As the name suggests, "irregular" galaxies have no fixed shape, and so the group contains a very wide selection of objects. In fact, there are two types of irregular galaxy. Type I's are usually single galaxies of peculiar appearance. They contain large numbers of young stars, and show the glowing gas clouds that are also visible in spiral galaxies. Type II irregulars include the group known as interacting or disrupting galaxies, in which two or more galaxies are colliding, merging or interacting. Type II's appear to contain a large amount of dust.
Some structure is seen in the type I galaxies. In fact, they are most closely related to spirals, containing both discs and bulges. This is the only similarity though, as the discs of irregular galaxies have no spiral structure, and the galactic bulges are not at their centre. The type I irregular galaxy NGC 55 shown left, and imaged by the Anglo-Australian Observatory, clearly shows the galactic bulge, to the right of "centre". NGC 55 is actually quite similar to the Large Magallenic Cloud (LMC) shown at the top of this page, but the similarity is not immediately obvious because we see the LMC "face on", whereas NGC 55 is seen edge-on. Also visible in this image are dark patches (dust lanes) and light "spots", which are glowing nebulae.
Type I Irregulars contain mostly hydrogen and helium, while galaxies like the Milky Way contain many heavier elements that have been formed in stars (see the section on stellar evolution).
TType II irregular galaxies are unusual, but often spectacular, objects. They are formed when two galaxies interact.
The idea of collisions between galaxies might at first seem unlikely. If galaxies were evenly scattered there would be so much space between them that only one collision would occur in around 100 times the age of the Universe. Galaxies often exist in clusters, however, where they are much closer together than average. Collisions are therefore more likely, allowing the objects we see to be created.
Astronomers have simulated such collisions between galaxies using powerful computers to try to model how the galaxies behave when they interact. You can watch a movie of these simulations by clicking here (courtesy of the Space Movie Archive). In this simulation, astronomers have modelled the collision of two galaxies of equal mass. The shapes that are formed bear a remarkable resemblance to some type II irregular galaxies actually observed.
The simulation also shows regions of star where the gas is compressed and heated. The red regions signify high rates of star formation, whilst blue areas are less active. (This research was carried out by Chris Mihos and Lars Hernquist of University College, Santa Cruz). The simulation represents a total time of around 1.5 billion years.
One of the best known interacting galaxies is called the Antennae. This image of its centre was taken by the Anglo-Australian Observatory. This object is made up of two "NGC" (New General Catalogue) galaxies: NGC 4038 and NGC 4039. The scale of this image is huge, with the two cores separated by a distance of approximately 65,200 light years. Not visible in this picture are two huge streaks of dust and gas which make up the tips of the Antennae. The tips are separated by some 500,000 light years. There are many regions of star formation occurring in this object, especially in the core.
The Hubble Space Telescope also imaged a very unusual galaxy known as the Cartwheel (right, courtesy of STScl/NASA). This is also a product of galactic collision. In this case, a small galaxy (which may be one of the objects on the right of the ring) passed through the middle of the main spiral galaxy, causing the compression of gas and dust. The ‘wave’ produced then moved towards the outside edge of the galaxy, leaving newly formed stars in its wake. It is estimated that billions of stars were created in this collision. You can find out more about this object by reading the original press release.
Another well known example of an interacting galaxy is the Whirlpool, or M51, which is shown on the left (SEDS archive). This image clearly shows the two galaxies that make up M51. The large, face on spiral is NGC 5194, estimated to have a mass of some 100,000,000,000 solar masses. The smaller galaxy (appearing as a bright patch directly above the main object) is NGC 5195. This galaxy seems to be developing a spiral structure, although the shape is very difficult to detect, and at present it is more properly classified as an irregular.
|Image courtesy of the Isaac Newton Group of Telescopes, La Palma|
Click on the links below to find out more about galaxies.
Author: Nigel Bannister
Updated by: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001