Peculiar Galaxies: The Oddballs of the Universe

The peculiar galaxy M82.
The galaxy M82 is a good example of a peculiar galaxy; it is also classified as an irregular galaxy. It has undergone huge bursts of star formation and appears to have a huge cloud of gas surrounding and extending out from its core. NASA, ESA, & Hubble Heritage Team (STScI/AURA) Ack: J. Gallagher (Univ. of Wisconsin), M. Mountain

Exploring Peculiar Galaxies

There is a wide range of galaxy types out there in the universe. Some are spiral galaxies, like our own Milky Way. Others are elliptical galaxies, while others are called "irregulars".  Back when astronomer Edwin Hubble was first classifying galaxy shapes, these were the main types. But, as astronomers refined the classification of galaxies over the years, they began to notice oddly shaped ones that didn't seem to fit in any category. So, they called them "peculiar" galaxies. Not only do they have strange shapes, but they also have other features that distinguish them from other galactic types. So, the generally accepted definition of "peculiar galaxy" is one that has something unusual about its size, shape, or composition. 

Now, that being said, peculiar galaxies do have things in common with several different galaxy types, such as size and the types of stars they contain. They may have an active nucleus, as many others do, which indicates the presence of a supermassive black hole that's ejecting material into the intergalactic medium.

Formation of Peculiar Galaxies

Less than 500 galaxies are official classified as peculiar, and not all catalogs agree on their classification. With the advent of very deep surveys of the cosmos taken by such observatories as the Hubble Space Telescope, astronomers can see many more strange and peculiar galaxies in the very distant universe. So, there are many more to study and understand. 

The prevailing wisdom about these objects is that they are the result of recent galaxy mergers  between two or more spiral or elliptical galaxies. We know that mergers are the primary way that galaxies grow and mergers are seen throughout the history of the more recent universe.

During collisions, the galaxies involved experience a huge spike in star formation or ignition of the nucleus of one or both galaxies. This is a common property of peculiar galaxies as well and is another bit of evidence pointing to mergers being part of the history of peculiars.

Difference Between Irregular and Peculiar Galaxies

The difference between an irregular and peculiar galaxy is not entirely clear. In fact, some catalogs differ in opinion about the actual classifications of the two types. In theory, while peculiar galaxies are the result of a recent merger of two "normal" galaxies, it may be that irregular galaxies are created simply by gravitational interactions (but not collisions) between galaxies.

For this reason, irregular galaxies would be expected to be smaller and distorted by the nearby presence of a much larger galaxy. The Large and Small Magellanic clouds (in the Southern Hemisphere skies) are examples of irregular galaxies.

The merger of two galaxies, like the expected collision of the Andromeda galaxy with the Milky Way galaxy, could lead to a peculiar galaxy in a few billion years. However, this prediction is up for debate, as many researchers believe that an irregular galaxy would be formed initially, not a peculiar. 

Snapshot of a Galaxy Merger

Here's another way to think of peculiar galaxies: they may be snapshots of galaxy mergers in the first millions of years after the collision. That's when the resulting galaxy is in an active state and still maintains some common features of the host galaxies.

Then, over time, as the galaxies become more entwined, and the activity level drops they take on a more irregular appearance. Finally, some theories suggest that collisions between some galaxies, such as the merger of two similarly sized spiral galaxies, will eventually lead to the production of an elliptical-type galaxy.

However, some challenge even this, arguing that the classification of irregular galaxies should be limited to those galaxies that have no distinguishing features what so ever and also have small size, perhaps a hundred or a thousand times smaller than normal spiral and elliptical galaxies (the Magellanic Clouds, again, being prime examples). And, therefore, every other galaxy that exhibits, well, peculiar properties should be formally classified as a peculiar galaxy.

As yet, the re-classification based on size alone has not been widely accepted. However, it seems logical, at least to me, that the distinction be made on activity and features, and not simply on size. This especially holds since it can be difficult to identify the cause of the distortions (mergers versus simply gravitational distortion). It's clear there's a lot of work to be done yet in understanding and classifying galaxies that don't fall into the "normal bins" of spiral and elliptical shapes. 

Edited and updated by Carolyn Collins Petersen.