The applet opens with an example rotation curve plotted. The x axis is distance from the center of the galaxy (in kiloparsecs) and the y axis is rotation speed (in km/s). The data points are shown as vertical hashes, where the size of the hash reflects the accuracy of the measurement (i.e., these are "errorbars").
You have the following controls:
- Galaxy:
- this is a pull-down menu which allows you to select different galaxies to model.
- Disk Mass-to-Light ratio:
- a slider bar which controls the mass-to-light ratio of the disk. The other properties of the disk (central luminosity density and radial scale length) are set by the observational data, and are already factored in to the calculation.
- Halo Central Density:
- a slider bar which controls the central density of the isothermal halo.
- Halo Size:
- a slider bar which controls the core radius of the isothermal halo.
First try simply playing with the disk mass-to-light ratio. This will attempt to fit the data with only a disk model. Then play with the halo parameters. As you do this, you will see three curves plotted over the data:
Blue curve : rotation curve predicted by disk model (only)
Red curve: rotation curve predicted by halo model (only)
Black curve: rotation curve predicted by disk+halo model.
Finally at the bottom of the plot is the chi-square value of the fit, a measure of how good the fit is. Better fits have lower chi-square values. See if you can minimize this.