Transcript The Spatially-Resolved Scaling Law of Star Formation

GALAXY FORMATION AND
EVOLUTION - 2
DISCOVER Magazine’s
2007 Scientist of the Year
David Charbonneau, of the Harvard-Smithsonian
Canter for Astrophysics (CfA), for his studies of alien
worlds, in search of Earth-like planets, and the
possibility that they may be hosting life.
Using the `transiting planet’
technique, he examined the
gases of an exo-planet’s
atmosphere. He also obtained
infrared spectra with the Spitzer
Space Telescope.
M81 group
Billions of stars all tug on each other instead of just one
planet tugged by the gravity of the Sun.
The ideas of galaxy formation
• Spiral galaxies form from the collapse of
spinning gas cloud
• Elliptical galaxies form from the mergers
of disk galaxies, or from clouds with low
spinning
• Peculiar galaxies are formed through
the interaction of galaxies
Which type of matter
dominates?
4% Baryonic Matter
22% Dark Matter
74% Dark Energy
-----100%
Dark Energy does not interact with ordinary matter; its
existence is known indirectly
Dark matter in galaxies
• What is dark matter?
It is the matter that cannot be
observed through light
• How can we infer its existence?
Through its gravity
Rotation Curves
v2
A rotation curve is just a
plot of rotational velocity vs
distance for objects in
(roughly) circular orbits.
v1
M
For a set of objects orbiting
a common point, what is
the orbital velocity for
different objects at
different distances from
the common point?
Measuring the Rotation
Rotation Curve for a Merry-GoRound
Straight because objects are
rigidly held.
Rotation Curve for our Solar System
Curved because mass
of Sun is much, much
greater than planets.
Rotation Curve for our Galaxy
(assuming the bulk of
the mass is in the middle)
V  r-1 --- What you expect if
you assume all the mass is
where the light is being emitted
The rotation curves are flat at
large radii
Rubin 1980
NGC 2915 in optical and 21 cm
visible matter
dark matter
So what is this missing mass?
• The only way to explain
the rotation curve of our
galaxy is to say that there
is lots and lots of mass
that is not emitting light.
• The halo of our galaxy
must be full of it. The
halo outweighs the disk
by a factor of 10.
• As far as we can tell, this
mass doesn’t emit any
light at any frequency.
What is the form of the missing mass?
Dark Matter Possibilities
• Here is the first lists of candidate
materials for the dark matter that
dominates the mass in our galaxy.
– Black Holes
– Black Dwarfs
– Brown Dwarfs
Baryonic Matter
(e, p, n … the same stuff
that we’re made of)
Evidence for Dark Matter
• Evidence #1 - The amount of mass that
we can “see” in a galaxy is not enough
to account for the observed gravitational
pull on the stars or on the gas.
Our picture of the mass around
galaxies now looks something like this
Evidence for Dark Matter
• Evidence #2 - The amount of mass that
we can “see” in a cluster of galaxies
does not account for the observed
gravitational pull on the galaxies in the
cluster (as evidenced by the large
variation in velocities of the galaxies).
Hercules
Cluster
Galaxies in a cluster have
more erratic velocities than
we expect from the amount
of matter we can “see” in
the cluster
Evidence for Dark Matter
• Evidence #3 - The intracluster gas is too
hot to be held by the visible matter in a
cluster.
Cluster of galaxies: Hydra A
Optical
X-ray
Using gravitational lensing to
measure the mass of clusters
Just as black holes and other massive
objects curve space around them,
galaxies and galaxy clusters curve space.
When the alignment of a large mass and
background objects is right, a dramatic
effect is observed. This is known as a
gravitational lens.
Lensing by a dense
cluster of galaxies.
Using Einstein’s laws
of general relativity,
we can estimate the
mass of the lens.
About 10 times more
mass is present than
one would estimate by
summing the mass of
the visible galaxies.
Foreground
Cluster of
Galaxies
Background
lensed galaxies
Dark
Matter
Dark Matter Summary
• 83% of the matter in the universe does not emit
detectable radiation at any wavelength.
• Evidence to date suggests that only a very small
fraction of the dark matter is made of familiar
matter (baryons).
• Dark matter is not stars or stellar remnants,
galaxies, dust clouds, or anything else made of
protons, neutrons, or electrons – as far as we
can tell, it is an as of yet undiscovered form of
matter.
This is one of the biggest mysteries facing astronomers –
What is our Universe made of?