GALAXY FORMATION ANDGALAXY FORMATION ANDEVOLUTION - 2EVOLUTION - 2

DISCOVER Magazine’s2007 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 Matter74% 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

M

v2 A A rotation curverotation curve is is just a plot of just a plot of rotational velocity rotational velocity vs distance for vs distance for objects in (roughly) objects in (roughly) circular orbits.circular orbits.

For a set of objects orbitinga common point, what isthe orbital velocity for different objects at different distances from the common point?

v1

Measuring the Rotation

Rotation Curve for a Merry-Go-Round

Straight because objects arerigidly held.

Rotation Curve for our Solar System

Curved because massof Sun is much, much greater than planets.

Rotation Curve for our Galaxy

(assuming the bulk ofthe mass is in the middle)

V r-1 --- What you expect if you assume all the mass is where the light is being emitted

Rubin 1980

The rotation curves are flat at large radii

NGC 2915 in optical and 21 cmvisible 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?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 stuffthat 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 amountof 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 densecluster of galaxies.

Using Einstein’s laws of general relativity,we can estimate themass of the lens.

About 10 times moremass is present thanone would estimate bysumming the mass ofthe visible galaxies.

ForegroundCluster ofGalaxies

Background lensed galaxies

Dark

Matter

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?