ASTR 222 - Homework #2

1. Studying the Galaxy with Globular Clusters

Here is a list of (real!) data for globular clusters.

The data give

We are going to use this data to learn about the distance to the galactic center, and the rotation speed of the Milky Way.

First, transform the observed data (l, b, R) into cartesian coordinates (X,Y,Z). +X should point towards the galactic center (l=0), +Y should point in the direction of rotation (l=90), and +Z should point above the plane (positive b). Defined this way, we have

Calculate and plot the X,Y,Z positions to get a feel for the distribution of globular clusters in the Galaxy. Plot the X-Y and X-Z projections so that you can see how things are distributed along each axis. Now, in the 1910s, Harlow Shapley derived the distance to the Galactic Center by using the positions of the globular clusters. Now it's your turn: Finally, we can use the observed velocities of the globular clusters to get the rotation speed of the Galaxy. So we should be able to fit a sine function to the data: Vr = -Vc,proj sin(l). Once you have solved for Vc,proj, remember we have to deproject this value to get the true rotation speed. Since Vc,proj = Vc<cos(b)>, averaging over all b's, we have Vc,proj = (2/pi)Vc. So to correct for projection, Vc = (pi/2)Vc,proj. Now, what is the velocity dispersion of the globular cluster system? That is, if the globular clusters weren't moving at all, the sine fit would work perfectly, and go through all the data points (projection effects aside). It clearly doesn't -- there is a lot of scatter around that line. That tells us that the clusters themselves are moving as well, but in a random sense, rather than as a rotating system. The scatter of the velocities around the sine fit gives us an estimate of how fast the clusters are moving in this random motion, which is referred to as their velocity dispersion.

You can estimate the scatter around a fit by calculating the the standard deviation of the residuals. In other words, for each data point, calculate the residual as residual = Vr - Vr,fit = Vr + Vc,proj sin(l), and then calculating the standard deviation of the residuals. Again you'll want to correct your scatter for projection effects, this time by multiplying your std deviation by root(3) since we're only seeing one of the three velocity components of each cluster's motion.

2. A simple Oort limit

This dataset contains apparent magnitude (V), radial velocity (vrad), and absolute magnitudes (MV) for a sample of K giant stars along a line of sight at galactic latitude b ~ -90 (in other words looking directly out of the plane of the Milky Way's disk). We are going to use this dataset to measure the mass density of the Milky Way's disk in the solar neighborhood. Remember how we do this, by using the Oort limit, which means we need the velocity dispersion and  scale height of the stars.

Scale Height

Calculate the distance to each star from its apparent and absolute magnitude. Because these stars are all at b ~ -90, their Z distance from the plane is pretty much given by the distance of the star, once we correct for the distance of the Sun from the galactic plane. Since the Sun is "above" the disk plane (at Z=+30 pc) and these stars are seen "below" the disk plane, their distance from the disk plane is simply given by Z = distance - 30 pc. 

Then find the number of stars (N) as a function of Z by making bins of, say, 200 pc size and counting the number of stars in each bin. Make a plot of ln(N) (y axis) vs Z (x axis) and find the scale height of the stars (how?). When fitting, remember to make smart choices about which bins to include in your fit; including all your bins is probably not a good choice!

Velocity Dispersion
Just like we said the distance was roughly the Z coordinate (once we corrected for the Sun's Z coordinate), we can also say the radial velocity is roughly the Z velocity once we correct for the Sun's Z velocity.

From the distribution of velocities, calculate the Sun's Z velocity (how?). Then subtract that from all the velocities to get the Z velocity of each star. Then bin up the stars in a few bins of Z, calculate the dispersion of each of the bins (how?), and then make a plot of velocity dispersion as a function of Z height. How and why does it change with Z?

The Oort limit
Given the scale height and velocity dispersion  (which velocity dispersion do you use, and why?), estimate the mass density of the disk using the method shown in class. Describe sources of error and how they may have affected your result.