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?
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