Lecture 12

Astronomy 1001

6/27/07

Looking Back Through Time

• Recall that looking at distant objects is the same as looking backwards through time

• The most distant galaxies we see are 13 billion lightyears away

• We can assemble a history of galaxies by looking at similar galaxies of different ages

Galaxy Formation

• We can’t witness galaxy formation due to a lack of stars– Extensive use of computer simulations

• Assume that Hydrogen and Helium gas was spread (almost) uniformly throughout the universe

• Rotation of the galactic disks was established after the first generation of stars

Galaxy Differences• Protogalactic Cloud Differences

– Protogalactic spin might have varied– Protogalactic density might have varied

Galaxy Interactions

• Galaxies rarely evolve in isolation– In the past the galaxies were

even closer together

• Galaxy collisions take hundreds of millions of years to be completed

• Gas settles in the center of collision and forms many new stars

Clusters of Galaxies

• Many clusters have galaxies have huge elliptical galaxies that are probably the result of many collisions

• This could explain how spiral galaxies become ellipticals

Starburst Galaxies• If a galaxy is forming

stars at a prodigious rate, we call it a Starburst Galaxy– Cannot sustain this rate

of star formation forever

• Probably associated with galaxy collisions– All of the gas is used up

in star making very quickly

Active Galactic Nuclei

• Some (very distant) galaxies emit tremendous amounts of radiation and possibly jets of radiation from their center– The very brightest AGNs are

called quasars

• Recall that supermassive black holes live at the center of galaxies– Dropping matter onto dense

objects liberates their potential energy

Discovery of Quasars• In the 1960s, astronomers

were attempting to correlate radio and visible light sources– A galaxy was discovered with a

spectrum that didn’t correspond to any element

• Using Hubble’s Law and the inverse square law of light, the galaxy was determined to be 100x as bright as the Milky Way

The Study of Quasars• Quasars are almost always

very distant– Nearby mini-Quasars (Seyfert

Galaxies) are a useful source of information

• The light emitting source is very small

• The signal from quasars is so crisp that the light emitting source is probably about the size of our solar system

Radio Galaxies and Jets

• Some galaxies emit unusually strong radio waves– Jets and radio lobes are

associated with these objects

• Radio galaxies are also powered by a very small source– A donut-shaped ring of

molecular clouds surrounds the source

The Power Source• The only explanation that fits is

matter falling onto a supermassive black hole– More mass than is in the Sun has to

be falling onto the black hole each year

• Jets are more difficult to explain– Produced by twisted magnetic

fields?

• Not known where the supermassive black holes come from

More Evidence for Supermassive Black Holes

• We can observe rings of gas orbiting something invisible in M87

• Astronomers have found that the size of the bulge of a galaxy and its supermassive black hole are related– Whether stars came first or the

black hole came first is a mystery

Group Work

• Lets pretend that our measurements of the orbital velocity of the clouds at the center of M87 were off by a factor of 2. Following the example on page 655, halve the velocity of the cloud and figure out how much mass is residing inside of the clouds. What does this say about how sure astronomers are that supermassive black holes exist?

Quasars as an Observational Tool

• Quasar spectra tell us information about protogalactic clouds

• Still a new method, but it indicates that our picture of galaxy formation is generally correct

The Universe

• What exactly the universe is made out of is unknown– Most of it is NOT atoms or the

particles we have been talking about

• Dark Matter and Dark Energy are the dominant constituents– We know about how much of

each there is– What they are exactly is

mysterious

Evidence for Dark Matter

• Recall that how quickly things orbit depend on how much mass is present

• We construct rotation curves to graphically show this point

• The Milky Way’s rotation curve shows that the mass of the galaxy does NOT follow the light of the galaxy

• Can use a mass to light ratio to clarify things

Evidence in Clusters

• Recall that clusters of galaxies are groups of galaxies held together by gravity

• You can measure the speed of the galaxies by using the Doppler effect

• The galaxies are moving much more quickly than they should

Hot Gas

• There is a lot of very hot gas in between galaxies in a cluster

• Gas is in gravitational equilibrium– The average kinetic energy is

balanced by gravity

• By measuring the average temperature, the amount of mass matches with the mass determined by other methods

Group Work

• Following the example on page 669, figure out how much mass a cluster must have if the temperature of its gas is 100 million Kelvins (1 x 108 K)

Using Einstein

• Recall that gravity bends light

• How much light is bent depends on how much mass there is

• We find that light is being bent by pretty much exactly as much as you expect if the earlier results were correct

What is Dark Matter?

• Things made up of ordinary matter are called baryons

• There could conceivably be a ton of not so bright things accounting for Dark Matter

• MACHOs– Brown Dwarfs, black

holes, neutron stars– Ruled out due to lensing

Nonbaryonic Possibilities

• WIMPs– Interact weakly– Very small

• Hot Dark Matter vs Cold Dark Matter

Dark Matter and Galaxies

• Dark Matter had to be the primary gravity source in galaxy formation

• How clustered things are is a result of dark matter

• Matter appears to be arranged in huge chains (or walls) and thus there are large voids