Chapter 15.3 Galaxy Evolution

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



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.


After a few million years, these supernovae seeded the galaxy with first heavy elements and heated the surrounding gas.


This heating slowed the collapse and the rest of the gas settled slowly into a rotating disk.


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


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


Galaxies don’t evolve in perfect isolation! They interact, collide and even merge!


~ 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.


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!

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.

•  Gravitational potential energy of matter falling into black hole turns into kinetic energy.

•  Friction in accretion disk turns kinetic energy into thermal energy (heat).

•  Heat produces thermal radiation (photons).

•  This process can convert 10-40% of E = mc2 into radiation.

Energy from a Black Hole

Jets are thought to come from twisting of magnetic field in the inner part of accretion disk.

Do supermassive black holes really exist?

Do supermassive black holes really exist?

•  Measuring the orbits of stars at the center of the Milky Way indicate a black hole of mass of ~4 million MSun

Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion MSun

•  Many nearby galaxies (perhaps all of them) have supermassive black holes at their centers.

•  These black holes seem to be dormant active galactic nuclei.

•  All galaxies may have passed through a quasar-like stage earlier in time.

•  [movie]

Black Holes in Galaxies

Chapter 15.3 Galaxy Evolution

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