Gravitational lensing by field halos: a clean test of dark matter models:A clean test of dark matter models

The Cold Dark Matter (CDM) model for galaxy formation is currently the most successful at reproducing a large variety of observations, but it remains largely untested in the non-linear sub-galactic regime. A clear prediction of this model is that a significant number of low-mass halos should populate any line-of-sight. As most of these objects are expected to be completely dark, strong gravitational lensing provides a unique channel to detect them and determine the properties of dark matter by constraining the halo-mass function at the low-mass end. However, the available data for known gravitational lens systems currently lack the sensitivity to probe the halo mass function at the regime where different dark models significantly differ. This proposal requests 27 orbits to re-observe a carefully selected sample of 11 highly magnified gravitational arcs/rings. The high redshift (z 2-3) of the uv- bright background galaxies will allow the dark matter distribution in the non-linear regime to be probed over a sizeable line-of-sight volume. A published analysis of the single-orbit imaging for these lenses does not detect any low mass halos. Given the expectation value is 1+/-1, this result is (just) consistent with CDM. However, given the low expectation value, further observations are required to test the robustness of this result. Simulations predict that 9+/-3 low-mass halos will be detected in the higher signal-to-noise ratio dataset requested here, for a CDM Universe. A null-detection from these new data will, therefore, rule out the CDM model for dark matter at more than the 3 sigma level.