Astron. Astrophys. 363, 401-414 (2000)

1. Introduction

The study of the weak lensing distortion of background galaxies is a powerful tool for measuring the mass distribution of galaxy clusters. It has long been recognized (Tyson et al. 1984) that the tidal field of a cluster acts by slightly modifying the images of distant, background galaxies. By measuring these small distortions the projected mass distribution of the lensing cluster can be derived in a model independent fashion (Kaiser & Squires 1993), in contrast with other mass estimators (X-ray measurements of intra-cluster gas density and temperature, galaxy dynamics).

To date, there are approximately 20 clusters which have been well studied with lensing techniques (see, e.g., Mellier 1999 for a review). In several cases the resulting masses are claimed to be larger (by a factor of two) than those obtained using different methods. At present, it is not known if this discrepancy is due to biases and inaccuracies in the X-ray and dynamical estimates, or to the lensing data reduction method. As a matter of fact, it is not even clear whether there is a real discrepancy (see Wu et al. 1998; see also Allen 1998). For example, Boehringer et al. (1998) have shown that different mass estimators are in excellent agreement when very good quality X-ray and optical data are used.

In this paper, we study the mass distribution of the cluster MS10081-1224 (in the following, for simplicity, MS1008) using high quality imaging data obtained during the Science Verification of FORS1 at the VLT. The depth and seeing quality of these observations in four different bands (BVRI), make this dataset ideal for weak lensing mass reconstruction. The large surface density of background galaxies in these images, as well as the sharpness of the PSF and its small variations across the field, significantly improve the accuracy in measuring the distortion field from object ellipticities. As a result, high signal-to-noise shear maps can be obtained. This allows us to better investigate the effect of systematics involved in the mass inversion procedure, which in this case exceed statistical errors. Moreover, the availability of radial velocities for 65 galaxies in MS1008 from the CNOC survey (Carlberg et al. 1996), as well as X-ray spectroscopic observations, provide independent estimates of the total mass.

The paper is organized as follows. In Sect. 2 we introduce the basic lensing relations and the notation used throughout the paper. The observations and the imaging dataset are briefly described in Sect. 3. Sect. 4 is devoted to the weak lensing analysis of the cluster, where we describe in detail the mass reconstruction method and the results obtained. A virial analysis based on the CNOC redshift survey of MS1008 is provided in Sect. 5. A comparison between the light and the mass distribution of MS1008, and in particular its mass to light ratio, are discussed in Sect. 6. Finally, we draw the conclusions of this study in Sect. 7.

An independent lensing study using the same dataset has recently been presented by Athreya et al. (2000 preprint); moreover, the depletion effect on this cluster has been recently investigated by Mayen & Soucail (2000, preprint). We briefly compare our results with these studies in the conclusions.

Throughout the paper we adopt , , .

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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