Lecture

Galaxies: Classification, Formation, and Evolution

Introduction

Required Reading
Chapters 17 and 19 (See course calendar for optional sections.)
Review and Thought Questions
Ch. 17, p. 401: 1, 2
Ch. 19, p. 461: 1, 3, 5, 7, 10

At the turn of the century, the Milky Way was our ENTIRE universe! Then came the Shapley-Curtis Debate in 1920 that opened up the possibility that the Milky Way was just one of many galaxies. The "spiral nebulae" (as the galaxies were called then) were actually galaxies like ours, only very distant. The question was resolved in the mid-1920's when Edwin Hubble, using the 100-inch Hooker Telescope at Mt. Wilson, identified Cepheid variable stars in the Andromeda Galaxy, M31.

    The Shapley-Curtis debate makes interesting reading even today. It is important, not only as a historical document, but a lso as a glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly faulty. This debate illustrates forcefully how tricky it is to pick one's way through the treacherous ground that characterizes research at the frontiers of science. Shu, F., 1982, The Physical Universe, An Introduction to Astronomy, (University Science Books, Mill Valley, California) p. 286

Learning Objectives

After listening to the lecture, reading the text and these on-line notes, and completing the lab assigned with this lesson, you should be able to:

  1. List characterizing features of spiral, barred spiral, lenticular, elliptical, and irregular galaxies.
  2. State the evidence that collisions between spiral galaxies may form elliptical galaxies.
  3. Briefly describe how the gravitational contraction of a protogalactic cloud explains the different characteristics of the disk- and spheroidal-populations of stars.
  4. Summarize the "nature" versus "nurture" arguments for why a galaxy ends up being what it is.
  5. Explain what is meant by the "lookback time" and how we use this concept to follow the evolution of galaxies.
    Following Objectives are Optional (depending on the quarter):
  1. Explain what is meant by the Hubble Classification Scheme for galaxies
  2. Given an image of a galaxy, name its Hubble type.
  3. State how the star formation rate differs from an Sa to an Sc spiral galaxy.
  4. Explain why spiral arms are so prominent in spiral galaxies.
  5. Explain what is meant by the spiral arm "wind up conundrum."
  6. List some of the reasons why we think spiral galaxies form bars.
  7. Distinguish between disk-population stars and spheroidal-population stars in terms of their ages, masses, colors, and orbits.
  8. Outline the major advances that have been made in our understanding of galaxy formation and evolution through the research done with the Hubble Space Telescope.
Terms you should know:


spiral galaxy
barred spiral
elliptical galaxy
irregular galaxy
spiral density wave
cannibal galaxies
dwarf elliptical
local group
cluster
supercluster
void
Hubble classification scheme

Concepts Covered

Classifying Galaxies

The method most commonly used to classify galaxies was instigated by Edwin Hubble and is based on the morphology of the galaxy.

Edwin Hubble's discovery of Cepheid variables in the Andromeda Galaxy put to rest any discussion about the Milky Way being the "entire universe." At the time, Hubble was also using the 100-inch Hooker telescope to take thousands of photographic plates of the nebulous objects now known as galaxies. As biologists classify fauna, as paleontologists seek the evolution of life through fossils, as our government seeks to gain the demographics of our society through census, Hubble classified the galaxies based on how they looked so that he could hope to get an insight as to their characteristics, their evolution, their numbers as a function of type.

A good deal of research, observations, and theory have been based upon understanding just why there is a sequence. With the deep field images of the Hubble Space Telescope, and with nearly a century of research since the creation of the "Hubble Sequence," astronomers today question even the relevance of the sequence. The diversity of galaxies both near and far is such that the categories are becoming more and more narrow. One is hard-pressed to stack the galaxies into a few, orderly bins.

Having now questioned the basis, purpose, and relevance of the Hubble sequence, this lecture will introduce you to the various types of galaxies and their defining characteristics. We need this information as we go on to the second part of the lecture: the formation and evolution of galaxies. Just as the classification of stars led us to understanding their birth, life, and death, the Hubble sequence does hold clues to help us understand the birth, life, and death of the galaxies that hold those stars. Reference: Sky & Telescope, May 2000

Types of Galaxies

Spiral Galaxies

  • Formation and Evolution: spiral galaxies may have been the first to form from the huge clouds of material, clumps left in the overall structure of the universe. As a cloud collapsed, it spun up. The rapid spin would force some of the material onto a disk, much like the material in a protoplanetary disk. Initially, the stars would all be made of just hydrogen and helium. The galaxy would become slowly enriched as massive stars went supernova and spilled heavy elements back into the interstellar medium.
  • Milky Way Galaxy: fast or slow collapse? One of the pursuits of today's study of the Milky Way is to determine if it underwent rapid or slow collapse. It is a goal to understand how we got here. Astronomers look at the characteristics of the globular clusters and the disk and bulge to help them solve the puzzle. This question is not answered yet. Astronomers are also observing globular clusters to see if maybe some of them are actually captured satellites.
  • Relationship between "tightness of arms" and "size of bulge": as you study the morphology of the various types of spiral galaxies, look for a relationship between the tightness of the "winding of the arms" and the size of the bulge. Yet another question: Why is there this pattern? What is different about these spirals that produce such a range of characteristics?
  • Why aren't spiral galaxies "all wound up"? If the arms were produced by the rotation of the galaxy, why aren't they tightly wound? The stars at the Sun's distance from the center take about 230 million years to move around the center of our galaxy, and our galaxy is thought to be between 12 and 15 billion years old. A low-mass star, having been around since the start, would have circled over 50 times. Surely the galaxy would be as tight as a spring! But the Milky Way and other spirals are not all wound up. Review the density-wave theory for a possible solution to this conundrum.
  • Spiral Arms: one reason why arms are so noticeable in a spiral galaxy is that they trace massive, very luminous stars that form and die quickly. Inter-arm space is filled with old, low-mass, dim stars that live a long time. There are nearly as much matter in between the arms as in them, but this matter is not nearly as luminous and therefore not nearly as noticeable.

Let's think of a down-to-Earth analogy. You are in a classroom having two exit doors. The classroom is full. Your instructor turns out the lights, asks you all to stand up, spread yourselves in a circle, run through the classroom, out one door, through the hall, and back in the other door. There are so many students that you always jam up as you reach each door. Since you want to get around and finish this silly game, you each get out your lighter and give the persons in front of you hot-seats to hurry them along. Now, the students are spread around evenly, but what would an outsider, viewing this fiasco from above, really see?

Barred Spirals

  • Formation and Evolution: barred spirals probably formed much like the spirals, collapsing early on in the history of our universe. But why do barred spirals have a "bar"? Do they contain much less massive halos? Is there a need to transfer angular momentum from the bulge, and does the bar become an effective way to transfer that angular momentum outwards? Some recent research has shown that barred spirals have a higher central concentration of gas than unbarred spirals. Some Astronomers believe that galaxies may alternate between a barred and an unbarred state; tidal interactions form a bar and then the natural "self-destruction" blends it away. Still another theory states that bars form either when two galaxies collide or when they nearly miss each other as they drift through space.
  • Star Formation: barred spirals are often great "nurseries for star formation. Noticeably luminous regions are often seen at the ends of the bar.

Elliptical Galaxies

  • Formation and Evolution: one of the many burning questions in astronomy has been whether or not ellipticals are "Nature" or "Nurture." Did they form from primordial clouds that perhaps did not have any overall rotational motion? Did they form from mergers of spiral galaxies? Both? If both methods have been present, do those ellipticals that formed initially differ in characteristics from those that formed from mergers?
  • Characteristics: ellipticals have no spiral arms. We may be seeing the same overall shape (football) but just from different view points.

  • Stars: ellipticals contain old, low-mass stars. They have no free gas and dust to form new stars.
  • Colors: ellipticals look reddish-orange because they are made up of mostly old, low-mass stars. There is usually no current star formation, and no excess gas and dust between the stars.
  • Elliptical Sub-Types
    • Cannibals (cD) (eating) the largest galaxies
    • Dwarf Ellipticals (being eaten) the smallest galaxies

Lenticular (S0) Galaxies

  • These are basically spiral galaxies without spiral structure.
  • Interstellar matter has been all used up.
  • Old population stars dominate these galaxies.
  • Lenticular galaxies are of ften indistinguisable from ellipticals.

Irregular Galaxies

  • Galaxies that don't fit anywhere else are tossed into the "irregular" category.
  • Peculiar shapes abound.
  • These galaxies often look disturbed, distorted
  • Unlike lenticulars, irregular galaxies are often places of rampant star formation. A good example is a companion to our Milky Way -- the Large Magellanic Cloud. It is thought that the LMC may be producing new, young globular clusters (a bit of an oxymoron).

The Hubble Sequence of Galaxies

Take a closer look at the various classifications of galaxies, and then visit the SEDS web sites to get more information and even more images for each of the galaxies.

Why is there a Hubble Sequence Anyway?

Good question, actually. In his article, "Beyond the Hubble Sequence" (Sky & Telescope, May 2000), Professor Gregory Bothun addressed the meaning of the classification scheme. Is it representing evolution? intrinsic physical properties? formation?

At first, the sequence was thought to represent the evolution of galaxies. While some thought that the order was from elliptical to spirals, others thought that it was the opposite order (recall: 2 astronomers, 3 opinions). It appears now, however, that the Hubble "tuning fork" is not any kind of temporal evolution since galaxies across types are all approximately the same age. If one takes into account star formation rates, one does see a pattern form with Sa galaxies having low present-day formation rates and Sc having a higher star formation rate. In this scenario, Sa galaxies would have used up the free gas needed to form stars and star formation has decreased.

Consideration of intrinsic physical properties brings out 4 possibilities: the galaxy's total mass, the mass of its bulge, its angular momentum, and the angular momentum of its dark-matter halo. Bothun rules out the galaxy's total mass, questions the bulge mass, and dismisses the angular momentum of the disk as possible determinants of a galaxy's Hubble type. The investigation of the angular momentum of the halo is still in its beginning stages. What is interesting about this theory, however, is that the morphology of a galaxy is the result of something we cannot yet detect, dark matter!

The formation of a galaxy and the environment in which it forms both seem to have an effect on the type of galaxy we see today. The image at the right is taken from the article by Bothun, and illustrates how an elliptical versus a spiral might form. This is no doubt an over-simplified picture since galaxies are clustered and do not form in isolation. With the strong evidence of mergers occurring at all epochs of the universe, what a galaxy is today is a combination of "nature" and "nurture" -- how it was born and how it was "raised." One of the key insights made as to the types of galaxies and the environment was that ellipticals dominate in high density clusters while spirals dominate in low-density environments.

Bothun concluded that "...there is no one dominant physical mechanism that determines the morphological properties of a galaxy. One overall idea has emerged, however: the amount of gas left over after the formation of the first generation of stars is likely the most important parameter guiding a galaxy's subsequent evolution. The problem is that we just don't know what physical processes determine how much gas is left over in the first place!"

Basic Galaxy Properties

Trait Spiral/Barred Spiral Galaxies
(S, SB)		 Elliptical Galaxies (E) Irregular Galaxies (Irr)
Shape and structural properties Highly flattened disk of stars and gas, containing spiral arms and thick central bulge. Sa and SBa galaxies have largest bulges, the least obvious spiral structure, and roughly spherical stellar halos. SB galaxies have an elongated central "bar" of stars and gas. No disk. Stars smoothly distributed through an ellipsoidal volume ranging from nearly spherical (E0) to very flattened (E7) in shape. No obvious substructure other than a dense central nucleus. No obvious structure. Irr II galaxies often have "explosive" appearance.
Stellar content Disks contain both young and old stars; halos consist of old stars only. Contain old stars only Contain both young and old stars.
Gas and dust Disks contain substantial amounts of gas and dust; halos contain little of either. Contain little or no gas and dust. Very abundant in gas and dust.
Star formation Ongoing star formation in spiral arms. No significant star formation during the last 10 billion years. Vigorous ongoing star formation.
Stellar motion Gas and stars in disk move in circular orbits around the galactic center; halo stars have random orbits in three dimensions. Star have random orbits in three dimensions. Stars and gas have very irregular orbits.

Galaxy Formation and Evolution

To some degree, the formation and evolution of galaxies were covered under each of the individual topics above. Here we will take a look at what astronomers' have to say about galaxy formation and evolution based upon the images taken by the Hubble Space Telescope.

Galactic Building Blocks

September 4, 1996 Photo No.: STScI-PRC96-29a

Embedded in this Hubble Space Telescope image of nearby and distant galaxies are 18 young galaxies or galactic building blocks, each containing dust, gas, and a few billion stars. Each of these objects is 11 billion light-years from Earth and much smaller than today's galaxies.

At this distance, the universe was only about 16 percent of its current age. The 18 young galaxies were found within an area about 2 million light-years across, which is about the distance between our Milky Way and the Andromeda galaxy. Some astronomers believe the young objects are the ancient building blocks of today's galaxies, because they are close enough in space to eventually collide or merge with each other. At 2,000 to 3,000 light-years across, each building block is larger than a normal star cluster - as seen in our galaxy - but smaller than a present-day galaxy, which typically is about 30,000 to 100,000 light-years wide. They are located in a small region of sky in the northern part of the constellation Hercules, near the border of Draco. The image covers a diameter that is 13 times smaller than that of the full moon.

Hubble Web Site where you should go to complete the discussion on the formation of galaxies.

Galaxies in their Pre-Adolescent Years

PHOTO CAPTION NO.: STScI-PRC94-39B FOR RELEASE:JULY 24, 1995

This is a NASA Hubble Space Telescope image of a variety of galaxies with irregular and peculiar shapes. These galaxies are so far away that they are seen when the universe was a fraction of its current age. The bright blue regions indicate a rapid episode of star formation. Hubble reveals that these objects once far outnumbered large galaxies like our Milky Way, but have faded or self-destructed by today.

This image is part of a serendipitous sky survey which has been conducted over the past three years by Professor Richard Griffiths and colleagues at the Johns Hopkins University, Baltimore, MD, with a team of astronomers in the United States and Britain.

The survey is one of the key projects for Hubble. Over the past three years the deep survey has uncovered a bizarre variety of shapes and structures in distant galaxies, which previously appeared as fuzzy blobs from ground-based telescopes.

Credit: Richard Griffiths (JHU), The Medium Deep Survey Team, and NASA Faint Irregular Galaxies

Galaxies: Snapshots in Time (A Family Album?)

PHOTO RELEASE NO.: STScI-PR94-52C EMBARGOED UNTIL: 1:00PM (EST) DECEMBER 6, 1994

This sequence of NASA Hubble Space Telescope (HST) images of remote galaxies offers tantalizing initial clues to the evolution of galaxies in the universe.

[far left column] These are traditional spiral and elliptical-shaped galaxies that make up the two basic classes of island star cities that inhabit the universe we see in our current epoch (14 billion years after the birth of the universe in the Big Bang). Elliptical galaxies contain older stars, while spirals have vigorous ongoing star formation in their dusty, pancake-shaped disks. Both galaxies in this column are a few tens of millions of light-years away, and therefore represent our current stage of the universe s evolution.

[center left column] These galaxies existed in a rich cluster when the universe was approximately two-thirds its present age. Elliptical galaxies (top) appear fully evolved because they resemble today's descendants. By contrast, some spirals have a frothier appearance, with loosely shaped arms of young star formation. The spiral population appears more disrupted due to a variety of possible dynamical effects that result from dwelling in a dense cluster.

[center right column] Distinctive spiral structure appears more vague and disrupted in galaxies that existed when the universe was nearly one-third its present age. These objects do not have the symmetry of current day spirals and contain irregular lumps of starburst activity. However, even this far back toward the beginning of time, the elliptical galaxy (top) is still clearly recognizable. However, the distinction between ellipticals and spirals grows less certain with increasing distance.

[far right column] These extremely remote, primeval objects existed with the universe was nearly one-tenth its current age. The distinction between spiral and elliptical galaxies may well disappear at this early epoch. However, the object in the top frame has the light profile of a mature elliptical galaxy. This implies that ellipticals formed remarkably early in the universe while spiral galaxies took much longer to form.

Credit: A. Dressler (Carnegie Institutions of Washington), M. Dickinson (STScI), D. Macchetto (ESA/STScI), M. Giavalisco (STScI), and NASA Galaxies in the Young Universe

QUASARS: the Equivalent of the Teen-Age Years?

Space Telescope Science Institute, Baltimore, MD Press RELEASE: 96-244

Two teams of astronomers are releasing dramatic Hubble Space Telescope images today, which show that quasars live in a remarkable variety of galaxies, many of which are violently colliding. This complicated picture suggests there may be a variety of mechanisms -- some quite subtle -- for "turning on" quasars, the universe's most energetic objects. The Hubble researchers are intrigued by the fact that the quasars studied do not appear to have obviously damaged the galaxies in which they live. This could mean that quasars are relatively short-lived phenomena which many galaxies, including the Milky Way, experienced long ago.

Discovered only 33 years ago, quasars are among the most baffling objects in the universe because of their small size and prodigious energy output. Quasars are not much bigger than Earth's solar system but pour out 100 to 1,000 times as much light as an entire galaxy containing a hundred billion stars. A super massive black hole, gobbling up stars, gas and dust, is theorized to be the "engine" powering a quasar. Most astronomers agree an active black hole is the only credible possibility that explains how quasars can be so compact, variable and powerful. Nevertheless, conclusive evidence has been elusive because quasars are so bright they mask any details of the "environment" where they live.

Here's another multiple-choice quiz with instant feedback (note, not all questions are relevant for every quarter):

Galaxies

(Click on 'reload' if the first question does not appear.)

Relevant Links


Last updated on: