Chapter 15.3 Galaxy Evolution
• Are there any connections between the three types of galaxies?
• How do galaxies form? How do galaxies evolve?
P.S. You can find all the pictures/ movies either on NASA’s website, or in the textbook.
Elliptical Galaxies Spiral Galaxies Irregular Galaxies
Deep observations not only look to great distances, but also look back in time.
This means we see distant galaxies as they were when they were much younger.
• How do we observe the life histories of galaxies?
Hubble Ultra Deep Field
• How do we observe the life histories of galaxies?
Grouping by types allow us to study how a particular type of galaxies evolves in time.
Observing first galaxies/ stars require larger telescopes and sensitive to infrared light, because of the extreme redshift.
• How did galaxies form?
The best models for galaxy formation assume:
1. Matter originally filled all of space when the universe was very young
2. The distribution of matter was not perfectly uniform–
some slightly denser than the others– and this slightly greater pull of gravity wins the expansion.
In about 1 billion years, protogalactic clouds formed.
0.5 billion years 0.5-1 billion years
• How did galaxies form?
• How did galaxies form?
Matter is initially almost uniform
Dense regions form protogalactic clouds
Gravity pulls galaxies together to form galaxy clusters.
Protogalactic clouds cooled and the first generation of stars formed.
• How did galaxies form?
After a few million years, these supernovae seeded the galaxy with first heavy elements and heated the
surrounding gas.
• How did galaxies form?
This heating slowed the collapse and the rest of the gas settled slowly into a rotating disk.
• How did galaxies form?
Questions:
• Where did these slight density enhancements come from?
• Why are there elliptical galaxies and irregular galaxies?
Spiral galaxies have more gas, more young stars and look bluer.
Elliptical galaxies have less or no gas, more old stars and look redder.
• Why do galaxies differ?
Initial conditions in the protogalactic clouds are different.
1. Protogalactic spin:
Faster rotation (more initial angular momentum): Spiral Galaxies
Slower rotation (little/no initial angular momentum): Elliptical Galaxies
• Why do galaxies differ?
Initial conditions in the protogalactic clouds are different.
2. Protogalactic density:
High gas density results in quicker cooling, faster star formation before gas settled into a disk: Elliptical Galaxies
Low gas density, less star formation, gas settled into a disk: Spiral Galaxies
• Why do galaxies differ?
Galaxies don’t evolve in perfect isolation!
They interact, collide and even merge!
• Why do galaxies differ?
~ 2500 miles
HI CA
Stars:
Galaxies:
~ 9 ft
Average distances between galaxies are not much larger than their sizes, and collisions between them are inevitable!
More often in the past, since the universe was smaller.
• Why do galaxies differ?
Computer simulations show that two spiral galaxies collide can form an elliptical galaxy in about one billion years!
Prof. Josh Barnes in IfA, UH www.ifa.hawaii.edu/~barnes 1. Galaxies approach into orbit around each other due to gravity.
2. Distorted as collision continues, gas collapse towards the center.
3. Gravitational force pulls out long tidal tails and features.
4. Centers of the galaxies merge into an elliptical.
• Why do galaxies differ? Galaxy collisions!
• Why do galaxies differ? Galaxy collisions!
• Why do galaxies differ? Galaxy collisions!
• Why do galaxies differ? Galaxy collisions!
Giant elliptical galaxies grew hugely by “eating”
other galaxies, and become the central
dominant galaxies in the center of galaxy clusters.
• Observational evidences
Visible light X-ray
- Groups of young stars forming found in old elliptical galaxies.
- Some gas and stars show reverse orbits.
- Shells formed when gas stripped out of a galaxy during collision.
• Observational evidences
- In the very early universe, H/ He filled out the space with slightly non-uniform distribution. These matter then condense through gravitational force to form protogalactic clouds in about 1 billion year.
- Protogalactic clouds then collapse to form disk galaxies.
- The difference of galaxies come from two sources:
1. The initial conditions of the protogalactic clouds are different, either the spin or the density, which results in different type of galaxies.
2. Later interactions or collisions of galaxies can also affect their evolution.
• Quick Review
Galaxy collisions ignite huge bursts of rapid star formation!
Most “normal” galaxies form about 1-4 stars per year (MW:
1 star per year), but these galaxies form >100 stars per year!
These galaxies run out of gas in just a few million years.
A very temporary phase in a galaxy’s life.
• Starburst Galaxies
Bubbles of hot gas from supernovae blow out bursts through the gas disk and form galactic winds that flow outward.
• Starburst Galaxies
Starburst galaxies represent an important role in helping us understanding the galaxy evolution.
The collisions we observe nearby
trigger bursts of star formation.
Modeling such collisions on a computer shows that
two spiral galaxies can merge to make an elliptical.
Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical.
Note: The stars themselves do not collide,
but the galaxiesʼ gas clouds do.
Collisions
may explain why elliptical galaxies
tend to be found where galaxies are closer
together, namely in galaxy
clusters.
Giant elliptical galaxies at the centers of clusters
seem to have consumed a number of smaller
galaxies.
Intensity of supernova explosions in
starburst galaxies can drive galactic winds.
Intensity of supernova explosions in
starburst galaxies can drive galactic winds.
X-ray image
• Angular momentum may determine size of disk
• Density of protogalactic cloud may determine how fast a galaxy forms
• Collisions shape galaxies early on
– Mergers of small objects make halo & bulge
– Mergers of larger objects make elliptical galaxies
• Relatively undisturbed galaxies can still have disks
Why do galaxies differ?
15.4 Quasars and
Other Active Galactic Nuclei
Learning goals
• What are quasars?
• What is the power source for quasars and other active galactic nuclei?
• Do supermassive black holes really exist?
If the center of a galaxy is
unusually
bright we call it an active
galactic nucleus
Quasars are the most
luminous examples.
Active Nucleus in M87
The highly redshifted spectra of quasars indicate large distances. (Discovered in the 1960s)
From brightness and distance we find that luminosities of some quasars are >1012 Lsun
Variability shows that all this energy comes from region smaller than solar system
What is the origin of a quasar?
Galaxies around quasars
sometimes appear
disturbed by
collisions.
An active galactic nucleus can
shoot out blobs of plasma
moving at nearly the speed of
light.
Speed of ejection suggests that a black hole is
present.
Accretion of gas onto a supermassive black hole appears to be the only way to explain
all the properties of quasars.
