Clusters - Lab Applications Clusters Home

Start with the Coma cluster. Use the mouse to "observe" a few galaxies and then flip over to "Analyse" to see the dataset. Keep adding galaxies to your dataset and watch how it builds up.

  • How many galaxies do you need to get a good estimate of the cluster velocity and velocity dispersion?
  • How have you defined "a good estimate"?
Use the limits windows underneath the velocity histogram to set the limits over which the mean velocity and velocity dispersion are calculated.
  • How does changing the limits change your estimates?
  • How would you set the limits to give you the best estimates?
  • Why is it okay to set limits on the velocities used?
Choose a Hubble constant and calculate the mass of the Coma cluster.
  • How does it compare to the range of values astronomers have derived for the mass of the Coma cluster
    (MComa = 1-3x1015 Msun)
  • One problem with our applet image is that it only shows the very central region of the cluster. How does this affect your estimate of the cluster mass?
  • Does it matter that we haven't measured *every* galaxy in Coma? Why or why not?
  • How does the mass change if you use a different Hubble constant? Why does it change like this?
Coma has a luminosity of LComa = 5x1012 Lsun.
  • Calculate the total mass-to-light ratio of Coma.
  • If galaxies have a mass-to-light ratio of 10, what is the mass of galaxies in Coma?
  • If Coma has a Xray gas mass of MX = 3x1014 Msun, what is the total amount of "missing mass" in the cluster?
Now try another cluster. Observe galaxies and calculate the cluster mass.
  • Compare it to Coma. Is it more or less massive than Coma?