# Dark Matter

Try this!

Picture a rocket being launched into space. How fast do you think it must travel to escape the earth's gravitational pull? You can determine this by using the escape velocity equation,

with m as the mass of the earth and r as the radius of the earth. Calculate how fast the rocket must travel to escape from the earth.

Is it:

How does this apply to dark matter? One of the most important uses of X-ray observations of clusters of galaxies has been the determination of mass estimates for these systems.

The fundamental assumption is that the hot, X-ray emitting gas between the galaxies in the cluster is trapped in the gravitational well of the cluster and roughly in hydrostatic equilibrium. Both of these assumptions have been shown to be fair, based on the observational measurements to date. The X-ray observations from satellites such as ROSAT and ASCA can be used to determine the gas density profile and temperature. These values are then fed into a mathematical model to determine the total mass of the cluster. This value can then be compared to the observed luminous mass (i.e., mass of galaxies plus hot gas as determined from visible light observations).