Comparison of spiral, elliptical, and irregular galaxies
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As the name suggests, "irregular" galaxies are those which have no specific form, and so the group contains a very diverse selection of objects. In fact, there are two types of irregular galaxy. Type I's are usually single galaxies of peculiar appearance. They contain a large fraction of young stars, and show the luminous nebulae which are also visible in spiral galaxies. Type II irregulars include the group known as interacting or disrupting galaxies, in which the strange appearance is due to two or more galaxies colliding, merging or otherwise interacting gravitationally. Type II's appear to contain a large amount of dust.
Type I Irregulars
In the light of the name "irregular", it is perhaps surprising that some systematic structure is observed in the type I galaxies. In fact, they are most closely related to spirals, and this is a relation which is alluded to in the presence of disks and bulges like their more orderly counterparts. However, it is here that the similarity ends, since the disks of irregular galaxies certainly show no sign of the spiral structure, and the galactic bulges are not even located at the centre of the object ("centre" in this case being a somewhat loose definition !). Examination of the type I irregular galaxy NGC 55 shown above, and imaged by the Anglo-Australian Observatory, clearly shows the presence of a galactic bulge, to the right of "centre". Although it bears little resemblance to the image of the Large Magellanic Cloud (LMC) at the top of this page, the two galaxies are somewhat similar. The similarity is not immediately apparent, because we see the LMC from what may be regarded as 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 may in some ways be regarded as "primitive". 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 form the luminous nebulae when heated by nearby stars).
Type II irregulars: galaxies in trouble
Type II irregular galaxies are remarkable, and often very spectacular, objects. They can be formed by several mechanisms, and one of the most common is a gravitational interaction with another nearby galaxy.
The idea of collisions between galaxies might at first seem unlikely. In general, galaxies are widely scattered, and there exists so much space between one and the next, that only one collision would occur in around 100 times the age of the universe, so that we would not expect to see any of these objects. However, it must be remembered that galaxies often exist in clusters, where the average spacing between cluster members is much smaller. In these circumstances, collisions are far more likely - so we have a mechanism for creating the amazing objects that we see through our telescopes.
Before examining some real examples of such objects, we can look at simulations which astronomers have constructed using very powerful computers. These simulations try to work out the effect on stars in one galaxy when another passes close by, and the results are very interesting. In the above simulation, astronomers have modelled two galaxies of equal mass, in collision. The various shapes which are formed are remarkably similar in appearance to some of the type II irregular galaxies observed in reality.
Another remarkable feature of this simulation is that it shows regions where star formation is triggered by the compression of gas in the objects. Look out especially for the regions which are coloured in red; these signify high rates of new star formation, whilst bluer areas are less intense. (This research was carried out by Chris Mihos and Lars Hernquist of University College, Santa Cruz). The time period from beginning to end of the simulation represents a total timespan of around 1.5 billion years.
One of the most well known of these interacting galaxies is called the antennae because of its distinctive appearance. The image on the left was taken by Richard Bright and has been subsequently enhanced to show the antennae more clearly. This object is made up of two "NGC" (New General Catalogue) galaxies: NGC 4038 and NGC 4039. You may notice that it bears a striking resemblance to some of the stages presented in the simulation given above. The scale of this image is staggering: the distance between the centres of the two cores is approximately 65200 light years, and yet the tips of the antennae (which actually extend beyond the edge of this image are separated by some 500,000 light years. There are many regions of star formation occurring in this object, especially in the core, which has been imaged by the AAO and is shown on here.
Another example of this type of galaxy is M82, a galaxy which displays all the hallmarks of an interaction with a neighbouring object (in this case, M81 at the bottom of the image). Dark lanes of dust are visible, and the blue colour is characteristic of a fairly recent and prodigious episode of star formation (a so-called starburst). M82 is also a very strong radio source.
Both of these images are reproduced courtesy of the SEDS archive.
In its typical, spectacular way, the Hubble Space Telescope imaged a very unusual galaxy known as the Cartwheel (left, courtesy of STScI / NASA). This is another result of the collision of galaxies; 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 galaxy (which is thought to have been a spiral), causing gas and dust to be compressed. This "wave" then moved towards the outside edge of the galaxy, leaving behind it newly formed stars. 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 (see link to Hubble news centre on the links page).
Another well known example of an interacting galaxy is the Whirlpool, or M51, which is shown here on the left (SEDS archive). M51 is of great historical interest, since it was observed by Lord Rosse through his 72" reflecting telescope at Birr Castle in Ireland, in 1845; this was the first time that a spiral character had ever been detected in the "nebulae" as they were called at the time. The object is approximately 37 million light years distant.
M51 is in fact made up of 2 galaxies as this image clearly illustrates. 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 into a spiral structure, although the shape is very difficult to detect, and at present it is more properly classified as an irregular on the basis of its appearance. In fact, studies have shown that if two uniform disk galaxies (i.e. with no spiral shape) are in collision, spiral structures do develop; it is therefore possible that the spiral structure of the larger galaxy exists as a direct consequence of this event.
Also clearly visible on the image is the bar of material connecting the component galaxies; dark lanes of dust are visible where this bar connects with the smaller galaxy, and areas of recent star formation adorn the analogous site in the main object. This bar consists of stars, dust and gas - material drawn off the galaxies by their gravitational interaction.
Bear in mind that our own galaxy is itself subject to gravitational interactions with nearby objects, although this interaction is far less powerful than it is in the cases discussed on the page. The Milky Way has two "satellite" galaxies, called the large and small Magellanic clouds. These galaxies are themselves irregulars, and are very spectacular naked eye sights for observers in the southern hemisphere, appearing as bright patches in the milky way as it runs through the sky. The largest of the clouds (the LMC) has a mass estimated at one twentieth of the Milky Way's. The average distance between the Milky Way and these clouds is roughly four times the diameter of the Milky Way itself, and it is this small separation which causes the systems to interact. This interaction results in a connecting stream of gas running from the clouds to our own galaxy; this stream is detected by astronomers observing at radio wavelengths. Whilst nowhere near as dramatic as the bar seen in images of the Whirlpool galaxy above, this is an analogous structure.