An irregular galaxy is a galaxy with no rotational symmetry. Irregular galaxies get their odd shapes in many ways. One way irregular galaxies are formed is when galaxies collide or come close to one another, and their gravitational forces interact. Another source of irregular galaxies may be very young galaxies that have not yet reached a symmetrical state. Also, in some irregular galaxies, like M82, young stars eject energetic bubbles gas, giving the galaxy a blobby look.
The stars are bunched up but the patches are randomly distributed throughout the galaxy. Some irregulars have a lot of dust and gas so star formation is possible. Some are undergoing a burst of star formation now, so many H II regions are seen in them. Others have very little star formation going on in them (even some of those with a lot of gas and dust still in them).
Most irregulars are small and faint. The dwarf irregulars may be the most common type of galaxy in the universe (or maybe the dwarf ellipticals are). The estimates of the number of dwarf irregulars and dwarf ellipticals are based on the proportions of these types of galaxies in nearby groups. The dwarf galaxies far away are too faint to be seen and are, therefore, overlooked in surveys of the sky. Perhaps if the dwarf galaxies were brighter, Hubble would have arranged the galaxies in a different sequence instead of the two-pronged sequence. Examples of irregular galaxies are the Large and Small Magellanic Clouds (two small irregulars that orbit the Milky Way).
A peculiar galaxy is an irregular galaxy that has an abnormal shape (neither elliptical, spiral, nor lenticular) and/or has another unusual characteristic, like jets of gas spewing from the nucleus, unusual amounts of dust, low surface brightness, etc. They are probably formed as galaxies collide or have companion galaxies that influence them. Peculiar galaxies are generally smaller and optically dimmer than regular galaxies. They have the designation p or pec at the end of their name. In 1966, the astronomer Halton Arp published a catalogue of 338 of these galaxies in his “Atlas of Peculiar Galaxies.” Arp suggested that peculiar galaxies create stars in intense bursts. Ring galaxies are a type of peculiar galaxy
Type I Irregulars
NGC 55 Despite the name “irregular”, some systematic structure is observed in the type I galaxies. In fact, they are most closely related to spirals, with discs and bulges like their more orderly counterparts. Here the similarity ends, however, as the discs of irregular galaxies show no sign of spiral structure, and the galactic bulges are located away from the centre of the object (“centre” in this case being a somewhat loose definition!). The type I irregular galaxy NGC 55 shown right, and imaged by the Anglo-Australian Observatory, clearly shows the presence of a galactic bulge, to the right of “centre”. NGC 55 is actually quite similar to the Large Magellanic Cloud (LMC) shown at the top of this page, although the similarity is not immediately apparent because we see the LMC from a “face on” perspective, whereas NGC 55 is seen edgeways-on. Also visible in this image are dark patches (dust lanes) and light “spots”, which are luminous nebulae.
These galaxies can be regarded as “primitive” as they are relatively poor in “heavy” elements (i.e. elements higher up the periodic table than Helium). In contrast, galaxies like the Milky Way are rich in these elements, which have been manufactured by stars in a process called nucleosynthesis (see the section on stellar evolution). Irregulars are also very rich in clouds of hydrogen, which, when heated by nearby stars, glow to form the luminous nebulae.
Type II irregulars: galaxies in trouble
Type II irregular galaxies are remarkable, and often very spectacular, objects. They can be formed by several mechanisms. One of the most common is a gravitational interaction with another nearby galaxy.
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 the average spacing between cluster members is much smaller. Collisions are therefore more likely, allowing the creation of the amazing objects that we see through our telescopes.
Astronomers have simulated such collisions between galaxies using powerful computers to try to determine the effect on stars in one galaxy when another passes close by. 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 where star formation is triggered by the compression of gas in the objects. The red regions signify high rates of star formation, whilst blue areas are less intense. (This research was carried out by Chris Mihos and Lars Hernquist of University College, Santa Cruz). The simulation represents a total duration of around 1.5 billion years.
The Antennae galaxies, NGC 4038 and 4039 One of the best known interacting galaxies is called the Antennae. This image of its core (left) 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 (below 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. The Cartwheel galaxy 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. 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.
The Whirlpool Galaxy
Image courtesy of the Isaac Newton Group of Telescopes, La Palma Studies have shown that the collision of two uniform disc galaxies (with no spiral shape) may cause a spiral structure to develop. It is therefore possible that the spiral structure of the larger of the Whirlpool galaxies was caused by the collision.
The Milky Way has two irregular “satellite” galaxies, called the Large and Small Magellanic Clouds. These can easily be seen with the naked eye from the southern hemisphere, appearing as bright patches in the Milky Way. The LMC has a mass approximately one twentieth of the Milky Way’s. The average distance to these clouds is about four times the diameter of our galaxy, and it is this small separation that causes the systems to interact. This interaction results in a connecting stream of gas running from the clouds to our own galaxy, which is detected by astronomers observing at radio wavelengths.