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 %

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 rotation curve is just a plot of rotational velocity vs distance for objects in (roughly) circular orbits. For a set of objects orbiting a common point, what is the 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 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 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 cm visible matter dark matter

The existence of dark matter V 2 =GM(r)/r Mass within radius r: M(r)=V 2 r/G

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 OpticalX-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. 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

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?