Detection of (dark) matter concentrations via weak gravitational lensing

The distortion of images of faint background galaxies by (weak) gravitational lensing can be used to measure the mass distribution of the deflector. Reconstruction methods for the mass profile of lensing clusters have been developed and successfully tested. Alternatively, the image distortions can be used to define a weighted mean of the mass inside a circular aperture, as was first suggested by Kaiser. This 'aperture mass' has the advantage that strict error bars can be obtained from the data itself, and that a strict lower limit of the lens mass inside a circle can be obtained. The aperture mass can thus be used to detect dark matter concentration. Keeping in mind that wide-field cameras will become increasingly available, this method can be used to search for mass concentrations on wide-field images. To do this, the aperture mass measure is generalized to account for different weighting functions. For each such weighting function, a signal-to-noise ratio can be calculated. For an assumed mass profile of the density concentrations, the weighting function can be chosen such as to maximize the resulting signal-to-noise ratio. Assuming that dark matter halos can be approximated by an isothermal profile over a large range of radius, a weighting function is constructed which is adapted to this density profile and which yields a smooth signal-to-noise map. Numerical simulations which adopt parameters characteristic of 4-m class telescopes are then used to show that dark halos with a velocity dispersion in excess of #approx# 600 km/s can be reliably detected as significant peaks in the signal-to-noise map. The effects of seeing and an anisotropic PSF are then investigated and shown to be less important than might be feared. It is thus suggested that the method of aperture mass measures developed here can be used to obtain a mass-selected sample of dark halos, in contrast to flux-selected samples. Shear fields around high-redshift bright QSOs as detected by Fort et al. provide a first successful application of this strategy. The simplicity of the method allows its routine application to wide-field images of sufficient depth and image quality. (orig.)Available from TIB Hannover: RR 4697(917) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman