Discuss two ways of detecting dark matter in distant galaxies and clusters
What will be an ideal response?
The luminosity of a galaxy is proportional to the amount of visible mass it contains. The absolute luminosity of a galaxy can be used to determine an estimate for the mass of visible matter in the galaxy. The actual mass of the galaxy can be measured through several methods (the rotation curve method, the velocity dispersion method). The difference between the actual mass and the estimated mass is the amount of dark matter in the galaxy.Dark matter in a cluster can be detected by measuring the orbital motion of the galaxies in the cluster, and the amount of mass in each galaxy. The cluster must have enough mass to hold the galaxies in their orbits, allowing astronomers to determine that dark matter must be present in the cluster to hold it together.Many clusters are filled with hot gas, visible in X-ray observations. The gas is moving rapidly, and the cluster must have enough dark matter to keep the hot gas held in the cluster.Albert Einstein described the ways that matter curves space, and the path of photons will be bent around a mass to create gravitational lensing. Gravitational lensing distorts the images of objects behind a concentration of dark matter, and can be used to map and detect dark matter.
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