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HW #13 (Due 12/02/09)
1) List the three main components of a disk galaxy and describe the difference between
the stars that are present in the components.
The three main components are the disk, the bulge and the halo. The disk has
stars, gas and dust. It is the location where star formation is occurring. Because
of this the light coming from the disk of the galaxy is extra blue in color. This is
due to the presence of young, high-mass, high-luminosity stars that only live a
short period of time.
2) Explain how galaxies interacting can produce spiral density waves.
This is a tidal effect. When two galaxies pass by each other, the gravity is stronger
on the side that is closest to the passing galaxy, and it is weakest on the side that is
farthest away. This allows a very stretched out tidal bulge to develop on the side
near the passing galaxy, because of the stronger gravity, and a bulge on the opposite
side, because that part of the galaxy didn’t move as much as the center of the galaxy.
This produces two prominent spiral density waves.
3) Write out Kepler’s third law and explain why a flat (constant) orbital velocity for
stars in a galaxy, indicates that there has to be Dark Matter contributing to the total
mass of galaxies.
(Minterior + Mstar)P2 = (4π2/G)r3 We can bring the P to the other side of the
equation and realize that (4π2r2/P2) = v2. The velocity squared. Solving for velocity we
get
𝒗 = √(𝑴𝒊𝒏𝒕𝒆𝒓𝒊𝒐𝒓 𝒙 𝑮/𝒓)
The flat rotation curves means that the orbital velocity is constant as you move farther
and farther away from the center of the galaxy. But notice that r is growing as you move
farther from the center. The only way that v can remain constant, is if Minterior is
growing at the same rate as r. This increase is due to dark matter.
4) Explain why there are giant elliptical galaxies at the center of all galaxy clusters.
The center of galaxy clusters has a large density of galaxies. In other words, there
are lots of galaxies in a small volume of space. In this type of location, galaxy
mergers are very frequent. When spiral galaxies merge together their stars are
thrown into a more spherical distribution. So merging spirals make elliptical
galaxies. And this occurs most frequently at the center of galaxy clusters.
5) Explain the modern day model for how the Milky Way formed. Also explain how
this model is related to the images of extremely distant galaxies.
Small galactic fragments condensed within their own dark matter halos after the Big Bang.
These fragments not only made their own stars but also merged to form bigger galaxies.
The stars from the merger where thrown around the new galaxy to make a halo of stars.
Since these were the first stars to form, the halo has is made of very old, metal-poor stars.
The gas in the merger is able to lose energy and spin into a disk. There new star formation
occurs.
This is related to the Hubble image of extremely distant galaxies, because we are looking
back to the time when galaxies were merging and making large galaxies. This is an image
that probably looks very similar to the formation of the Milky Way.