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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,

v=sqrt(2Gm/r)

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:

  1. 5 km/sec
  2. 8 km/sec
  3. 11 km/sec
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).

Think About!

If the hot gas is observed to be moving at an average velocity of 18,600 km/sec and still remain gravitationally bound to the cluster of galaxies, how much mass must be attracting it? The radial extent of the cluster is 300 kiloparsecs.

Compare your result to the observed luminous mass of 2 x 1012 solar masses. The difference in mass is due to dark matter, that is, non-luminous mass. What percentage of the galaxy we have been working with (NGC2300) must be composed of dark matter?


This activity was developed by Jacqueline Slay, Largo High School, Largo, MD

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This page last updated: Wednesday, 17-Nov-2004 15:33:04 EST