Unit 15: External Galaxies
 
 

OVERVIEW

The classification by Hubble of normal external galaxies is described (ellipticals, spirals, SOs, irregulars). Their range of properties (mass, luminosity, gas and dust, age, chemical composition, stellar population, shape and symmetry, rotation) is discussed. Observations of peculiar galaxies are considered, along with possible causes for peculiar galaxies. Results on the clustering of galaxies is reviewed, including evidence for voids, filamentary-like structures and superclusters. The various types of clusters are described. Evidence for the expansion of the Universe is discussed. The redshift-distance relation (Hubble's Law) is discussed. The concept of observing objects at high redshift and looking back in time is discussed. The method of using this and other techniques to study the formation and evolution of galaxies and active galaxies is reviewed.

LEARNING OBJECTIVES

At the end of this unit you should be able to:

1. Describe Hubble's classification scheme for normal galaxies, along with evidence for peculiar galaxies.

2. Discuss the properties of normal galaxies.

3. Discuss what might cause the formation of a peculiar galaxy.

4. Review the evidence for clustering: the Local Group, the Local Supercluster, voids, filamentary structures, etc.

5. Describe the properties of different kinds of clusters.

6. Review the method of determining distances using the Cepheid period-luminosity relationship.

7. Review the evidence for the expansion of the Universe.

 8. Discuss Hubble's Law (a redshift-distance relation) and how it is used to determine distances.

9. Discuss how we observe the formation and evolution of galaxies in the Universe and why this is possible.

 KEY WORDS external galaxy

properties (total mass, luminosity, gas content, dust content, age, chemical composition, stellar population, large-scale shape and symmetry, rotation),

Hubble classification of galaxies

tuning-fork diagram

elliptical

S0

spiral

irregular

peculiar galaxy

collisions between galaxies

mergers between galaxies

interacting galaxies

Local Group

Local Supercluster

clusters

rich clusters

poor clusters

irregular clusters

regular clusters

superclusters

filamentary-like large-scale structures

voids

non-luminous dark matter

expansion of the Universe

redshift

blueshift

Doppler redshift

gravitational redshift

cosmological redshift

Cepheid variable star

Cepheid period-luminosity relation
 
Hubble Law

Hubble constant

 redshift-distance relation

look-back time

active galaxies

quasars

matter falling into massive black holes

age of the Universe

nature of non-luminous dark matter

redshift of galaxy formation

gravitational instabilities
 
 

 WRITTEN NOTES An external galaxy is any galaxy other than our own Milky Way Galaxy.

External galaxies can be identified by looking for large collections of stars which lie outside the boundaries of the Milky Way Galaxy.

The properties of external galaxies vary greatly in terms of their total mass, their total luminosity, the amount of gas and dust present, their age and chemical composition, their stellar population, their large-scale shape and symmetry, and their rotation as a whole.

In 1925 Hubble set up a classification scheme for galaxies. This scheme is still used today. The Hubble classification is primarily based on a galaxy's appearance. In our discussion of normal galaxies we will consider four major types: elliptical, S0 (pronounced S-zero), spiral, and irregular. There are many subtypes of galaxies which we will not consider.

 Types of Normal Galaxies According to Hubble Classification 1. Elliptical Galaxies. Ellipticals range from being giant collections of stars (totaling 1013 solar masses) to dwarf collections of stars (totaling 106 solar masses). Their shapes range from circular to elongated. They have little, if any, gas and dust. Their stellar population is old. They rotate very slowly or not at all.

2. Spiral Galaxies. The Milky Way Galaxy is a good example of a spiral galaxy. Spirals contain a bulge, a disk with spiral structure, and outer parts (halo and corona) which are difficult to study in external galaxies since little light is emitted there. Spirals range in mass from about 1012 down to 109 solar masses. To varying degrees (which is one of the criteria for subtyping), gas, dust, star formation, and spiral structure are present in the disks. Spirals are fast rotators. Some spiral galaxies exhibit a bar-like structure across their disks (called barred spirals), while others don't (called normal spirals).

3. S0 Galaxies. SOs have some of the properties of ellipticals and some of the properties of spirals. For example, they have a bulge and a disk component, but they have little gas, dust or young stars and they show no spiral structure.

4. Irregular Galaxies. A few percent of galaxies show no regular shape and they are classified as irregulars. They have large amounts of gas, sometimes have dust, and exhibit varying degrees of star formation. They do not rotate.

 Peculiar Galaxies About 10% of the external galaxies cannot be classified according to these normal types. These are called peculiars. Some peculiar galaxies look as if explosions occurred in them, but that is probably not what happened. Other peculiar galaxies have rings of stars and gas and dust encircling them in unusual directions. Peculiar galaxies may be the result of collisions or mergers between galaxies. Processes in Galaxies. Hubble's classification scheme for galaxies is normally represented in terms of the so-called "tuning-fork diagram." Originally, it was thought that there was evolution from one galaxy type to another along the tuning-fork during a galaxy's life, but this fell out of favor. More recently, however, studies have shown that colliding spiral galaxies may merge to eventually become S0s or ellipticals (this is important!). During such a merger, gas and dust are converted to stars rapidly (so after a time there are no young stars) and the rotational motion of the spirals is dissipated. Infrared observations of external spiral galaxies allow us to study star formation in them. Extremely large bursts of star formation in galaxies are often initiated by interactions between galaxies that are close to each other.

Models of interacting galaxies (based on the theory of gravity) show that some of the structures seen in external galaxies are explainable in terms of an interaction.

There is evidence for massive black holes (>106 solar masses) in the centers of most external galaxies.

 Clusters of Galaxies The Universe is thought to be 13 to 15 billion years old. At the Universe's present age, galaxies are known to be clustered.

The Milky Way Galaxy belongs to a group of about 25 galaxies known as the Local Group. Its size is about 3 million light years across.

Clusters of galaxies are larger than groups of galaxies. Typically they have hundreds to thousands of members.

The Local Group is a member of the Local Supercluster of galaxies which is about 100 million light years across. The Local Supercluster contains many groups and clusters of galaxies.

Clusters and superclusters are arranged in flattened or filamentary-like large-scale structures.

In between the clusters are voids where very few galaxies are found. The large-scale structure of the voids is bubble-like, and the clusters and superclusters form the boundaries (surfaces) of the voids.

From the motions of the individual galaxies in a cluster, the mass of the cluster can be determined. These studies indicate that as much as 95% of the mass of the Universe is in the form of the non-luminous dark matter.

 The nature of the dark matter is not known. For example, it could be in the form of small black holes, brown dwarfs (objects not massive enough to form stars), large planet-sized bodies, exotic subatomic particles, neutrinos, etc. Types of Clusters When clusters of galaxies are studied one finds that some contain very few galaxies (called poor clusters) while others contain many (called rich clusters). Rich clusters of galaxies often contain a large amount of hot ionized gas that emits X-rays. The mass of the hot gas may exceed the total mass of the individual galaxies which make up the cluster.

If clusters are classified according to the types of galaxies they contain and their shapes, one finds that there are two types.

1. Irregular Clusters are irregularly shaped and contain a mix of all galaxy types (spirals, ellipticals, etc.).

2. Regular Clusters are spherically shaped and they contain only ellipticals. Mergers and interactions in regular clusters are more common. This probably explains why they don't contain spirals.

 The Expansion of the Universe The spectra of external galaxies which do not belong to the Local Group are redshifted. This means that all of the components of a galaxy's electromagnetic spectrum (from the X-rays to radio) are found at longer wavelengths than might be expected. The redshift is most easily measured by observing the wavelengths of emission lines or absorption lines.

The term "redshift" refers to short wavelengths becoming longer. For example, the blue part of a spectrum might be shifted to the red part of a spectrum. Note that if the blue part of a spectrum were shifted to the yellow part of a spectrum, this would also be a redshift, only the amount of the redshift would not be as large.

If an object is moving away from an observer, the observer will see that light redshifted. However, if an object is moving toward an observer, the observer will see that light blueshifted. This is known as the Doppler effect. The Doppler redshift should not be confused with the gravitational redshift.

 The redshifts seen in external galaxy spectra arise because the galaxies are moving away from our Milky Way Galaxy. More distant galaxies are moving away faster.

The expansion of the Universe is what causes galaxies' redshifts. An expanding Universe means that the volume or size of the Universe is increasing with time. Since galaxies occupy a particular part of the Universe, it is natural for an expanding Universe to cause a redshift.

In an expanding Universe, more distant objects will naturally be moving away from the Milky Way Galaxy faster than nearer objects.

The redshift caused by the expansion of the Universe itself is referred to as a cosmological redshift. This is because the study of the large-scale structure of the Universe (and its expansion) is called cosmology.

The cosmological redshift is a very special type of Doppler redshift. This is because distant galaxies are not really moving very fast with respect to the space they occupy. It is the space itself that is growing.

Therefore, large distances in the Universe (on a scale which corresponds to the distances between clusters of galaxies) can be determined by measuring redshifts. The larger the redshift, the more distant the object.

 The Relation Between Distance and Redshift On size scales which are too small (e.g., the distances between individual galaxies), the redshift can not be used to measure distance, because the gravity of an individual galaxy will have an influence on galaxies near it. For example, the spectrum of the Andromeda galaxy (the other large galaxy in the Local Group aside from the Milky Way) is blueshifted, indicating that it is moving toward the Milky Way. The relation between distance and redshift can be determined using bootstrapping methods. For example, the distances to the nearest stars are determined directly from their trigonometric parallax. This allows us to determine the luminosity or intrinsic brightness of different types of stars. In general, by observing objects of known luminosity and applying the inverse square law, the distances to these objects can be determined.

Many intermediate steps of understanding how the luminosity of the most intrinsically bright objects (like Cepheid variable stars, novae, HII regions, supernovae, and galaxies themselves) compare to the luminosity of the least intrinsically bright objects (like nearby stars) are required in order to determine the relation between distance and redshift.

Two of the most important methods developed to do this are the Cepheid period-luminosity relation and the Tully-Fisher relation.

The relation between distance and redshift is known as the Hubble Law in honor of the astronomer who did the most fundamental work in this area. However, there is disagreement on how fast the Universe is expanding, and so the redshift tells us the distance to a very distant galaxy only to an accuracy of about 15% to 20%.

Due to the expansion of the Universe, Hubble's Law tells us that space is growing at a rate such that two points separated by one million light years are, on average, becoming 20 km (12 miles) further apart each second. An astronomer would say this by noting that the Hubble constant is 70 km/sec/Mpc.

This is actually a small amount in one sense: two points in space separated by 1 km (0.6 miles) are, on average, becoming further apart by the thickness of a piece of notebook paper every 1,500 years.

 Look-Back Time and the Redshift Distance Relation

Since the speed of light is finite (300,000 km per second or 186,000 miles per second), astronomers see distant objects as they appeared in the past.

Therefore, by studying distant objects (clouds of gas, galaxies, active galaxies, quasars) which have larger and larger redshift, astronomers can see further and further back into time. Given the distances involved, the study of objects at high redshift allows us to see what the Universe was like when it was less than about 10% of its current age.

Objects at the highest redshifts are often said to be at the "edge of the known Universe." However, this is a very simplistic statement given our knowledge of cosmology (the Universe has no center).

The value of studying objects at different redshifts is to probe how galaxies and other matter in the Universe evolve with time from the origin of the Universe itself.

 The Origin of Galaxies and Active Galaxies When energetic activity such as strong radio, optical or X-ray emission is associated with a galaxy (or a small region in a galaxy) that object is called an active galaxy.

As we look back into time to larger and larger redshifts (e.g., when the Universe was 50% of its current age), it is clear that energetic activity associated with galaxies was much more frequent than it is now. (However, when the Universe was about 10% of its current age there were not many active galaxies.) A quasar (Chapter 32) is thought to be one type of active galaxy.

Activity in galaxies may be caused by collisions between them (mergers) or large amounts of matter falling into massive black holes in the galaxies' centers.

 Galaxy Formation There is no consensus on the details of how galaxies form.

However, it is clear that gravity plays a role in the eventual collapse of gas into a galaxy full of stars.

One theory shows that large masses may naturally undergo gravitational instabilities and collapse to form clusters of galaxies, voids, and individual galaxies.

 Another theory suggests that while gravity is important, it does not provide the trigger to start galaxy formation. It is proposed that supernovae of extremely massive objects which existed in the early Universe (such objects have not been observed) cause gas to be swept up, and then gravity takes over causing galaxies to form.

The nature of the non-luminous dark matter which makes up probably 95% of the mass of the Universe must influence how galaxies form.

Important: Collisions (mergers) between galaxies also influence galaxy formation.

Constraints on how galaxies form come from making observations which pertain to different redshifts (look-back times). Some fundamental questions are: At what redshift can galaxies first form? (That is, when did galaxies first form?)  How does the clustering of galaxies change with redshift (i.e., over time)?  How does the activity in galaxies change with redshift (i.e., over time)?

 Discovery of Distant Normal Galaxies Recently, some of the most distant non-active (normal) galaxies were discovered at very high redshift. They formed when the Universe was only about 10% of its current age (i.e., when the Universe was less than a few billion years old). This type of information constrains theories of how galaxies form and the nature of the dark matter in the Universe. In some theories, with specific types of dark matter, galaxies need more than a few billion years to form.   READING ASSIGNMENT

Chapter 19

HOMEWORK

No H.W., but an in-class extra-credit assignment.