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Galactic Evolution. AST 112. Four Types of Galaxies. Spiral Elliptical Lenticular Irregular Why?. Hubble Sequence. The Hubble Sequence suggests a path of evolution for galaxies. The Hubble Sequence is ABSOLUTELY WRONG! (But good first try.). Formation of Galaxies.

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four types of galaxies
Four Types of Galaxies
  • Spiral
  • Elliptical
  • Lenticular
  • Irregular


hubble sequence
Hubble Sequence

The Hubble Sequence suggests a path of evolution for galaxies.

formation of galaxies
Formation of Galaxies
  • Galaxies begin as a protogalactic cloud
  • We will assume a “top-down” theory of galaxy formation
    • Large, simultaneous collapse
elliptical vs spiral
Elliptical vs. Spiral

What determines whether a galaxy willbecome a spiral or an elliptical?

spiral or elliptical
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
formation of galaxies1
Formation of Galaxies
  • Keep this in mind:
    • Spiral galaxies have gas-rich disks
    • Ellipticals do not
protogalactic conditions
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
spirals halo bulge and disk
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
spirals halo bulge and disk1
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
spirals halo bulge and disk2
Spirals: Halo, Bulge and Disk
  • What causes star formation to ramp up in the disk?
  • Why is the bulge yellow?
  • 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
lenticulars anemic spirals
Lenticulars: “Anemic Spirals”?
  • Lenticular galaxies could be former spirals that have lost their cold gas
    • Used up
    • Lost via collision
globular clusters
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
globular clusters1
Globular Clusters
  • Two possible origins for globular clusters:
    • As the protogalactic disk collapsed, larger fragments formed globular clusters


    • These are the bulges of much smaller galaxies that were absorbed into larger galaxies
galactic collisions mergers
Galactic Collisions / Mergers
  • If the approaching galaxies are moving fast, they may collide and keep going
  • Otherwise, they may collide repeatedly and merge
galactic collisions
Galactic Collisions
  • The Whirlpool Galaxy (Messier 51) is a classic example of colliding galaxies
galactic collisions1
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)
galactic collisions2
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)
galactic collisions3
Galactic Collisions
  • Tidal forces tear the disks apart
  • Orbits get randomized
  • Trails of stars form after collisions
  • Supernovae and winds blow gas away
galactic collisions4
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 multiplelarge-scale collisions?

galactic collisions5
Galactic Collisions

What’s goingon here?

galactic collisions6
Galactic Collisions
  • M81 / M82 have been duking it out for millions of years
  • M82 is undergoing starburst because of this
sagittarius dwarf galaxy
Sagittarius Dwarf Galaxy
  • The Milky Way is colliding with the Sagittarius Dwarf Galaxy!
  • 80,000 LY away (behind the bulge)
collision with andromeda
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
observing galaxies through time
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)
observing galaxies through time1
Observing Galaxies Through Time
  • The circled galaxy is seen when it was 800 million years old
observing galaxies through time3
Observing Galaxies Through Time
  • Due to the expansion of the Universe, collisions between galaxies used to be more frequent
observing galaxies through time4
Observing Galaxies Through Time
  • Newly formed galaxies show huge starburst activity
  • They are warped, bright, chaotic
    • Little to no structure
  • 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
late bloomer
Late Bloomer
  • Zwicky 18 resembles newly formed galaxies that we observe with Hubble
  • A “late bloomer”?
life in the cluster
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?
life in the cluster1
Life in the Cluster
  • Messier 87: The showpiece of dense-cluster evolution!
  • Gigantic.
  • Elliptical.
life in the cluster2
Life in the Cluster

Clusters (50-1000 galaxies) do contain spirals but they tend to live on the edges.

Galaxy Groups(5-50 galaxies)are often dominated by spirals.

life in the cluster3
Life in the Cluster
  • Central Dominant Galaxy of a cluster is usually a Giant Elliptical
  • Most massive galaxies in the universe.


life in the cluster4
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
  • 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
  • Young galaxies are usually irregular starburst galaxies
  • Earlier, quasars (galaxies with AGN) were more common
  • 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
down to earth
Down to Earth

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