Astronomers have seen many galaxies throughout the universe, most of which can be divided into categories, but along the way they have also discovered some unusual galaxies!
In the mid-1920s, Edwin Hubble, observing some objects called nebulas, realized that most of them have stars and are outside our galaxy, and since then people have made great strides in understanding different types of galaxies and their evolution.
While galaxy was being hunted for decades, astronomers classified certain types of galaxies as "pecular" galaxies.
If someone asks you what a galaxy is, and if you have some idea of astronomy, you'll probably come to mind the image of a disk filled with gas and stars.
So what's so special about the unusual galaxy that it's allowed them to be singled out, and why were they marked as "pecular"?
Before we sink into a world of unusual galaxies, it would be interesting to first look at different types of galaxies, which will help us understand the meaning of special galaxies.
Hubble sequence
The galaxies are usually classified according to a system known as the Hubble classification or the Hubble sequence, in which conventional galaxies are classified on the basis of their shape: elliptical, linzolike, spiral and incorrect.
Elliptic galaxies look smooth, unspecific and have elliptical shape; spiral galaxies are flat, and the distribution of stars in them is such as to make them like spirals; spiral galaxies sometimes have a core structure in the center; these spirals are called spiral galaxies with lock-in.
Linzooid galaxies are galaxies with a lens structure. They consist of an extended disk that surrounds the bulge in the center. Sometimes the lens galaxies have an object in the centre shape of a rod. Such tape circulars are called ribbons with a lock. In the Hubble sequence, the tape circulars are described as S0, and the ribbons with a lock are described as SB0.
Finally, there are wrong galaxies.
What about unusual galaxies? You can say that galaxies that don't fit into any of Hubble's sequence categories can be one of those galaxies, but it looks like the Hubble sequence covers all possible configurations and shapes that might have galaxies.
Definition of pecular galaxies
It turns out there are some exceptions. When Halton Arp, an American astronomer well known for his work on pecular and interacting galaxies, tried to understand galaxies with spiral sleeves, he eventually compiled a list of galaxies that did not really fit into Hubble's sequence.
In his atlas, Arp listed and photographed some 330 galaxies, which he thought were unusual, then divided these galaxies into four major groups according to their characteristics, as well as a fifth group containing six galaxies, which he called "Diverse".
Arp continued to study the sky to gather more information about spiral sleeves and to understand more about unusual galaxies. In 1977, he and Barry Mador published another list containing 25 categories of such galaxies.
Using the observations of the Hubble Space Telescope, several astronomers reported that the percentage of unusual galaxies was significantly higher at long distances, but the interpretation of what they were was subjective and varied from one astronomer to another, thus the need to develop an objective definition for such galaxies.
In 1996, A. Naim and O. Lahav tried to formulate some parameters of what might be an unusual galaxy, and in their study they noted the subjective nature of classifications based on their characteristics.
Naim and Lahav pointed out that features can be both weak and powerful, and they got this by looking at the data from the Arp Atlas. Weak features can just be a small change from the models that we usually see in the galaxies, while strong features can be much larger variations in which galaxies look very different from what we see in other cases.
However, they noted that these definitions often lead to confusion because they depend on individual observers, and there are a large number of galaxy images that make visual classification much less feasible. They have tried to develop a more reliable definition using quantitative parameters that could describe characteristics.
Trying to quantify features
One of the quantitative methods proposed by them is a comparison of templates. Here they compare images of unusual galaxies with several balloon galaxies. These model galaxies have a clear classification in the Hubble sequence. The difference between images of unusual and model galaxies can be quantified, which gives an indication of the degree of unusualness.
Another way was to measure the light concentration index and the asymmetries index. The difference between these values for normal and special galaxies can give us a degree of specificity. In both cases we see that there is a comparison with normal galaxies.
Naim and Lahav, in their 1996 study, tried to quantify the degree of unusualness, but one of their main findings was that such features usually arise as asymmetries and humps, which may be strong in some galaxies, but very thin in others; the line between normal and unusual galaxies is not sharp, and the degree of deviation is as fundamental as the deviations themselves.
Another attempt at quantitative description was made by Lior Shamir and John Wallin in 2014, where they used Sloan's data from the sky digital survey and developed an algorithm to automatically identify unusual galaxies.
The algorithm could not distinguish the background noise from the real galaxies; nor did some galaxies detected by the algorithm show any features; however, it did identify exactly about 500 candidates for pecular galaxies, which were then grouped according to their morphology.
Groups include vapours distorted by the tide, collision galaxies, blue galaxies with unusual morphology, galaxy with built-in point sources, galaxy visible from the rib, and linear features.
There have also been discussions about the relationship between pecular galaxy and active galaxy nuclei. ANAG is a galaxy whose centres are much brighter than the rest of the galaxy. While earlier reviews were not very clear, recent surveys and Hubble Space Telescope show that pecular galaxies tend to have a more active core.
Other properties of pecular galaxies
Some studies and observations show that star formation in pecular galaxies is more unusual than in conventional galaxies, and one study has shown that such abnormal rates of star formation are linked either to the interaction of galaxies or to the collision with each other.
The environment of the pecular galaxy is usually quite aggressive, because these galaxies can throw out the jets of matter or interact with other galaxies in their properties, especially in their morphology, and these changes appear as deviations from what we usually see, what makes galaxy unique.
With this large dynamic interaction and the collision of gas and matter in pecular galaxies, one effect is usually the increased rate of star formation; another effect of the interaction of galaxies with each other is increased nuclear activity.
In view of the above, it seems clear that there are differences between pecular and normal galaxies. Although the definition of the extent of distortion is based on intuition and depends on the observer, there is general agreement on what will constitute a galaxy: interoperable galaxies, galaxies with optical jets, collapsing galaxies, and so on.
So it's true that pecular galaxy has its characteristics. Understanding its formation and characteristics will give us a lot of information about the evolution of such objects. It means that different galaxies have been formed in different ways. The universe is really amazing by its diversity of galaxies and all the other celestial objects that we have discovered so far.