5.1.3 The Formation and Evolution of Galaxies

Objectives

- distinguish between the top-down and bottom-up models of the formation of galaxies

- explain the monolithic collapse model of galaxy formation

- describe the hierarchical model of galaxy formation

- identify the role of supermassive black holes and dark matter in galactic evolution

- discuss the evolution of galaxies through galaxy mergers

Top-Down Model

States that galaxies formed from a vast cloud of gas and dust.

This cloud broke into smaller clouds.

Gravity compressed these clouds into galaxies.

Bottom-Up Model

States that instead of a vast cloud of gas, galaxies formed from small clumps of matter.

Gravity compressed these clumps into dwarf galaxies that merged to form larger galaxies.

This model is accepted by most astronomers today!

Monolithic Collapse Model

A top-down model.

States that galaxies formed quickly and all at once.

"Monolithic" = all galaxies formed from a common initial material or source.

This model fails to explain:

- the variety of galactic shapes and sizes

- the presence of supermassive black holes in the centers

- the large-scale structure of the universe.

Hierarchical Model

A bottom-up model.

States that galaxies formed from clumps of matter that gained mass and then grew into larger structures.

Smaller galaxies combined to form larger galaxies.

This is widely accepted because observational evidence supports it.

Formation of Different Types of Galaxies

Spiral Galaxies - stars form in central disk, then fans out. Older stars are near the core, and younger stars are on the arms.

Elliptical Galaxies - when most stars form at the same time, the galaxy is a disk shape. Spiral galaxies can also merge and become elliptical galaxies.

Irregular Galaxies - protogalaxies, extremely old and eventually merge into other types of galaxies.

Lenticular Galaxies - early and disk-shaped, merged into other galaxy types

Dark Matter

Dark Matter - a mysterious kind of invisible matter, does not emit electromagnetic waves, cannot be seen or studied.

Cold Dark Matter model - dark matter mixed with visible matter when atoms formed after the Big Bang and later helped in the formation of larger structures.

Black Holes

According to the hierarchical model, supermassive black holes might have formed from the accretion of material at the center of a protogalaxy or by the direct collapse of a gas cloud due to gravity.

These SMBH skip stages of star formation and were formed in a short time (a few million years).

Energy and radiation from SMBH heat up and push out gas, which stops galaxies from collapsing and creating new stars. This affects the shape and structure of galaxies.

Merging Galaxies

Galaxies are continuing to merge, even today! They are moving through space. One galaxy's gravity will pull on another galaxy's, and eventually collide.

This can cause new stars to form, it can change the orbit of stars, and it can fling stars into space.

The Milky Way and Andromeda are expected to collide in 4 to 5 billion years.

Great job!

Today, you learned:

- the difference between the top-down and bottom-up models

- the monolithic collapse model of galaxy formation

- the cold dark matter (CDM) model of galaxy formation

- the role of dark matter and supermassive black holes in galactic evolution

- how galaxies merge