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

Elliptical vs. Spiral What determines whether a galaxy will become a spiral or an 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 Galaxies Keep this in mind: Spiral galaxies have gas-rich disks Ellipticals do not

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 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 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 Disk What causes star formation to ramp up in the disk? Why is the bulge yellow?

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

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

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 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

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 The Whirlpool Galaxy (Messier 51) is a classic example of colliding galaxies

A Galactic Merger

Another galactic merger.

A Galactic Collision

Cartwheel Galaxy

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

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?

Galactic Collisions What’s going on here?

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

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

Sagittarius Dwarf Galaxy

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

Galactic Collisions: Simulation

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 Time The circled galaxy is seen when it was 800 million years old

Observing Galaxies Through Time

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

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

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

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

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 Cluster Messier 87: The showpiece of dense-cluster evolution! Gigantic. Elliptical.

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

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

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

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

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

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?