Direct Detection of Cold Dark Matter Substructure

We devise a method to measure the abundance of satellite halos in gravitational lens galaxies, and apply our method to a sample of 7 lens systems. After using Monte Carlo simulations to verify the method, we find that substructure comprises fraction f=0. 02 (median, 0. 006<f<0. 07 at 90% confidence) of the mass of typical lens galaxies, in excellent agreement with predictions of CDM simulations. We estimate a characteristic critical radius for the satellites of 0. 0001<b/arcsec<0. 006 (90% confidence). For a satellite mass function of dn/dM Mˣ with x=-1. 8 and Mₗ<M<Mₕ, the critical radius provides an estimate that the upper mass limit is 10⁶Msun < Mₕ < 10⁹Msun. Our measurement confirms a generic prediction of CDM models, and may obviate the need to invoke alternatives to CDM like warm dark matter or self-interacting dark matter.