Clusters - Involved Physics Clusters Home

Most of the physics of this applet is actually covered in the discussion of the virial theorem. Be aware, though, that there are many more sophisticated ways of calculating a cluster mass, which are a bit more accurate than what we've used. The basic concepts, though are pretty much the same. See the links section for pointers towards other mass estimators.

As far as how the applet runs, we basically use a database of galaxy positions, magnitudes, and radial velocities culled from the literature. We combine these with a wide field image of each cluster so that we can "point and click" our galaxy data off the cluster image.

The velocity dispersion and mean velocity are derived NOT from fitting the binned histogram but from the statistical mean and standard deviation of the dataset (i.e., the individual values) as it is built up.

For calculating the luminosity distribution, we simply convert the galaxy magnitudes into fluxes via the standard expression:

f ~ 100.4m

Once a Hubble constant is entered, we calculate the distance from the recessional velocity of the cluster, then convert fluxes to luminosities and angular size to physical size, and finally calculate the mass and mass to light ratio as spelled out in the virial theorem discussion.

It's all really just a database applet...