Introduction to Stellar, Galactic, & Extragalactic Astronomy
Lecture 43: Dark Matter
Dark Matter is matter we cannot see directly
- Detected only by its gravity
- >90% of the Mass of the Universe?
Evidence for Dark Matter:
- Flat Galaxy Rotation Curves
- Galaxy Motions & Hot Gas in Clusters
Baryonic & non-Baryonic Forms
Galaxy Rotation Curves revisited
Spiral Galaxies rotate such that the rotation speed:
- rises with radius from the center in the inner parts of the
- becomes roughly constant (flat) with radius in the outer parts
as far as can be measured in the disk.
Mass Distribution in Galaxies
Most of a galaxy's light is in the inner 10 kpc
If stars provided all of its mass we expect:
- rotation curve will rise in the inner parts to a maximum speed
- fall steadily with radius outside ~10 kpc
But the curve stays flat!
- Need more mass at large radii than is observed in the stars
Dark Matter Halos
Q: Where is the extra mass if it is not coming from stars?
A: Galaxies have extended "dark halos"
- Contains >90% of the galaxy's mass
- more extended than the starlight component
- orbits of satellite galaxies suggest it may extend out as
far as 200 kpc or more!
Fritz Zwicky's "Missing Mass"
1933: Fritz Zwicky measured the peculiar velocities of
galaxies in the Coma cluster
- Found peculiar velocities of ~1000 km/sec.
- This is greater than the escape velocity computed if you just
add up the light of the galaxies in the cluster.
Zwicky suggested that "missing mass" adds extra gravity
to hold the cluster together.
Dark Matter in Clusters
Subsequent observations showed that galaxy clusters are 90-99%
- Clusters are filled with hot X-ray emitting intergalactic
gas. Without dark matter, this gas would dissipate away.
- Gravitational lensing of background galaxies by clusters requires
dark matter in clusters.
It is "Dark" because it cannot be detected directly
using light (of any kind).
Detected only because of the effects of its gravity on
- It makes the outer parts of galaxies rotate faster than expected
from their starlight.
- It makes galaxies in clusters orbit faster than expected from
the total starlight of the galaxies.
What is it?
Baryonic Dark Matter?
- Ordinary matter ("baryons") made of protons and
Non-Baryonic (Particle) Dark Matter?
- Exotic subatomic particles that only interact with matter
via Gravitation (or maybe the Weak Force)?
Some mixture of the two?
Baryonic Dark Matter
- Brown Dwarfs and Jupiter-sized planets.
- Diffuse gas clouds
- cold stellar remnants (black holes, neutron stars, and white
- Frozen hydrogen snowballs
- All are hard to keep from making light.
What if a Brown Dwarf, Jupiter, or dark stellar remnant in the
halo passes between Earth and a more distant star?
- GR predicts: the dark object's gravity bends the starlight
of the more distant star.
- This makes a "Gravitational Microlens" that magnifies
the star momentarily as it passes.
- Chance alignment between the dark object and the distant star
makes a gravitational lens.
- Motion of both the lens and the distant star makes it sweep
over the Earth.
- As the lens sweeps past, we see the distant star brighten
suddenly at all wavelengths, then fade to normal brightness.
Chance alignments are very rare.
Need to monitor millions of stars for many years.
- A number of large international collaborations are monitoring
the LMC and Galactic Bulge.
- So far, they have collectively found ~ a dozen microlensing
Non-Baryonic Dark Matter
Fundamental particles which only interact via gravitation or the
- Produced in large numbers in the Big Bang
- Moving very fast ("Hot" dark matter)
Exotic new particles:
- Predicted by speculative particle theories.
- Massive "cold" dark matter particles.
Particle Dark Matter Searches
Searches for direct detection of dark matter particles.
- Attempts to measure the mass of the neutrino
- Particle Collider experiments to create new fundamental massive
particles in collisions.
- Searches for "cold dark matter" particles hitting
the Earth from space.
A (Very) Radical Suggestion:
Some have suggested that Dark Matter does not exist, but rather
our theory of gravity is wrong on large scales!
- None of the alternative theories of gravity have survived
tests of detailed predictions.
- Hard to reconcile these theories with the observed gravitational
How much Dark Matter is there?
D and He from Primordial Nucleosynthesis predicts W in baryons
Galaxy halo masses from rotation curves gives
- Some of the dark matter is non-Baryonic (??)
- The Universe is Open if all dark matter is only in
Updated: 1997 November 24