Normal Galaxies

Chapter 24, Galaxies, Building Blocks of the Universe

Chapter 25, Galaxies and Dark Matter

HUBBLE TUNING FORK DIAGRAM

Galaxies are clusters of stars, gas, dust, and dark matter whose emission is dominated by the light from its stars; their spectra are sum of the individual stars. Galaxies serve as markers in our study of the structure of the Universe. They are interesting in their own right, however. Hubble developed the following morphological (based on their appearance) classification scheme for galaxies.

Elliptical Galaxies

Galaxies with no spiral arms and no obvious gas disks and little or no gas and dust. For the most part they contain older, reddish stars with little internal structure. Elliptical galaxies shown a large range in size from Giant Ellipticals (~trillions of stars and ~million light years across) to Dwarf Ellipticals (as few as 1 million stars). Dwarf ellipticals are the most numerous.

Spiral Galaxies

Galaxies with strong spiral arms in disks, with halos, and a bulge. The disk and bulge contain significant amounts of gas and dust. The light from their disks is dominated by younger, more bluish stars. Type Sa, Sb, and Sc are defined by the sizes of the bulges (from large to small) and the tightness of the winding of their spiral arms (tight to loose).

Barred Spiral Galaxies

Similar to spirals except that they have elongated bulges which gives them a barlike appearance. They are classified as SBa, SBb, and SBc in a manner similar to that defined for regular spirals. The Milky Way is classified today as either an SBb or SBc galaxy.

S0 (Lenticular) Galaxies

Seemingly intermediate between E7 ellipticals and Sa and SBa galaxies. S0 galaxies or lenticulars show extensive stellar disks and flattened bulges but lack large amounts of gas and dust and spiral arms.

Irregular Galaxies

More amorphous in shape are the Irregular galaxies. Irr are gas and dust rich and are divided into two classes Irr I and Irr II. Irr I galaxies look like mutated spiral galaxies, the Large Magellanic Cloud (LMC) is an Irr I. Irr are usually smaller than spirals, 108-10 stars; the most common type are the small Dwarf Irregulars.

Schematically, the Hubble Tuning Fork diagram looks like

The preceding is not thought to be an evolutionary scheme. There, however, appears to be evolution between the different Hubble classes as a result of collisions between galaxies. The Milky Way galaxy is usually classified as an SBb galaxy in the Hubble scheme.

Despite the fact that the Hubble Sequence is based only on the appearance of galaxies (morphology of galaxies), several physical properties of galaxies vary smoothly along the sequence. We have, 

little gas and dust <----------------------> lots of gas and dust
mainly Pop II stars <----------------------> Pop I & II stars
Reddish <----------------------------------> Bluish
little ongoing star formation <------------> star formation
large bulge <------------------------------> small bulge
                      tight arms <---------> loose arms

Origin of Elliptical and Spiral Galaxies

The basic idea is that either an elliptical galaxy or spiral galaxy will form depending upon when star formation occurs in the galaxy formation process. Galaxies are thought to form from the collapse of low-density gas clouds. If the gas turns into stars during the early stages of the process, then we essentially have a bunch of BBs collapsing to form the galaxy. Because stars are small and they are far apart, they don't collide in the formation process. This allows the stars to maintain roughly their initial shape and to settle into a roughly spherical form. In this case, they become elliptical galaxies.

If the gas does not turn into stars quickly, then we have a system of collapsing gas clouds. The gas clouds are much larger than are stars and collide much more readily during the formation stage. The collapsing material thus runs into opposing material as it tries to pass through the equatorial plane (for an analogous situation, consider the star formation process). The collisions do not allow the collapsing material to pass through each other and the material is forced to settle into a disk. After the disk forms, star formation begins in earnest and a spiral galaxy is produced.

Galaxies in Collision

Galactic collisions are thought to be common, between 5 % to 25 % of galaxies have suffered mergers. This is well-understood. Galaxies are crowded into the Universe. Our local group of galaxies is around 3 million light years across and contains more than 50 galaxies. So, the average distance between galaxies in the Local Group is a few million light years. The Andromeda galaxy (M31), for example, is around 2.5 million light years away from us. The diameter of M31 is a few 100,000 light years or so. The size of M31 is thus only 20 or so times smaller than its distance from us. For the Universe this is close. In the Solar neighborhood our nearest neighbor, Proxima Centauri is 4.3 light years away. A typical star has a radius of a few light seconds! The size of Proxima Centauri is a few millionths of the distance form us and it is not likely that it will run into us. Galaxies are, in a real sense, crowded in the Universe, almost touching into each other while stars are widely separated and free to move without hindrance.

Cannibalilsm

Galaxies are often times found in rich clusters and because the Universe was smaller in the past, collisions between galaxies are likely to have been more important in the past. This is indicated by the observation that spirals are deficient in the central regions of rich clusters of galaxies and the existence of super-giant elliptical galaxies in the centers of clusters of galaxies. It is thought that the collision between two roughly equal mass spiral galaxies will lead to the formation of an elliptical galaxy while the interaction of a massive galaxy and a low mass galaxy leads to cannibalism where the more massive galaxy eats the less massive galaxy.

Collisions still occur today, however, and may be responsible for driving spiral arms in some galaxies, e.g., M51, the Whirlpool galaxy, and will likely occur in the future. For example, if a large galaxy is around 10 Million light years across and contains several thousand members then, on averae, there are one or two galaxies in every box of size 1 Million light years x 1 Million years x 1 Million years in the cluster. That is, galaxies are separted by hundreds of thousands of light years. A galaxy like the Milky Way has a diamter of 100,000 to 300,000 light years; this indicates that galaxies in clusters nearly overlap and so it is not so surprising that collisions are rather common.

Examples of merging galaxies:

Antennae Galaxy

Cartwheel Galaxy

Mice Galaxy

Milky Way and Andromeda Collision

The Milky Way and Andromeda (distance ~2.5 million light years) are the largest galaxies in the Local Group. Andromeda is approaching the Milky Way and is expected to interact in ~2-3 billion years. The first collision will not disrupt the galaxies but they will re-interact eventually merging in less than 5 billion years.

HUBBLE'S LAW

In the early 1900's (1910's), an important discovery was made by V. M. Slipher which was then followed up by Edwin Hubble. Slipher (and then Hubble) found that all distant galaxies were receding from the Earth. Actually, they found that all distant galaxies showed redshifts which they inferred to mean that the galaxies were moving away from us.

Question: How did Slipher and Hubble Accomplish this Feat?

Spectroscopes and Rainbows

Slipher and Hubble acquired spectra of distant galaxies by passing their light through a dispersive device (a device which separates a blend of light into its constituent colors), a spectroscope. Here, the prism acts as the dispersive device separating the light into its constituent colors.

The bottom picture of a rainbow shows a naturally occurring dispersive device, namely, droplets of water spread the white light from the Sun into its constituent colors.

Spectra and Absorption Lines

To the left are shown spectra of different types of stars. The dark absorption lines are the fingerprints of individual elements. Each pattern is unique to the element. absorption lines of hydrogen, helium, carbon, and other elements are shown. Because distant galaxies are made of stars, one expects that the light from a galaxy will appear as the combination of hundreds of billions of stars! Slipher and Hubble acquired spectra of many galaxies and noticed something quite interesting. They found that the light from distant galaxies was shifted to the red end of the spectrum, the light was redshifted.

Redshift, z

A redshift is a shift in the measured wavelength of some spectral feature to a value greater than its value as measured in a laboratory on the Earth. The redshift, z, is defined as

Here the Greek letter lambda represents the wavelength of the light. The redshift is then the relative change in the wavelength of the spectral line (feature). If the observer and the source are in relative motion, then z will be nonzero. If there is no relative motion then, z = 0.

Extragalactic Distance Ladder

Determining distances to objects in the Univere is difficult (and one of the most important) undertakings in observational astronomy. The basic idea is to find Standard Candles, objects whose properties are known well-enough so that their intrinsic luminosities may be accurately known (or deduced, such as Cepheids from the Period-Luminosity relation). The problem is that a whole set of Standard Candles must be discovered in order to allow us to determine distances to objects within our own Galaxy as well to objects at the furthest edges of the Universe.

Tully-Fisher Method

The stars and gas in the disks of spiral galaxies orbit about the center of the galaxy as evidenced by their rotation curves. Because of the motion of the gas and stars, the emission from the orbiting stars and gas suffers Doppler Shifts. Because the size of the Doppler Shifts,
, tells us how fast the source moves, we can infer information about the mass of the galaxy. This fact is exploited in the Tully-Fisher Effect

The Tully-Fisher Effect must be calibrated. This is done in a series of steps: trigonometric parallax ====> distance to nearby stars ====> distances to nearby clusters of stars (calibrates bright stars and Cepheids in clusters) ====> distances to nearby galaxies (calibrates Tully-Fisher Effect). This bootstrapping leads to what the text refers to as the Distance Ladder.

Tycho Supernova Remnant

Type Ia Supernovas

Starting in the mid-1990s, a promising (and important) standard candle has been used, Type Ia Supernovas (SNe). Type Ia SNe are very bright and can stand out against the backdrop of a galaxy, even though the light from a galaxy is produced by hundreds of billions of stars (see upper left panel). Type Ia SNe are produced by the explosion of an accreting massive white dwarf star as the white dwarf nears the Chandrasekhar Mass Limit (see upper right panel). This allows Type Ia SNe to be observed to large redshift z > 1.

Hubble's Law

Slipher and Hubble demonstrated that the larger the redshift, z, the greater the distance to the object. This can be easily seen in the above where the sources with the smaller redshifts present larger images on the sky. Since we know that angles (apparent sizes of objects on the sky) decrease as you move an object farther away roughly as angle ~ size/distance, smaller appearing objects must be farther away.

Slipher and Hubble found what is referred to as Hubble's Law. Algebraically, we have that

  • Hubble's Law ===> the redshift z is proportional to distance

Following Slipher and Hubble, if we interpret the redshift as due to motion, we can re-state Hubble's Law in its more familiar form. However, note that the redshift measured for distant galaxies is primarily due to the expansion of the Universe, and not to what are called peculiar velocities. An approximation to the redshift, z, driven by the expansion of the Universe is found to be v ~ cz when v is much smaller than cz. Here, c is the speed of light, 300,000 kilometers per second.

The best current estimate for the Hubble constant, where

cz = Ho x D
z is the reshdt, c is the speed of light, D is the distance and Ho is the Hubble constant is
Ho = 22 km per second per Million light years.

Look-Back Time and Distance

Redshift, Look-Back Time, and Distance

Because light travels with a finite speed, c = 300,000 kilometers per second, the light which we receive from distant objects must have left the objects sometime in the distant past. In the table to the right (taken from the text), we show the relationship between the redshift, current distance of the object and the look-back time for the object. Note that the Universe was smaller when the light left the distant objects.

Redshift, z

v/c

Current Distance

Look-Back Time

0.000

0.000

0 Ly

0 y

0.010

0.010

137 MLy

137 My

0.025

0.025

343 MLy

338 My

0.050

0.049

682 MLy

665 My

0.100

0.095

1.35 BLy

1.29 By

0.200

0.180

2.64 BLy

2.41 By

0.250

0.220

3.26 BLy

2.92 By

0.500

0.3850

6.14 BLy

5.02 By

0.750

0.508

8.64 BLy

6.57 By

1.000

0.600

10.8 BLy

7.73 By

1.500

0.724

14.4 BLy

9.32 By

2.000

0.800

17.1 BLy

10.3 By

3.000

0.882

21.1 BLy

12.5 By

4.000

0.923

23.8 BLy

12.1 By

5.000

0.946

25.9 BLy

12.5 By

6.000

0.960

27.5 BLy

12.7 By

10.00

0.984

31.5 BLy

13.2 By

50.00

0.999

40.1 BLy

13.6 By

100.0

1.000

42.2 BLy

13.7 By

infinity

1.000

47.5 BLy

13.7 By

Clustering Scales

Astronomers estimate that the Universe contains ~40 billion galaxies as bright or brighter than the Milky Way. These galaxies extend throughout most of the Universe and tend to clump, that is, they tend to cluster in space.

Solar System ===> ~ 0.5 light day

Galaxy Sizes (M101) ===> 0.1 Mly to 1 Mly

Local Group (several dozen members--Milky Way, Andromeda, M33, ...) ===> ~ 3 Mly

    Pictures of various members of the Local Group of galaxies can be found in the APOD archive.

Clusters (Virgo cluster contains 1,300, perhaps 2,000 members) ===> ~ 10 Mly

    Groups and clusters of galaxies range in size from as few as ten members to ones with thousands of members. Consequently, they typically contain 1014 to 1015 Solar masses of material with most of the material in the form of Dark Matter (up to 80-90 %). Further, even for the normal material, more of the material is in the form of hot (10-100 million Kelvin), intracluster gas than in galaxies! As a note, Dark Matter was first suggested based on observations of the Coma Cluster of galaxies in the 1930s by Fritz ZwickyZ.

    In rich clusters, there is a lack of spirals near their centers (galaxy interactions?) and often times, they contain super-giant Elliptical galaxies (such as M87 in the Virgo cluster) as a result of galactic cannibalism.

Clusters of Clusters (Hercules Super-Clusters) ===> ~ 300 Mly

    Our local group of galaxies is thought to be a part of the larger Virgo Supercluster of galaxy clusters.

Filaments, Great Wall, and Voids ===> 600 & 300 Mly (~ 0.6 and 0.3 Bly)

    The pie slice to the left is a redshitft map. The survey finds the number of galaxies in redshift intervals in given directions. To convert the redshifts to distances, Hubble's Law must be used.
Larger structures have not been detected yet (although, larger structures could have been detected with our present technology).

Structure Formation

There is large scale structure in the Universe. An important problem is the question of Where and when did this structure form? Although the cosmic microwave background radiation (CMBR) is very smooth ---> the Universe was very smooth at the time of formation of the CMBR (~380,000 years after the birth of the Universe), the Universe did show considerable structure even at this time. This means that the Universe started to generate its currently observed structures in the times before the formation of the CMBR (before the Epoch of Recombination). ( Computer simulation of structure formation).

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