Download Chapter 15.3 Galaxy Evolution

Chapter 15.3
Galaxy Evolution
Elliptical Galaxies
Spiral Galaxies
Irregular Galaxies
•  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.
•  How do we observe the life histories of 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.
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.
•  How did galaxies form?
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?
Matter is initially almost uniform
Dense regions form protogalactic clouds
Gravity pulls galaxies together to form galaxy clusters.
•  How did galaxies form?
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.
Questions:
•  Where did these slight density enhancements come from?
•  Why are there elliptical galaxies and irregular galaxies?
•  Why do galaxies differ?
Elliptical galaxies have
less or no gas, more old
stars and look redder.
Spiral galaxies have more
gas, more young stars and
look bluer.
•  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?
Stars:
HI
Galaxies:
~ 2500 miles
CA
~ 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?
Galaxy collisions!
Computer simulations show that two spiral galaxies collide
can form an elliptical galaxy in about one billion years!
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.
Prof. Josh Barnes in IfA, UH
www.ifa.hawaii.edu/~barnes
•  Why do galaxies differ?
Galaxy collisions!
•  Why do galaxies differ?
Galaxy collisions!
•  Why do galaxies differ?
Galaxy collisions!
•  Observational evidences
Giant elliptical galaxies
grew hugely by “eating”
other galaxies, and
become the central
dominant galaxies in the
center of galaxy clusters.
Visible light
X-ray
•  Observational evidences
-  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.
•  Quick Review
-  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.
Galaxy collisions ignite huge bursts of rapid star formation!
•  Starburst Galaxies
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 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.
X-ray image
Intensity of supernova explosions in
starburst galaxies can drive galactic winds.
Why do galaxies differ?
•  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
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.
Energy from a Black Hole
•  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.
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
Black Holes in Galaxies
•  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]