In this problem you will use some real data to measure the structure of the Milky Way in the vertical direction (away from the plane) and constrain the existence and scale height of the thick disk.
The table below shows the derived
numbers of stars of various absolute magnitudes in a field of area
18.24 square degrees towards the South Galactic Pole by Kiwi
astronomer Gerry Gilmore and British astronomer Neill Reid. Because we
are looking almost exactly at the SGP, the line-of-sight
distances of the stars are almost exactly their z distances (measured
perpendicular to the galactic plane).
(a) Think about the Hipparcos H-R diagram we looked at in class last
semester (Kutner Fig 3.11) When does an absolute magnitude translate
cleanly into a certain evolutionary state, and when might two
different evolutionary states both have the same absolute magnitude?
if we are interested in studying the structure of the *old* stars in
the Galaxy's disk, what absolute magnitude range would be the safest
to use? Why?
(b) Make a plot of the number of stars in this range
per unit volume (what would be a good choice of units here?) as a
function of height above the galactic plane z.
(c) In class we
talked about models of galaxy disks where stellar density decreases
exponentially with height above the plane. How would it be best to
redraw the plot of (b) to make it easy to fit one or more exponential
functions to the data by eye?
(d) Redraw the plot and fit it with
one or more exponential functions as needed. What scale height(s) do
the function(s) have? Comment on any evidence in the data for the
existence of a thick disk. How do your answers compare with the
numbers quoted in the notes for the scale heights of the Galaxy's two
disk populations? Did Gilmore and Reid's data support the best
estimates available (ie given in my notes) some 20 years later?