2002ApJ...572...25D

We devise a method to measure the abundance of satellite halos in gravitational lens galaxies and apply our method to a sample of seven lens systems. After using Monte Carlo simulations to verify the method, we find that substructure comprises f_sat=0.02 (median, 0.006<f_sat<0.07 at 90% confidence) of the mass of typical lens galaxies, in excellent agreement with predictions of cold dark matter (CDM) simulations. We estimate a characteristic critical radius for the satellites of 0".0001<b<0".006 (90% confidence). For a dn/dM∝M^–1.8 (M_low<M<M_high) satellite mass function, the critical radius provides an estimate for the upper mass limit of 10⁶M_☉≲M_high≲10⁹M_☉. Our measurement confirms a generic prediction of CDM models and may obviate the need to invoke alternatives to CDM such as warm dark matter or self-interacting dark matter.