Lecture 36:Lecture 36:

Galaxy Formation Galaxy Formation

and Evolutionand Evolution

Galaxy Formation and Galaxy Formation and EvolutionEvolution

How do galaxies form, and how do How do galaxies form, and how do they change over time?they change over time?

Why do we see so many different Why do we see so many different kinds of galaxies? Are their kinds of galaxies? Are their differences a result of ‘nature’ or differences a result of ‘nature’ or ‘nurture’?‘nurture’?

Galaxy Formation and Galaxy Formation and EvolutionEvolution

How do the properties of galaxies How do the properties of galaxies depend on their environment?depend on their environment?

How do galaxies observed at How do galaxies observed at different epochs relate to one different epochs relate to one another?another?

let’s start from the let’s start from the beginning…beginning…

the Universe started out very dense the Universe started out very dense and very smooth (the `Big Bang’).and very smooth (the `Big Bang’).

most of the mass was in the form of most of the mass was in the form of invisible invisible dark matterdark matter, and the rest , and the rest was hydrogen and helium gas. was hydrogen and helium gas.

Initially, the dark matter and gas Initially, the dark matter and gas were distributed in the same way were distributed in the same way throughout space.throughout space.

let’s start from the let’s start from the beginning…beginning…

there were small lumps, where some there were small lumps, where some places were slightly denser than places were slightly denser than othersothers

as the Universe expanded, these as the Universe expanded, these lumps grew larger and denser lumps grew larger and denser because of the force of gravity because of the force of gravity

expanding

collapsing

collapse of density collapse of density perturbationsperturbations

clumps gain angular clumps gain angular momentum from ‘tidal momentum from ‘tidal torques’torques’

gas collapses to form a gas collapses to form a diskdisk

Spiral armsSpiral arms

molecular molecular hydrogen hydrogen in our Galaxyin our Galaxy

The Winding ProblemThe Winding Problem

all stars move at almost the same all stars move at almost the same orbital speed, but stars near the orbital speed, but stars near the center of the galaxy don’t have as center of the galaxy don’t have as far to gofar to go

if spiral arms rotated along with if spiral arms rotated along with the galaxy, they would quickly the galaxy, they would quickly “wind up” and become washed out“wind up” and become washed out

we don’t see any galaxies with we don’t see any galaxies with such tightly wound armssuch tightly wound arms

Spiral Density WavesSpiral Density Waves

instead of being a fixed pattern, instead of being a fixed pattern, spiral arms are like waves moving spiral arms are like waves moving through the oceanthrough the ocean

they are places where the gas has they are places where the gas has “piled up” “piled up”

therefore we see a lot of new star therefore we see a lot of new star formation and dust along the spiral formation and dust along the spiral armsarms

Density waves are like a traffic Density waves are like a traffic jamjam

Star formation in galaxiesStar formation in galaxies

If the gas has enough angular If the gas has enough angular momentum, the disk is stable, and momentum, the disk is stable, and star formation proceeds gradually star formation proceeds gradually over a long time period over a long time period spiral spiral galaxygalaxy

otherwise, the gas may collapse still otherwise, the gas may collapse still further, forming stars rapidly and further, forming stars rapidly and consuming all the remaining gasconsuming all the remaining gas elliptical galaxy or spheroid?elliptical galaxy or spheroid?

A very old galaxy at high A very old galaxy at high redshiftredshift

when galaxies collide…when galaxies collide…

galaxy collisions can trigger galaxy collisions can trigger powerful powerful starbursts, starbursts, leading to star leading to star formation rates 10-100 times formation rates 10-100 times higher than before the collisionhigher than before the collision

if the two galaxies have similar if the two galaxies have similar mass, the disks of both galaxies mass, the disks of both galaxies will be destroyed, leaving behind a will be destroyed, leaving behind a spheroidal remnantspheroidal remnant

M51: a nearby interactionM51: a nearby interaction

M81 groupM81 group

21 cmoptical

elliptical galaxy with elliptical galaxy with ‘shells’‘shells’

perhaps collisions were perhaps collisions were more common in the past?more common in the past?

Galactic cannibalismGalactic cannibalism

‘‘central dominant’ central dominant’ (cD) galaxies in (cD) galaxies in clusters ‘eat up’ clusters ‘eat up’ hundreds of galaxies hundreds of galaxies that fall into the that fall into the center of the clustercenter of the cluster

we can still see the we can still see the ‘partially digested’ ‘partially digested’ lumps of stars!lumps of stars!

Infra-red starburst Infra-red starburst galaxiesgalaxies

galactic winds galactic winds

summarysummary

galaxies form in places where the galaxies form in places where the density becomes large due to density becomes large due to gravitational collapsegravitational collapse

collapsing gas has angular momentum collapsing gas has angular momentum so naturally forms disksso naturally forms disks

two ways to make spheroidstwo ways to make spheroids not enough angular momentum in gasnot enough angular momentum in gas galaxy collisionsgalaxy collisions

collisions also produce starbursts, collisions also produce starbursts, galactic winds, etc.galactic winds, etc.