Galaxies and Dark Matter

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.

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!