Dark Matter
X-ray observations of the hot plasma trapped in the gravitational field of the Dark Matter in clusters of galaxies remain one of the most powerful techniques to map the location of the Dark Matter and constrain its interaction with normal matter. Constellation-X will for the first time bring the spectral resolution and collecting area required to map the velocity field of the plasma on scales of a hundred of km/s, the relevant velocity scale for these systems. By deriving precise mass profiles and directly comparing the baryonic component of clusters Constellation-X will provide a direct measurement of the amount and distribution of dark matter to a unprecedented level or precision and allow accurate comparisons with weak and strong lensing measurements and determinations of the gas content via the Sunyaev-Zeldovich effect. There are no comparable X-ray facilities planned with similar capabilities.
Warm Dark Matter has become a viable "alternate" to the standard cosmological structure formation scenario, as it may resolve many problems in structure formation. Sterile neutrino dark matter, in the standard production scenarios, is detectable or potentially excludable with Constellation-X and by no other means. These particles are expected to decay, but with rather long time scales into two photons. The present best limits on these particles, if they are to represent the bulk of the dark matter, is between 1-20 keV. Constellation-X will be able to improve on the present limits by a factor of over 30, definitely either detecting or ruling out sterile neutrinos as the dark matter.
An additional interesting measurement that will come from these Cluster measurements will be constraints on the mass of the neutrino. The neutrino mass density originates primarily from the fact that the cluster of galaxies X-ray Luminosity Function provides a robust constraint on sigma-8 for a given value of Omega matter, while the CMB data predict sigma-8 as a function of the neutrino mass. So combining the two provides constraints on the neutrino mass (Allen 2003). Constellation-X data combined with Planck can be expected to place more accurate constraints on the neutrino mass.
