Lectures 21 & 22: Galaxy Evolution

Ancient light

Light that left its source long ago, allowing us to see the universe as it was in the distant past.

Protogalactic clouds

Dense regions forming from gravitational collapse. H and He gases in these clouds formed first stars.

Elliptical galaxy formation Idea 1

Initial angular momentum of protogalactic cloud could determine size of resulting disk. Zero spin —> no rotating gas disk —> no galactic disk

Elliptical galaxy formation Idea 2

Denser protogalactic cloud could cool and form stars before gas settled into disk. All stars might have formed while gas was still falling to the center (like halo stars in spiral galaxy)

Conservation of angular momentum

Gas settled into spinning disk.

Hubble's tuning fork diagram

Classification system for galaxy shapes and types.

Distant red ellipticals

Stars formed early, observed as red due to age.

Tidal Tails

Stretched stars caused by gravitational interactions of galaxies.

Gravitational collapse

Process where matter pulls together to form structures.

Density fluctuations

Small variations in matter density in early universe.

Galaxy collisions

Interactions between galaxies affecting their structures. Triggers bursts of star formation.

Spiral galaxies —> Elliptical galaxy

Two spiral galaxies can merge to make an elliptical galaxy.

Stellar Orbits

Paths stars follow around a galaxy's center.

Shells of Stars

Remnants of past galaxy collisions observed in ellipticals.

Nature vs. Nurture Models

Theories explaining galaxy formation through inherent traits or environment. Nature: Spin, density. Nurture: Galaxy collisions.

Starburst Galaxies

Galaxies forming stars rapidly, using up all gas in less than 1 billion years.

Galactic Wind

Outflow of gas driven by supernova explosions.

Quasars

Bright source in radio wavelength, unresolved point of light in optical wavelength, looks like stars. Powerfully radiate energy over very wide range of wavelengths, indicating that they contain matter with wide range of temps. Most luminous type of active galactic nucleus.

Luminosity Variability

Brightness changes in quasars occurring over days.

Redshifted Spectra of Quasars

Indicates large distances and rapid movement away from Earth.

Quasi-Stellar Object (QSO)

Another name for quasars, indicating star-like appearance.

Galactic Fountains

Gas outflows from galaxies due to supernova activity.

Elliptical Galaxy Characteristics

Random stellar orbits and often found in clusters.

Galaxy Formation Evidence

Observations supporting theories of galaxy collisions.

Hot Gas X-ray Emission

X-rays emitted from gas heated by supernovae.

Energy Source of Quasars

Energy emitted from regions smaller than solar system.

Deep imaging of quasars

Galaxies around quasars sometimes appear disturbed by collisions.

Active Galactic Nucleus

Bright center of a galaxy with high energy.

Radio Galaxy

Galaxies with active nuclei shooting out vast jets of plasma that emits radio waves coming from electrons moving at near light speed. Speed suggests massive black hole.

Cygnus A & Centaurus A

Example of a powerful radio galaxy.

Accretion Disk

Disk of gas spiraling into a black hole.

Energy from black hole

Gravitational potential energy of matter falling into black hole turns into kinetic energy. Friction (collisions) in accretion disk turns kinetic energy into thermal energy (heat). Heat produces photons (thermal radiation). This process can convert 10-40% of mass energy into radiation.

Jets

Thought to come form twisting of magnetic field in inner part of accretion disk.

Supermassive Black Hole

Black hole with mass millions to billions of solar masses.

Energy Ejection

Active galactic nucleus ejects plasma at light speed.

M87

Galaxy with a notable active nucleus and supermassive black hole.

Absorption Lines

Spectral lines indicating gas presence between Earth and quasars.

Expansion of the Universe

Galaxies moving away from Earth at varying speeds.

Black Hole Mass Relation

Central black hole mass relates to galaxy bulge mass.

Dormant Active Nuclei

Inactive supermassive black holes in many galaxies.

Quasar Spectra

Light spectra from quasars revealing cosmic information.

Origin of supermassive black holes

Mass of galaxy’s central black hole is closely related to mass of its bulge. Development of central black hole must be related to galaxy evolution.