Constraints on the identity of the dark matter from strong gravitational
  lenses
  The cold dark matter (CDM) cosmological model unambigously predicts that a
large number of haloes should survive as subhaloes when they are accreted into
a larger halo. The CDM model would be ruled out if such substructures were
shown not to exist. By contrast, if the dark matter consists of warm particles
(WDM), then below a threshold mass that depends on the particle mass far fewer
substructures would be present. Finding subhaloes below a certain mass would
then rule out warm particle masses below some value. Strong gravitational
lensing provides a clean method to measure the subhalo mass function through
distortions in the structure of Einstein rings and giant arcs.Using mock
lensing observations constructed from high-resolution N-body simulations, we
show that measurements of approximately 100 strong lens systems with a
detection limit of $10^7 h^{-1} M_{\odot}$ would clearly distinguish CDM from
WDM in the case where this consists of 7 keV sterile neutrinos such as those
that might be responsible for the 3.5 keV X-ray emission line recently detected
in galaxies and clusters.
