Galactic Evolution: Birth, Life, and Death Notes

What are the two main competing theories of galactic birth?

The density-angular-momentum theory and the collision-merger theory

According to the density-angular-momentum theory, what two factors determine a galaxy's type at birth?

The density and angular momentum of the protogalactic gas cloud

What type of galaxy forms from a high-density, low-angular-momentum protogalactic cloud?

An elliptical galaxy (early-type)

What type of galaxy forms from a low-density, high-angular-momentum protogalactic cloud?

A spiral galaxy (late-type)

What does low angular momentum in a protogalactic cloud imply about the gas motion?

The gas moves randomly with little net rotation

How is the analogy between stars and galaxies used to describe the density-angular-momentum theory?

Just as a star's position on the main sequence depends on its mass, a galaxy's position on the Hubble sequence depends on the density and angular momentum of its protogalactic cloud

Why is galactic evolution considered poorly understood?

Because galaxies are vast, dynamic systems with billions of stars, making their formation and evolution highly complex

What happens if a protogalactic cloud has high density?

Stars form quickly, move in random orbits, and the gas is consumed early, forming an elliptical galaxy

Why do elliptical galaxies tend to be round in shape?

Because the stars formed early on have mostly random, unordered orbits

Why do elliptical galaxies lack ongoing star formation?

Most of the dense gas was consumed early in a burst of star formation

What happens if a protogalactic cloud has low density?

Star formation is delayed until the cloud collapses and local regions become dense enough

What is the effect of high angular momentum on a collapsing protogalactic cloud?

The cloud flattens into a rotating disk perpendicular to its angular momentum axis

How does high angular momentum influence the galaxy's shape?

It leads to a flat, rotating disk structure characteristic of spiral galaxies

What causes clumps in a low-density, high-angular-momentum cloud to form stars?

Collisions and gravitational attraction between denser clumps trigger star formation

What galaxy type results from a cloud with low density and high angular momentum?

A spiral galaxy

What does the law of conservation of linear momentum cause during protogalactic collapse?

It averages out random vertical motions through collisions, reducing motion along the angular momentum axis

How does conservation of angular momentum affect stellar orbits in a collapsing cloud?

It leads to more circular orbits, as collisions smooth out initially elliptical paths

What shape does the collapsing protogalactic cloud take and why?

It flattens into a rotating disk due to reduced vertical motion and increasingly circular orbits

In what direction does the disk form relative to the angular momentum axis?

Perpendicular to the angular momentum axis

What type of galaxy results from a flattened, rotating protogalactic cloud?

A spiral-disk galaxy

When do some stars form during protogalactic collapse?

Early, before the full disk develops, in regions dense enough to trigger star formation

What is a key characteristic of stars that form early in the collapse?

They have low metallicities, reflecting their formation before the interstellar medium was enriched by earlier generations of massive stars

Why do early stars have low metallicity?

Because they formed before supernovae from earlier stars enriched the gas with heavier elements

Where are Population II stars found in a spiral-disk galaxy?

In the galactic halo

How do Population II stars differ in their orbits compared to Population I stars?

They retain more random orbits

Why do Population II stars have more random orbits?

Because they formed early, before the protogalactic cloud collapsed into a rotating disk

Where are Population I stars found in a spiral-disk galaxy?

In the galactic disk

Why do most stars in a spiral-disk galaxy belong to Population I?

Because they formed later, after the protogalactic cloud collapsed and flattened into a rotating disk

Why does low angular momentum lead to an elliptical galaxy?

Because low angular momentum results in unordered gas motion and little net rotation, producing a round shape

Why does high angular momentum lead to a spiral-disk galaxy?

Because it causes the collapsing cloud to flatten into a rotating disk where star formation continues over time

According to the collision-merger theory, what type of galaxy do all galaxies begin as?

Spiral-disk galaxies

How do spiral-disk galaxies grow in the collision-merger theory?

By consuming nearby satellite galaxies

What results from the collision of two large spiral-disk galaxies?

A massive elliptical galaxy

Why do galactic collisions not result in many direct star-on-star impacts?

Because galaxies are mostly empty space and stars are very far apart

If galaxies are not solid, how can they merge and change shape?

Their mutual gravity during a collision disrupts stellar orbits and reshapes the structure

What happens to stellar orbits during a galactic collision?

They become randomized, destroying the organized spiral pattern

What type of galaxy forms when the organized structure of a spiral galaxy is destroyed in a collision?

An elliptical galaxy

What happens to two spiral-disk galaxies after they pass through each other in a collision?

Their mutual gravity slows them, pulls them back together, and causes them to collide repeatedly

How do repeated galactic collisions affect stellar orbits?

Each pass further perturbs stellar orbits, randomizing them and destroying spiral structures

What is the end result of repeated collisions between spiral-disk galaxies?

A single, round-shaped elliptical galaxy composed of stars on random orbits

Why are the terms "collision" and "merger" used interchangeably in galactic evolution?

Because collisions between galaxies ultimately result in mergers into a single, combined galaxy

What real galactic collision is expected in about five billion years?

The collision between the Milky Way and the Andromeda Galaxy

What will the Milky Way-Andromeda merger result in?

A large elliptical galaxy called the Milkdromeda Galaxy

Why were galactic collisions more frequent in the early universe?

Because the observable universe was smaller and galaxies were more crowded

What current behavior of the Milky Way supports the collision-merger theory?

The Milky Way is actively consuming small satellite galaxies

What do groups of stars within the Milky Way reveal about its past?

Some star groups were originally part of smaller galaxies that merged with the Milky Way

How do computer simulations support the collision-merger theory?

They show that when two spiral-disk galaxies collide, they merge into an elliptical galaxy

What pattern do astronomers observe in galactic clusters that supports the theory?

Spiral galaxies are found on the outskirts, while elliptical galaxies dominate the center, often with a giant elliptical at the core

What does the distribution of galaxy types in clusters imply about their evolution?

It suggests that central ellipticals formed through mergers of spiral galaxies over time

What observational contradiction challenges the collision-merger theory of galactic birth?

Elliptical galaxies formed from mergers should show increased star formation due to gas collisions, but they actually show reduced star formation

Why do spiral-disk galaxies have active star formation?

Because they contain abundant interstellar gas that can collapse to form new stars

What would we expect to happen to star formation when two spiral-disk galaxies merge?

We would expect star formation to increase due to colliding gas clouds becoming denser

What do we actually observe in elliptical galaxies that have formed from mergers?

Very little interstellar gas and low levels of star formation

How does this observation contradict the collision-merger theory?

It suggests that mergers don't trigger sustained star formation as expected, indicating the theory may be incomplete

Why do elliptical galaxies challenge the collision-merger theory of galactic birth?

Because they contain very little interstellar gas and have minimal star formation, despite supposedly forming from gas-rich spiral galaxies

What is a starburst galaxy?

An irregular galaxy formed during a collision that experiences rapid, intense star formation, consuming most of its interstellar gas

How does the starburst galaxy model attempt to save the collision-merger theory?

It proposes that spiral mergers first form a starburst galaxy, which quickly uses up its gas, then settles into an elliptical galaxy

What contradiction arises from the starburst modification?

Starburst galaxies produce many young stars, but elliptical galaxies are dominated by old Population II stars with low metallicities

What type of stars dominate elliptical galaxies?

Old, low-metallicity Population II stars

Why is the presence of old Population II stars in ellipticals a problem for the starburst model?

Because it suggests that little to no recent star formation occurred, contradicting the idea of a recent starburst phase

What two factors determine galaxy type in the density-angular-momentum theory?

The density and angular momentum of the protogalactic cloud

What kind of galaxy forms from a protogalactic cloud with high density and low angular momentum?

An elliptical galaxy

What kind of galaxy forms from a protogalactic cloud with low density and high angular momentum?

A spiral-disk galaxy

What kind of galaxy forms from a protogalactic cloud with both high density and high angular momentum?

A lenticular galaxy

Why are lenticular galaxies flat like spiral galaxies?

Because their high angular momentum flattens the cloud into a disk during collapse

Why do lenticular galaxies have little interstellar gas?

Because the high-density gas was used up in an early burst of star formation

Which theory gives a more consistent explanation for lenticular galaxy formation?

The density-angular-momentum theory

Why is the collision-merger theory weak in explaining lenticular galaxies?

Because it provides conflicting or unclear mechanisms for their formation

What is an active galaxy?

A galaxy with an extremely luminous center, known as an active galactic nucleus (AGN)

What distinguishes an active galaxy from a quiet galaxy?

Active galaxies have very luminous centers (AGNs), while quiet galaxies do not

What is an active galactic nucleus (AGN)?

The extremely bright, energetic center of an active galaxy, typically powered by a supermassive black hole

What are the two most common types of active galaxies?

Seyfert galaxies and quasi-stellar objects (QSOs)

How are active galaxies identified?

By their redshift and the Hubble law, which reveals their extreme luminosity and distance

Where are active galaxies located in relation to Earth?

No active galaxies are found within hundreds of megaparsecs from our location; only distant galaxies exhibit AGNs

What might be a common feature of both Seyfert galaxies and QSOs?

Both are types of active galaxies with very bright, energetic centers likely powered by supermassive black holes

Why are Seyfert galaxies and QSOs important in understanding galaxy formation?

They provide insight into the extreme conditions at the centers of galaxies, which may play a role in the evolution of galaxies

Who are Seyfert galaxies named after?

Seyfert galaxies are named after Carl Seyfert

How are Seyfert galaxies different from quasars (QSOs)?

Seyfert galaxies are active galaxies that are less luminous and less distant than quasars

What makes quasars appear star-like?

Quasars appear star-like due to their extreme distance and brightness

Why are quasars often brighter than their host galaxies?

Quasars have extremely luminous active galactic nuclei (AGNs) that can outshine their entire host galaxies

How do the luminosities of active galaxies compare to quiet galaxies?

Active galaxies, including Seyferts and quasars, are thousands of times more luminous than quiet galaxies, especially in the X-ray band

How does the visibility of a Seyfert galaxy's host galaxy differ from that of a quasar's host galaxy?

Seyfert galaxies are less luminous, so their spiral-shaped host galaxies are visible, while quasars are so bright that their host galaxies are obscured

What makes Seyfert galaxies and quasars particularly interesting to astronomers?

Both are active galaxies with extremely luminous centers, offering insight into the extreme environments around supermassive black holes

What key observation helped resolve the mystery of the energy source powering active galactic nuclei (AGNs)?

AGNs show luminosity variations on timescales shorter than one year, implying they must be smaller than one light-year across

Why does the rapid luminosity variation in AGNs suggest a small energy source?

Because nothing can travel faster than light, the rapid changes imply the AGN is smaller than one light-year across, likely comparable to the size of our Solar System

How can astrophysicists estimate the mass of an AGN?

By measuring the Doppler shifts (blueshifts and redshifts) of gas orbiting near the galactic center, which reveals the mass of the AGN

What mass range do AGNs typically have?

AGNs typically contain millions of solar masses packed into a very small space

Why was a supermassive black hole concluded to be the source of AGN energy?

Due to the AGN's compact size and extreme mass, it was concluded that a supermassive black hole must be the energy source, as black holes efficiently convert infalling matter into energy

How much mass is needed to power the intense luminosity of an AGN?

Just a few solar masses per year are enough to explain the intense luminosity of AGNs

What feature of quasars indicates the presence of powerful jets?

Quasars often produce narrow, high-speed jets extending millions of parsecs, which emit strong radio waves and form vast radio lobes when they collide with surrounding gas

How are X-ray binaries similar to quasars?

X-ray binaries can emit jets along the disk's rotation axis, resembling quasars on a smaller scale, and are therefore called microquasars

What is the relationship between jets and radio lobes in quasars?

The jets in quasars collide with surrounding gas, emitting strong radio waves and forming vast radio lobes

What do the immense jets from quasars strongly indicate?

The immense jets from quasars strongly indicate a massive accretion disk around a supermassive black hole

What model for active galactic nuclei (AGN) was developed by astrophysicists?

The model consists of a Solar System-sized accretion disk feeding a central supermassive black hole, where the infalling gas heats to millions of kelvins and emits intense X-rays

How does the accretion disk in an AGN power the active galactic nucleus?

The infalling gas in the accretion disk heats to millions of kelvins, emitting intense X-rays that power the AGN

Why do distant galaxies appear different from local ones?

When observing a galaxy 10 billion light-years away, we are seeing it as it was 10 billion years ago, likely in its early stages of formation

What does the observation of distant galaxies suggest about galaxy formation?

The observation suggests that all galaxies may have been active in their youth, with active galactic nuclei powered by supermassive black holes

What happens to a galaxy as its supermassive black hole consumes its accretion disk?

Over billions of years, the supermassive black hole consumes its accretion disk, and the galaxy transitions from an active to a quiet state

What happens as a supermassive black hole consumes its accretion disk?

As the supermassive black hole consumes its accretion disk, the active galactic nucleus quiets down, and the galaxy gradually transitions from an active to a quiet state

How are quasars and Seyfert galaxies different in terms of their evolution?

Quasars, being the most distant, represent galaxies in their early, highly active stages, while Seyfert galaxies, being closer, are observed later in their evolution when the black hole has consumed much of the accretion disk, making them less luminous