Galaxies and Dark Matter

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Questions and Answers

What distinguishes a spiral galaxy from an elliptical galaxy?

Elliptical galaxies contain more interstellar gas and dust.

Spiral galaxies have a prominent disc component. (correct)

Spiral galaxies are always smaller in size.

Spiral galaxies are formed from mergers of smaller galaxies.

Which process primarily leads to the formation of a spiral galaxy?

The collapse of a protogalactic gas cloud. (correct)

The merger of two large elliptical galaxies.

The gravitational collapse of individual stars.

The evaporation of gas clouds in space.

What is a key feature of barred spiral galaxies?

They are composed entirely of older star populations.

They lack a central bulge.

Their spiral arms do not originate from a bar.

They contain extra star formation regions within the bar. (correct)

Which of the following statements is true regarding the morphology of galaxies?

Morphology can provide insights into the formation history of galaxies. Signup and view all the answers

What primarily characterizes giant elliptical galaxies compared to dwarf elliptical galaxies?

Giant elliptical galaxies contain many more stars. Signup and view all the answers

What causes the spheroidal shape of elliptical galaxies?

Successive mergers of galaxies that alter star orbits. Signup and view all the answers

Who originally conceived the classification scheme for galaxy morphology?

Edwin Hubble Signup and view all the answers

Which of the following is NOT a characteristic of spiral galaxies?

Formation primarily through galaxy mergers. Signup and view all the answers

What did Fritz Zwicky observe that implied the existence of unseen matter?

The inferred masses of galaxy clusters were much greater than the total mass of visible stars. Signup and view all the answers

During galaxy cluster collisions, which component does not collide due to being diffuse?

Galaxies themselves Signup and view all the answers

What evidence suggests the existence of dark matter during galaxy cluster collisions?

The observed positional offsets of mass and gas Signup and view all the answers

How are mass maps of galaxy clusters inferred?

Through gravitational lensing of background galaxies Signup and view all the answers

What drives the growth of galaxies according to the information provided?

The formation of dark matter halos Signup and view all the answers

What types of gases formed the first stars according to the content?

Pristine, metal-free gas Signup and view all the answers

Which statement accurately reflects the growth of dark matter halos over time?

They grow through smooth accretion and mergers. Signup and view all the answers

At what redshift did the first stars form in relation to the Big Bang?

At a redshift of 100, about 400,000 years post-Big Bang. Signup and view all the answers

What evidence suggests the presence of dark matter in the Milky Way?

The flat rotation curve of the Milky Way Signup and view all the answers

How does dark matter affect the formation of galaxies?

Through gravitational interactions that shape the structure Signup and view all the answers

What characterizes the size and mass of galaxy clusters?

They can have hundreds to thousands of galaxies and weigh up to $10^{15}$ times the mass of the Sun Signup and view all the answers

What happens to the velocity of galaxies in more massive galaxy clusters?

They orbit faster around the cluster center Signup and view all the answers

What is a major characteristic of dark matter?

It interacts only through gravitational forces Signup and view all the answers

Why are galaxy clusters gravitationally bound?

Galaxies within clusters are attracted to each other through gravity Signup and view all the answers

How is the mass of a galaxy cluster estimated?

Through the velocity dispersion of the galaxies within the cluster Signup and view all the answers

What prevents galaxy clusters from growing larger?

Gravitational interactions that limit their mass accumulation Signup and view all the answers

Study Notes

Galaxy Formation

The learning goal is to explain the various pieces of evidence supporting the existence of dark matter in the universe. It also seeks to clarify the role of dark matter in galaxy formation and the composition of dark matter.
Different galaxy morphologies (shapes) suggest different formation processes.
Spiral galaxies originate from the collapse of a protogalactic gas cloud. This collapse spurred the formation of a rotating disk due to the conservation of angular momentum. Further growth comes from smooth gravitational infall of gas into the dark matter halo. These galaxies exhibit minimal merging compared to other types.

Structure of the Milky Way

The Milky Way displays a structure containing a halo, bulge, disk, spiral arms, and globular clusters.

Local Galactic Group

The Local Galactic group is comprised of numerous galaxies, including the Milky Way, Andromeda, and various smaller dwarf galaxies.

Morphology of Galaxies

Galaxies exhibit diverse morphologies, ranging from bulge-dominated (elliptical) to disk-dominated (spiral).
Hubble's classification scheme categorizes galaxies based on their shape and the presence or absence of a central bar.

Spiral Galaxies

The distinguishing feature of spiral galaxies is their presence of a disk.
Ionized gas regions are associated with active star formation.
Spiral galaxies can have central bars, the ends of which host spiral arms.

Barred Spiral Galaxies

Many spiral galaxies exhibit a central bar structure.
Spiral arms begin at the ends of these bars.

Elliptical Galaxies

Elliptical galaxies are composed solely of a spheroidal component (bulge).
Giant elliptical galaxies possess trillions of stars.
Dwarf elliptical galaxies possess hundreds of millions of stars.

Different Morphologies = Different Formation

Spiral galaxies: Collapse of a protogalactic gas cloud, generating a rotating disc via angular momentum conservation. Continuous growth from smooth infall of gas into dark matter halo. Rare mergers.
Elliptical galaxies: Repeated mergers of galaxies scrambling stellar orbits, resulting in a spheroidal formation.

Formation of Elliptical Galaxies

The stages transition from gas clouds collapsing to form, followed by disk galaxies with surrounding globular clusters.
Some disks collide and form elliptical galaxies or maintain original shape.

Further Evidence for Dark Matter: Galaxy Cluster Velocity Dispersions

Galaxy clusters are massive, gravitationally bound systems of hundreds to thousands of galaxies.
Their mass can be 10¹⁵ times the mass of the Sun.
Galaxies' random orbital velocities about the cluster center correlate with cluster mass estimates.
Astronomer Fritz Zwicky observed that the inferred mass of galaxy clusters significantly exceeds the visible mass of the stars, implying the existence of non-luminous matter.

Further Evidence for Dark Matter: Galaxy Cluster Collisions

Interactions and mergers frequently occur among galaxy clusters.
Collisionless behavior of dark matter is a crucial factor, preventing galaxies from colliding during mergers while gas clouds collide instead.
Observed positional offsets between mass (predominantly dark matter) and gas within colliding clusters support the existence of dark matter.
Mass estimations possible using gravitational lensing from background galaxies. Lensing distortions correlate with cluster mass inferred statistically through this distortion.

Further Evidence for Dark Matter: CMB

Dark matter halo formation drives galaxy growth.
Universe structure correlates with dark matter amount.
The CMB maps unveil universe structure, indicating the total composition.

The Formation of the First Stars

The earliest stars formed within dark matter halos at high redshifts (approximately 400,000 years after the Big Bang).
The gas forming these early stars was mostly pristine, leading to the creation of exceptionally massive stars (around 100 times the mass of our Sun).

The Formation of the First Galaxies

Dark matter halos continue to grow due to gravity, and this growth follows smooth accretion processes and mergers.
Large halos accumulate enough gas to generate numerous stars, leading to the formation of the first galaxies roughly 650,000 years after the Big Bang.

Cosmic Reionization

The first galaxies ignited the universe, ending its dark ages and initiating the ionization process
This second transition from a neutral state to an ionized one is known as cosmic reionization.
The universe remains overwhelmingly ionized.

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Description

This quiz explores the formation of galaxies, focusing on the role of dark matter. Students will learn about different galaxy morphologies and the structure of the Milky Way, as well as the Local Galactic Group. Test your knowledge of these fascinating cosmic structures!