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Brenda Frye (LBNL), T. Broadhurst (ESO), H. Spinrad (UC Berkeley)
We demonstrate with a simple new test for gravitational magnification that the mass profiles of rich clusters have a low central concentration of dark matter in reasonable agreement with recent N-body simulations (Navarro, Frenk & White 1995). Redshift samples have been obtained at the Keck for a magnitude-limited sample of objects (I<23) behind three clusters, A1689, A2390 and A2218 within a radius of 0.5 Mpc. For each cluster we see both a clear trend in increasing flux and redshift towards the center. This behavior is the result of image magnifications, such that at fixed redshift one sees further down the luminosity function (Broadhurst, Taylor & Peacock 1995). The gradient of this magnification is, unlike measurements of image distortion, sensitive to the mass profile in the interesting regime where N-body simulations of dissipationless cold dark matter predict shallow profiles for massive clusters (rs<0.5Mpch-1), departing strongly from a pure isothermal halo.
We have also found that VRI color selection can be used effectively as a discriminant for finding high-z galaxies behind clusters and present five 4.1 < z < 5.1 spectra which are of very high quality due to their high mean magnification of ~0, showing strong, visibly-saturated interstellar metal lines in some cases.
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The author(s) of this abstract have provided an email address for comments about the abstract: bfrye@lilys.lbl.gov