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The word "galaxy" brings to mind images of the Milky Way or perhaps the Andromeda galaxy, with their spiral arms and central bulges. These spiral galaxies are what people commonly imagine all galaxies look like. Yet, there are many types of galaxies in the universe and they're not all spirals. To be sure, we live in a spiral galaxy, but there are also elliptical (rounded without spiral arms) and lenticulars (sort of cigar-shaped). There's another set of galaxies that are rather shapeless, don't necessarily have spiral arms, but do have a lot of sites where stars are forming. These odd, blobby ones are called "irregular" galaxies. Sometimes they get lumped in with the so-called "peculiar" galaxies due to their unusual shapes or other characteristics.Hubble Space Telescope's deepest view of the cosmos. There are hundreds of galaxies of all shapes and sizes in this image. NASA/ESA/STScI
As many as a quarter of known galaxies are irregular. With no spiral arms or central bulge, they don't seem to visually share much in common with either spiral or elliptical galaxies. However, they have some characteristics in common with spirals, at least. For one thing, many have sites of active star formation. Some may even have black holes at their hearts.
Formation of Irregular Galaxies
So, how do irregulars form? It seems that they are typically formed through gravitational interactions and mergers of other galaxies. Most, if not all of them began life as some other galaxy type. Then through interactions with each other, they became distorted and lost some, if not all of their shape and features.Hubble Space Telescope looked at a pair of colliding galaxies that are tangling as they interact. The shock of the collision has produced blue streamers that look like clouds. They are actually giant starburst regions, where clusters of hot, massive young stars are being born. In the future, this may end up being an irregular galaxy for a time. NASA/ESA/STScI
Some may have been created simply by passing near another galaxy. The gravitational pull of the other galaxy would tug on it and warp its shape. This will happen particularly if they pass near larger galaxies. This is likely what happened to the Magellanic Clouds, the smaller companions to the Milky Way. It appears that they were once small barred spirals. Because of their close proximity to our galaxy, they were distorted by gravitational interactions into their current unusual shapes.
Other irregular galaxies seem to have been created through mergers of galaxies. In a few billion years the Milky Way will merge with Andromeda galaxy. During the initial time of the collision, the newly formed galaxy (which is nicknamed "Milkdromeda") may look to be irregular as the gravity of each galaxy pulls on the other and stretches them like taffy. Then, after billions of years, they may eventually form an elliptical galaxy.
Some researchers suspect that large irregular galaxies are an intermediate step between the merger of similarly sized spiral galaxies and their eventual final forms as elliptical galaxies. The most likely scenario is that two spirals either mingle together or simply pass very near each other, resulting in changes to both partners in the "galactic dance".
There is also a small population of irregulars that don't fit into other categories. These are called dwarf irregular galaxies. They also look a lot like some galaxies as they existed early in the history of the universe, without a definite shape and looking more like a "shred" of a galaxy. Does this mean that the irregulars that are observed today are more like early galaxies? Or is there some other evolutionary path that they take? The jury is still out on those questions as astronomers continue to study them and compare younger to the ones they see that existed many billions of years ago.
Types of Irregular Galaxies
Irregular galaxies come in all sorts of shapes and sizes. This is not surprising considering they may have started out as either spiral or elliptical galaxies and simply distorted through the merger of two or more galaxies, or perhaps by nearby gravitational distortion from another galaxy.
However, irregular galaxies can still all into a number of sub-types. The distinctions are usually associated with their shape and features, or lack thereof, and by their size.
Irregular galaxies, particularly the dwarfs, are still not well understood. As we've already discussed, their formation is at the heart of the issue, particularly as we compare old (distant) irregular galaxies to newer (nearer) ones.
Irregular I Galaxies (Irr I): The first sub-type of irregular galaxies are known as Irr-I galaxies (Irr I for short) and are characterized by having some structure, but not enough to classify it as a spiral or elliptical galaxies (or any other type). Some catalogs break this sub-type down even further into those that exhibit either spiral features (Sm) - or barred spiral features (SBm) - and those that have structure, but not structure associated with spiral galaxies such as a central bulge or arm features. These are therefore identified as "Im" irregular galaxies.
Irregular II Galaxies (Irr II): The second type of irregular galaxy does not have any feature what so ever. When they were formed through gravitational interaction, the tidal forces were strong enough to eliminate all identified structure of what galaxy type it may have been previously.
Dwarf Irregular Galaxies: The final type of irregular galaxy is the dwarf irregular galaxy mentioned above. As the name suggests, these galaxies are smaller versions of the two sub-types listed above. Some of them contain structure (dIrrs I), while others have no trace of such features (dIrrs II). There is no official cut-off, size-wise, for what constitutes a "normal" irregular galaxy and what is a dwarf. However, the dwarf galaxies tend to have low metallicity (that means that they are mostly hydrogen, with low amounts of heavier elements). They may also form in a different way than normal-sized irregular galaxies. However, some galaxies currently classified as dwarf Irregulars are simply small spiral galaxies that have been distorted by a much larger nearby galaxy.
Edited and updated by Carolyn Collins Petersen.