Galactic Dynamics

Spring Term 2010


Galaxies are collections of stars, gas and dust bound together by their mutual gravity, typically spanning a mass range from 100 million to 10 billion times that of the sun.

Galaxies comprise the basic building blocks of the Universe and can be seen a good fraction of the way across it . When galaxies were born and how they evolved is currently a very active topic. At present, the Universe is thought to have been created in the Big Bang, in which space and time were in a simple hot energetic, state, about 14 billion (14 x 109 ) years ago.

Five steps from the Big Bang to Galaxies

Step 1) During the first 10-43 seconds the four fundamental forces are unified (although no complete physical description of this era yet exists). Temperature 1032 Kelvin. 10-43 seconds defines the time when gravity splits from the other forces (weak, strong and Electro-magnetic).

Step 2) Up to 10-35 seconds, quarks and anti-quarks dominate the Universe. The strong force separates from the weak and electromagnetic forces. Temperature drops to 1027 Kelvin. At 10-12 seconds the four forces become distinct.

Step 3) At 0.01 seconds, electrons and positrons form as the temperature drops to 1011 Kelvin. After 1 second, the Universe becomes transparent to neutrinos, which from now on hardly interact further with matter.

Step 4) At three minutes after the big bang, the temperature has reached 109 K, protons and neutrons combine to form what will become the nuclei of elements mostly H and He).  After 300,000 years the temperature has dropped to 3000 K and the electrons are captured by nuclei to form neutral atoms.  The Universe becomes transparent to light (photons stop interacting with free electrons) resulting in the formation of the Cosmic Background Radiation.

Step 5) After 1 billion years, the temperature is 20 K and galaxies and stars have begun to form via gravitational contraction of over-densities in the initial Universe.  Af a few billion years our Galaxy forms, at about 10 billion years after the Big Bang the Sun and Earth form. After 15 billion years we reach the present and a background temperature of about 3 K.

Adapted from "Steven Hawking's Universe"

Encyclopedia of Applied Physics, Vol. 23 (Page 47 - 81), 1998 WILEY-VCH Verlag GmbH, ISBN: 3-527-29476-7


About 300,000 years after the Big Bang, there was the era of recombination in which protons and electrons combined to form neutral Hydrogen.  At this point, baryonic matter in the Universe consisted of about 75% Hydrogen and 25% Helium (by mass), with some small amounts of heavy elements (elements starting from Lithium). The distribution of this material was very close to, but not quite, uniform. These slight over- and under-densities were observed for the first time by the  COBE satellite (launched in 1989) and amount to only a few parts in 100,000. The variations were mapped out over the whole sky, shown in the next figure, on scales of about 7 degrees. The successor mission, WMAP, mapped the sky to much better resolution (about 0.25 degree), and has confirmed that a new element in the basic mix -- "dark energy".

Encyclopedia of Applied Physics, Vol. 23 (Page 47 - 81), 1998 WILEY-VCH Verlag GmbH, ISBN: 3-527-29476-7

WMAP image of the temperature perturbations on the sky. Source :

After recombination, the Universe entered a period called the "Dark Ages", until gravitational attraction had operated on very slight overdensities in the matter distribution, leading to the formation of light emitting stars and galaxies. The Universe was optically observable again!

Exactly how stars and galaxies formed, when the process started and how long it took is currently a major area of research.  A simple picture runs like this: about 1 billion years after the big bang the first star forming regions, conglomerates of perhaps 106 to 109 solar masses began to develop. Over the next several billion years, most of these merge to form larger units or are partially destroyed by the energetic supernovae which develop as a natural part of star formation. Within a few billion years most of these have developed into stable configurations of stars and gas and are recognisable as "galaxies".

Snapshots of galaxy morphology (or appearance) over several billion years, as seen by Hubble Space Telescope.
Source :

Probably most of the "mature" galaxies we see around us now originate from this epoch 10-12 billion years ago.  Classically (20 years ago) it was considered that most or all galaxies had formed at this time. However, many studies now show that galaxy formation is an on-going process, but at a much reduced rate in the present universe compared to about 10 billion years ago. The plot below is a recent estimate of the rate at which stars have formed (in galaxies) over the lifetime of the Universe.


The distribution of galaxies in space around us in the nearby Universe is currently a very active research area. Surveys have revealed a very detailed foamy structure in which galaxies are found along "walls" surrounding large "voids" which are relatively galaxy free. The precise form of this distribution places interesting constraints on the type of Universe we inhabit --- its total mass, expansion rate and ultimate fate.

A map of the nearby universe and south poles of the Milky Way. 9325 data points each represent a galaxy with a measured redshift, the most distance being about 400 Mpc from us. Regions outside the wedges were not surveyed because of obscuration by dust in the Milky Way. Galaxies are arranged in sheets or filamentary structures of great size, defining regions which contain few galaxies ("voids"). The "great wall" is a particularly well studied nearby feature og galaxy space --- it appears in the upper (Northern sector) stretching across the entire survey. A similar wall is seen in the South.

Encyclopedia of Applied Physics, Vol. 23 (Page 47 - 81), 1998 WILEY-VCH Verlag GmbH, ISBN: 3-527-29476-7
Margaret J. Geller, John P. Huchra, Luis A. N. da Costa, and Emilio E. Falco, Smithsonian Astrophysical
Observatory © 1994

Surveys like the one above map the local Universe by measuring the position in 3-D space of galaxies up to a few hundred Mpc away. The observable Universe has a diameter of order 10 Gpc. To find and observe galaxies at large distances (and therefore in the early Universe) requires large or special telescopes. The Hubble Space Telescope  has obtained one such view in a very narrow "pencil beam" (unlike the survey above which covers a large part of the sky) of the  Hubble Deep Field . Galaxies are visible in this image more than halfway across the visible Universe.

The Hubble Deep Field , showing some of the circa 100 billion galaxies in the Universe.

 Source :
© 1997 The Association of Universities for Research in Astronomy, Inc.

At these early times in the evolution of the Universe galaxies look quite different to what they do today, which is what we'll look at next.

Galaxy Morphology

Quite a number of schemes have been developed to classify galaxies by their shape (or morphology). These schemes are really only descriptive, and have become less useful in very recent times as the Hubble Space Telescope  has revealed that many (possibly most) distant galaxies are chaotic sites of star formation which defy simple descriptive classification.

Nearby galaxies can be very usefully classified as spiral, elliptical or irregular.

The following text is Copyright © 1997, Barbara Ryden, Ohio State University.

Spiral Galaxies

Spiral galaxies have rotationally flattened disks, and contain moderate amounts of gas and dust.

Our own galaxy is a spiral galaxy; so is the Andromeda Galaxy (M31), and the Whirlpool Galaxy (M51).
AAT17 - NGC 2997: Sc galaxy, seen face-on.
INT4 - NGC 892: Sb galaxy, seen edge-on.

© Anglo-Australian Observatory

All spiral galaxies have

      flat, rotating disks
      central bulges
      gas and dust in the disk
      star formation in spiral arms

The class of spiral galaxies is further subdivided into classes Sa, Sb, and Sc. Sa galaxies have big central bulges, tightly wound spiral arms, and a relatively small amount of interstellar gas. Sc galaxies have small bulges, loosely wound spiral arms, and a relatively large amount of gas. Sb galaxies are intermediate between Sa and Sc. Our own galaxy is an Sb galaxy, as is M31. M51, which has a smaller bulge, is an Sc galaxy.

A ``subspecies'' of spiral galaxy is the class of barred spirals. In a barred spiral galaxy, the spiral arms wind away from an elongated central bar rather than from a spherical central bulge. A picture is worth a thousand words: look at the examples of barred spiral galaxies in the Galaxy Gallery. Aside from the presence of the central bar of stars, barred spirals are very similar in their properties to ``normal'' spirals. (Although the textbook refers to spiral galaxies without bars as ``normal'' spirals, this name is a misnomer. The number of barred spirals is roughly equal to the number of spiral galaxies without bars - neither type is more ``normal'' than the other.)
AAT 97 - NGC 3351: SBb galaxy
AAT 8 - M83: SBc galaxy

© Anglo-Australian Observatory

A ``subspecies'' closely related to spiral galaxies is the class of S0 galaxies (pronounced ``S zero''). S0 galaxies have

      flat, rotating disks
      central bulges
      very little gas and dust in the disk
      NO spiral arms

The fact that S0 galaxies lack both gas and spiral arms has led to the hypothesis that interstellar gas is necessary for spiral arms to form.

Elliptical Galaxies

Elliptical galaxies are slowly rotating ellipsoids, and contain little gas and dust.

Elliptical galaxies are actually the most common type of galaxy. Randomly select a large number of galaxies. On average, you will find:

      60% elliptical
      20% spiral
      20% irregular

Elliptical galaxies contain very little gas and dust, and the gas that is present is very hot and diffuse. Consequently, there is no current star formation in elliptical galaxies. The stars in elliptical galaxies are old (termed "Population II") stars. Unlike the disks of spiral and S0 galaxies, elliptical galaxies are NOT rotating rapidly.

Elliptical galaxies were given their name because they appear elliptical on the sky. Elliptical galaxies are subclassified according to how flattened they appear. Let ``a'' be the diameter of an elliptical galaxy along its longest dimension (its major axis, in the language of mathematicians). Let ``b'' be the diameter along the shortest dimension. The elliptical galaxy is then given the label ``En'', where ``n'' is a number given by the formula:

n = 10(a-b)/a

For instance, a galaxy which appears circular has a=b, and hence n=0. Galaxies which appear circular are thus given the label ``E0''. Slightly more flattened galaxies are labeled ``E1'', and so forth, up to the most flattened elliptical galaxies, which are called ``E7''. The galaxy M87 (displayed in the Galaxy Gallery) is nearly circular, and hence is classified as E0.

Note that the above classification scheme for elliptical galaxies is fairly lame, since it relies on the projected shape of the galaxy not on the true three-dimensional shape. The projected shape of an object depends on the angle from which we view it, which is totally accidental. Unfortunately, it's impossible to determine the true 3-D shape of an elliptical galaxy - we can't, for instance, take a million-parsec-long journey in order to circumnavigate it and view it from every angle.

Since elliptical galaxies appear elliptical in the sky, they must be ellipsoidal in three dimensions. Just as an ellipse is a distorted circle, an ellipsoid is a distorted sphere. There are three types of ellipsoid:

      1 - a stretched sphere (like a hot dog)
      2 - a squashed sphere (like a hamburger)
      3 - a sphere squashed in one direction, stretched in another (like a baking potato)

Elliptical galaxies are probably a mix of all three shapes.

Elliptical galaxies have a very large range of sizes. Both the largest and smallest galaxies in the universe are elliptical. Giant elliptical galaxies contain 1 trillion stars or more. Dwarf ellipticals contains 10 million stars or less.
The images show the giant elliptical galaxy M87 (which is about 200 kiloparsecs in diameter, 10 times the Mily Way) and the dwarf elliptical galaxy Leo II (which is about 1 kiloparsec in diameter, and contains so few stars you can see right through it). Of the two dozen galaxies closest to us, a dozen are dwarf ellipticals.

© Anglo-Australian Observatory


Irregular galaxies are irregular in shape and dynamics, and contain lots of gas and dust.

Irregular galaxies contain lots of gas and dust; in extreme cases, as much as 90% of the total mass of an irregular galaxy is in the form of interstellar gas and dust. As a consequence, irregular galaxies contain copious star formation. The star formation is patchy (tending to occur in clusters), as is the distribution of dust. Therefore, irregular galaxies are given their characteristic irregular, patchy, raggedy appearance. The Large and Small Magellanic Clouds, about 50,000 parsecs away from our own galaxy, are examples of irregular galaxies.
UKS 14 - The Large Magellanic Cloud
UKS - 17 The Small Magellanic Cloud.

© Anglo-Australian Observatory

The class of irregular galaxies tends to be something of a ``garbage bin'' classification. If a galaxy is extremely bizarre looking, it tends to be labeled as an irregular.

Galaxy Classification Schemes

Edwin Hubble, who first made the classification scheme for galaxies, arranged them in a ``tuning fork'' diagram (seen below), with ``normal'' spirals on the upper tine of the fork, and barred spirals on the lower tine. As you go from left to right on the tuning fork diagram, you move in the direction of increasing amounts of gas & dust, and more vigorous star formation. (When irregular galaxies are placed on the tuning fork diagram, they are placed at the far right).

Above text : copyright © 1997, Barbara Ryden, Ohio State University


The gas, stars and dust content of the galaxy types mentioned above may not be the final word. For example, one recently discovered spiral galaxy consists almost entirely of gas with a very tiny central region containing stars. It is worth keeping in mind that there are probably still a few surprises out there.

This image shows the radio and optical maps overlayed for the galaxy NGC 2915. Blue is the radio map, yellow is the optical. This is probably a "failed" galaxy which has not (yet) turned from gas to stars.

Source, and Gerhardt Meurer (Johns Hopkins University)

Further superb images of Galaxies sorted by type can be found at the Anglo Australian Observatory  (