This time you should be able to achieve a pretty good match between your graph and the measured rotational speeds. Be sure that your graph graph doesn't tail off too much. Even at a distance of 60 pc, the rotational speeds are stiil around 200 km s-1.
It is quite easy to show that if the halo density distribution follows an inverse square law, the rotational speed will remain constant right out to infinity - but then, if that were really the case, the mass of the galaxy within a given radius R would increase without limit. (For the purposes of this exercise, the galaxy is deemed to end at 40 kpc.)
Now look at the masses of the various components of your galaxy summarised in the 'Galactic parameters' box. We do know that the halo exists because there are a significant number of objects which we can see in it. These include globular clusters and a group of stars known as 'Blue-horizontal branch' stars; but there are nowhere near enough of these to account for the huge quantity of matter needed in the halo to match up with the measured rotation curves. Effectively, everything in the halo is 'dark matter' - ie matter which gravitates but which we cannot see.
Note that for the purposes of this program and for consistency, the densities of both halo and disc are expressed in units of 10-21 kg m-3.
1 x 10-21 kg m-3 is equal to 0.015 MO pc-3