthermore, given both magnification and distortion measurements, the mass distribution may be constrained in the strong regime. Methods for obtaining the magnification using spectroscopic and/or photometric information are discussed, for object detection within a fixed isophote or to a given flux limit. A map of the magnification around A1689 is constructed from the observed depletion of background red galaxy counts.

astro-ph↗

arXiv1995

A Gravitational Lens Solution for IRAS F10214+4724

We show that the high redshift IRAS source F10214 is highly magnified by the gravitational field of an intervening elliptical galaxy, accounting for its many anomalous properties. Detailed radio and near-IR images identify the IRAS source with a symmetric arc, centered on a red object, or lensing galaxy. To explain the observed structures, the center of the source must much more highly magnified than its outer regions. Lensing predicts a small counterimage to the arc, which we find adjacent to the lensing galaxy. A red component in the observed spectrum suggests a lens redshift of unity, and the lens model yields a mass estimate of $M(r<3kpc)\approx10^{11}{M_solar}$, consistent with an ordinary elliptical galaxy. We present new high-resolution optical images which show a thin arc of emission, implying an intrinsically small source ($<0.5kpc$) which is highly magnified ($\sim20\times$). Since the optical is strongly polarized with a Seyfert~II spectrum, we propose that the optical arc is magnified image of the inner region of an obscured AGN. The obscuring ``torus'' will be similarly magnified, naturally accounting for the large IR flux. We show that finding objects like F10214+4724 in redshift surveys is probable, given the level of magnification bias expected for compact luminous IRAS sources. Such cases represent the obscured AGN counterparts to the lensed QSO population and, because of their extended sizes, are useful in determining the mass distribution in the lensing galaxies.

astro-ph↗

arXiv1994

Recent Developments in Weak Lensing

Measurement of the gravitational distortion of images of distant galaxies is rapidly becoming established as a powerful probe of the dark mass distribution in clusters of galaxies. With the advent of large mosaics of CCD's these methods should provide a composite total mass profile for galaxy haloes and also measure the power spectrum of mass fluctuations on supercluster scales. We describe how HST observations have been used to place the observational measurement of the shear on a quantitative footing. By artifically stretching and then degrading WFPC2 images to simulate ground based observing it is now possible to directly calibrate the effect of atmospheric seeing. Similar experiments show that one can remove the effect of artificial image anisotropy arising in the atmosphere or telescope. There have also been important advances in the interpretation of the shear: reconstruction techniques have been extended to encompass the strong distortion regime of giant arcs etc., progress has been made in removing a bias present in earlier reconstruction techniques, and we describe new techniques for `aperture densitometry'. We present some new results on clusters of galaxies, and discuss the intimate connections between weak lensing and deep spectroscopy.

astro-ph↗

arXiv1994

A Method for Weak Lensing Observations

We develop and test a method for measuring the gravitational lensing induced distortion of faint background galaxies. We first describe how we locate the galaxies and measure a 2-component `polarisation' or ellipticity statistic $e_α$ whose expectation value should be proportional to the gravitational shear $γ_α$. We then show that an anisotropic instrumental psf perturbs the polarisation by $δe_α= P^s_{αβ} p_β$, where $p_α$ is a measure of the psf anisotropy and $P^s_{αβ}$ is the `linearised smear polarisability tensor'. By estimating $P^s_{αβ}$ for each object we can determine $p_α$ from the foreground stars and apply a correction $-P^s_{αβ}p_β$ to the galaxies. We test this procedure using deep high-resolution images from HST which are smeared with an anisotropic psf and then have noise added to simulate ground-based observations. We find that the procedure works very well. A similar analysis yields a linear shear polarisability tensor $P^γ_{αβ}$ which describes the response to a gravitational shear. This calibrates the polarisation-shear relation, but only for galaxies which are well resolved. To empirically calibrate the effect of seeing on the smaller galaxies we artificially stretch HST images to simulate lensing and then degrade them as before. These experiments provide a rigorous and exacting test of the method under realistic conditions. They show that it is possible to remove the effect of instrumental psf anisotropy, and that the method provides an efficient and quantitative measurement of the gravitational shear.

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