1. Galactic Evolution
AST 112

2. Four Types of Galaxies
Spiral
Elliptical
Lenticular
Irregular
Why?

3. Hubble Sequence
The Hubble Sequence suggests a path of evolution for galaxies.

4. The Hubble Sequence is ABSOLUTELY WRONG! (But good first try.)

5. Formation of Galaxies Galaxies begin as a protogalactic cloud
We will assume a “top-down” theory of galaxy formation
Large, simultaneous collapse

6. Elliptical vs. Spiral
What determines whether a galaxy will become a spiral or an elliptical?

7. Spiral or Elliptical? Does it form as a spiral or elliptical?
Depends on protogalactic cloud conditions
Does it ever transform from one to the other?
Depends on interactions with other galaxies

8. Formation of Galaxies Keep this in mind:
Spiral galaxies have gas-rich disks
Ellipticals do not

9. Protogalactic Conditions
Protogalactic spin:
If it’s spinning, it will flatten
Protogalactic density:
Higher density: more rapid star formation, most stars formed during collapse
Less material left over
Lower density: fewer stars created during collapse
More material left over

10. Spirals: Halo, Bulge and Disk
Protogalactic cloud contracted
Formed massive 1st generation stars
Spin increases as cloud collapses
Protogalaxy flattens as spin increases
Gas contracts faster than the stars
Spirals start with low density
Lower rate of star formation during collapse
First generation stars exploded in a few million years as the protogalaxy was collapsing
Slowed collapse

11. Spirals: Halo, Bulge and Disk
What do we have?
A spherical distribution of stars left over from initial collapse and star formation
Bulge and Halo
A disk of gas and dust that was “preserved” due to lower rates of star formation

12. Spirals: Halo, Bulge and Disk
What causes star formation to ramp up in the disk?
Why is the bulge yellow?

13. Ellipticals Giant ellipticals across the Universe:
Nearly devoid of blue and white stars
Very red; entire galaxy is old
Suggests all stars formed at once
Nothing left to make a disk out of
Star formation ceased shortly after the galaxy formed

14. Lenticulars: “Anemic Spirals”?
Lenticular galaxies could be former spirals that have lost their cold gas
Used up
Lost via collision

15. Globular Clusters
Globular clusters live in the halos of large galaxies
Milky Way: 150+
Andromeda: 500+
M87: 13,000+
Some may have intermediate-sized black holes at their center

16. Globular Clusters Two possible origins for globular clusters:
As the protogalactic disk collapsed, larger fragments formed globular clusters OR
These are the bulges of much smaller galaxies that were absorbed into larger galaxies

17. Galactic Collisions / Mergers
If the approaching galaxies are moving fast, they may collide and keep going
Otherwise, they may collide repeatedly and merge

18. Galactic Collisions
The Whirlpool Galaxy (Messier 51) is a classic example of colliding galaxies

19. A Galactic Merger

20. Another galactic merger.

21. A Galactic Collision

22. Cartwheel Galaxy

23. Galactic Collisions
Galactic collisions play a major role in galactic evolution
“Rarely evolve in perfect isolation”
From the book:
If our galaxy is a grapefruit, Andromeda is another grapefruit 3 meters away (and several are closer)

24. Galactic Collisions
Collisions occur over hundreds of millions of years
Stars don’t collide much
Gas and dust DO collide
Triggers LOTS of star formation (100/yr)

25. Galactic Collisions Tidal forces tear the disks apart
Orbits get randomized
Trails of stars form after collisions
Supernovae and winds blow gas away

26. Galactic Collisions
Collisions tend to disrupt orbits and destroy disks in spiral galaxies. Regardless of what type of galaxy you start with, what type of galaxy likely results from multiple large-scale collisions?

27. Galactic Collisions
What’s going on here?

28. Galactic Collisions
M81 / M82 have been duking it out for millions of years
M82 is undergoing starburst because of this

29. Sagittarius Dwarf Galaxy
The Milky Way is colliding with the Sagittarius Dwarf Galaxy!
80,000 LY away (behind the bulge)

30. Sagittarius Dwarf Galaxy

31. Collision With Andromeda
The Milky Way will merge with Andromeda in 3 to 5 billion years
Star collisions won’t hurt us… gas collision could
We could get flung out of the galaxy

32. Galactic Collisions: Simulation

33. Observing Galaxies Through Time
Galaxies take billions of years to evolve…
So how can we hope to study it?
Recall that light takes time to travel from one location to another
It’s fast. But the distances involved are HUGE.
We are able to study galactic evolution because our large telescopes can look far away
And therefore into the distant past
We can see back to 13 billion years
Almost back to the beginning of the universe (13.7 billion years old)

34. Observing Galaxies Through Time
The circled galaxy is seen when it was 800 million years old

35. Observing Galaxies Through Time

36. Observing Galaxies Through Time
Due to the expansion of the Universe, collisions between galaxies used to be more frequent

37. Observing Galaxies Through Time
Newly formed galaxies show huge starburst activity
They are warped, bright, chaotic
Little to no structure

38. Quasars Distant, luminous objects that look like stars
These are galaxies that have an active galactic nucleus
An SMBH that is actively devouring material
Forms a very hot accretion disk
Most of them more than halfway to cosmological horizon

39. Late Bloomer
Zwicky 18 resembles newly formed galaxies that we observe with Hubble
A “late bloomer”?

40. Life in the Cluster What kind of galaxy results from a merger?
Rich clusters: few or lots of mergers?
So what kind of galaxy do we find many of in large clusters?

41. Life in the Cluster
Messier 87: The showpiece of dense-cluster evolution!
Gigantic.
Elliptical.

42. Life in the Cluster
Clusters ( galaxies) do contain spirals but they tend to live on the edges.
Galaxy Groups (5-50 galaxies) are often dominated by spirals.

43. Life in the Cluster
Central Dominant Galaxy of a cluster is usually a Giant Elliptical
IT ATE THE OTHER GALAXIES.
Most massive galaxies in the universe.
M87

44. Life in the Cluster Another way for a spiral to become an elliptical:
Clusters have lots of hot gas
Spiral can pass through it. Stars keep going, gas gets left behind
Becomes elliptical if disk hasn’t formed
Could become lenticular if disk has formed

45. Summary A galaxy will likely become elliptical if:
It had no rotation as it formed
It is dense (uses up all of the star stuff on initial collapse)
It undergoes numerous large collisions
A galaxy will likely become a spiral if:
It is rotating
It is less dense (so that it doesn’t use up all of the star stuff during the initial collapse)
It doesn’t suffer too many large collisions

46. Summary Young galaxies are usually irregular starburst galaxies
Earlier, quasars (galaxies with AGN) were more common

47. Summary Galaxy groups tend to be dominated by spiral galaxies
Galaxy clusters tend to be dominated by elliptical galaxies
Galaxies at the center of large clusters are giant ellipticals
Because of many, many collisions

48. Down to Earth
Here we are in a large spiral galaxy on the outer edge of the Virgo Supercluster. We orbit a stable, long-lived star. Make sense?