Astronomy 100 -- Rotation Curves and Dark Matter
ROTATION CURVES OF SPIRAL GALAXIES
A ROTATION CURVE is a graph of the
ROTATION SPEED at different
distances from the Center of Rotation.
Some Examples:
Solid Body Rotation: Wheel, Record, CD, etc.
Particles farther from the center must have a higher
speed in order to go around once in the same time period
as particles close to the center.
Keplerian Rotation: Planets around the Sun.
Closer planets have the highest speeds
while the most distant planet has the slowest.
This is because almost all the mass of the
solar system is in the Sun.
MILKY WAY ROTATION CURVE
From the 21 cm observations, we find:
IMPLICATIONS FROM A "FLAT ROTATION CURVE"
- Destruction of Galactic Star Clusters
In galactic clusters, the stars closer to the center of the
MW will move ahead of stars that are further away from the
center of the MW. This stretches out the cluster and it
will eventually come apart.
- Existence of DARK MATTER!
Since the Milky Way (and all other spiral galaxies!) have a
FLAT ROTATION CURVE, stars farther from the center feel a
gravitational pull from more matter than stars close to the center.
(If these stars felt the same mass, more distant stars would be
slower like the planets in the solar system). There is more and
more mass yet less and less gas and stars! Therefore, there must
be some other matter we call "DARK MATTER".
DARK MATTER
The "Dark" of "Dark Matter" means astronomers have not been able
to detect ANY Electro-Magnetic Radiation (such as X-rays, visible
light or radio waves) from it. It is only known to be there due
to its gravitational pull.
Dark Matter Candidates:
- Very low mass stars and Jupiter sized planets too faint
to be seen.
- Faint White Dwarf stars (also too faint).
- Mini-Black Holes or stellar sized Black Holes
- Neutrinos with some mass.
- Unknown types of particle.
The nature of Dark Matter is one of the greatest puzzles in
Astronomy today.
Hubble's Law and Large Scale Structure.
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