Lecture 36: Galaxy Formation and Evolution

Galaxy Formation and Evolution How do galaxies form, and how do they change over time? How do galaxies form, and how do they change over time? Why do we see so many different kinds of galaxies? Are their differences a result of ‘nature’ or ‘nurture’? Why do we see so many different kinds of galaxies? Are their differences a result of ‘nature’ or ‘nurture’?

Galaxy Formation and Evolution How do the properties of galaxies depend on their environment? How do the properties of galaxies depend on their environment? How do galaxies observed at different epochs relate to one another? How do galaxies observed at different epochs relate to one another?

let’s start from the beginning… the Universe started out very dense and very smooth (the `Big Bang’). the Universe started out very dense and very smooth (the `Big Bang’). most of the mass was in the form of invisible dark matter, and the rest was hydrogen and helium gas. most of the mass was in the form of invisible dark matter, and the rest was hydrogen and helium gas. Initially, the dark matter and gas were distributed in the same way throughout space. Initially, the dark matter and gas were distributed in the same way throughout space.

let’s start from the beginning… there were small lumps, where some places were slightly denser than others there were small lumps, where some places were slightly denser than others as the Universe expanded, these lumps grew larger and denser because of the force of gravity as the Universe expanded, these lumps grew larger and denser because of the force of gravity

expanding collapsing collapse of density perturbations

clumps gain angular momentum from ‘tidal torques’

gas collapses to form a disk

Spiral arms

molecular hydrogen in our Galaxy

The Winding Problem all stars move at almost the same orbital speed, but stars near the center of the galaxy don’t have as far to go all stars move at almost the same orbital speed, but stars near the center of the galaxy don’t have as far to go if spiral arms rotated along with the galaxy, they would quickly “wind up” and become washed out if spiral arms rotated along with the galaxy, they would quickly “wind up” and become washed out we don’t see any galaxies with such tightly wound arms we don’t see any galaxies with such tightly wound arms

Spiral Density Waves instead of being a fixed pattern, spiral arms are like waves moving through the ocean instead of being a fixed pattern, spiral arms are like waves moving through the ocean they are places where the gas has “piled up” they are places where the gas has “piled up” therefore we see a lot of new star formation and dust along the spiral arms therefore we see a lot of new star formation and dust along the spiral arms

Density waves are like a traffic jam

Star formation in galaxies If the gas has enough angular momentum, the disk is stable, and star formation proceeds gradually over a long time period  spiral galaxy If the gas has enough angular momentum, the disk is stable, and star formation proceeds gradually over a long time period  spiral galaxy otherwise, the gas may collapse still further, forming stars rapidly and consuming all the remaining gas  elliptical galaxy or spheroid? otherwise, the gas may collapse still further, forming stars rapidly and consuming all the remaining gas  elliptical galaxy or spheroid?

A very old galaxy at high redshift

when galaxies collide… galaxy collisions can trigger powerful starbursts, leading to star formation rates times higher than before the collision galaxy collisions can trigger powerful starbursts, leading to star formation rates times higher than before the collision if the two galaxies have similar mass, the disks of both galaxies will be destroyed, leaving behind a spheroidal remnant if the two galaxies have similar mass, the disks of both galaxies will be destroyed, leaving behind a spheroidal remnant

M51: a nearby interaction

M81 group 21 cmoptical

elliptical galaxy with ‘shells’

perhaps collisions were more common in the past?

Galactic cannibalism ‘central dominant’ (cD) galaxies in clusters ‘eat up’ hundreds of galaxies that fall into the center of the cluster ‘central dominant’ (cD) galaxies in clusters ‘eat up’ hundreds of galaxies that fall into the center of the cluster we can still see the ‘partially digested’ lumps of stars! we can still see the ‘partially digested’ lumps of stars!

Infra-red starburst galaxies

galactic winds

summary galaxies form in places where the density becomes large due to gravitational collapse galaxies form in places where the density becomes large due to gravitational collapse collapsing gas has angular momentum so naturally forms disks collapsing gas has angular momentum so naturally forms disks two ways to make spheroids two ways to make spheroids not enough angular momentum in gas not enough angular momentum in gas galaxy collisions galaxy collisions collisions also produce starbursts, galactic winds, etc. collisions also produce starbursts, galactic winds, etc.