GALAXY MERGERS AND GRAVITATIONAL LENS STATISTICS

We investigate the impact of hierarchical galaxy merging on the statis tics of gravitational lensing of distant sources. Since no definite th eoretical predictions for the merging history of luminous galaxies exi st, we adopt a parameterized prescription, which allows us to adjust t he expected number of pieces comprising a typical present galaxy at z similar to 0.65. The existence of global parameter relations for ellip tical galaxies and constraints on the evolution of the phase space den sity in dissipationless mergers, allow us to limit the possible evolut ion of galaxy lens properties under merging. We draw two lessons from implementing this lens evolution into statistical lens calculations: ( 1) The total optical depth to multiple imaging (e.g., of quasars) is q uite insensitive to merging. (2) Merging leads to a smaller mean separ ation of observed multiple images. Because merging does not reduce dra stically the expected lensing frequency, it cannot make; lambda-domina ted cosmologies compatible with the existing lensing observations. A c omparison with the data from the HST Snapshot Survey shows that models with little or no evolution of the lens population are statistically favored over strong merging scenarios. A specific merging scenario pro posed to Toomre can be rejected (95% level) by such a comparison. Some versions of the scenario proposed by Broadhurst, Ellis, and Glazebroo k are statistically acceptable.