This is a guest article by Kaushik Telidevara
How Are Galaxies Formed?
Previously, scientists thought that the factual model of galaxy formation was Monolithic Collapse, where a cloud of primordial gas would collapse inwards due to its gravity forming a new galaxy. This process was thought to have happened very early in the lifetime of the Universe and every part of the galaxy was formed at the same time.
Modern evidence rules out this model; the widely varying ages of different components like stars, and movement patterns and courses of different solar systems, stars, and planets of the Galaxy provide a counter-argument for this theory which led to the development of the current model of galaxy formation, the Hierarchical Merger model of galaxy formation, which supports, and is heavily reliant on the existence of Galactic Interactions, collisions, and “cannibalism”, thus these components of different ages and movement patterns are from different galaxies.
In this model, galaxies are believed to grow by ingesting smaller, dwarf galaxies and the regions of dark matter that envelop them. In the process, some of these dwarf galaxies are shredded by the gravitational forces when they move too close to the center of the “host” galaxy’s enormous halo. This, in turn, leaves star-streams behind, as relics of the original event and these star-trails are of the many pieces of evidence for this theory.
There are many isolated galaxies, and these are usually very large and are either “Hermit” galaxies, which are solitary galaxies drifting through space, or “Cannibal” galaxies which undergo a process called Galactic Cannibalism whereby a large galaxy, undergoes gravitational interactions with a companion galaxy which results in the merger of both to form a larger galaxy. In essence, there is a possibility that any galaxy that is isolated, has in fact merged with its neighbors and is now one giant galaxy.
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Galactic cannibalism is a type of a Galactic interaction, which happens when some galaxy’s gravitational fields result in a disturbance of one another. The effect of these interactions varies as well, ranging from instances where the galaxies simply pass by each other and slightly change their course or other major effects like the spiral arm of a galaxy being ripped off due to an interaction.
The most common result of this process is an irregular galaxy of one form or another, although elliptical galaxies could also be formed. The Whirlpool, the Mice, and the Antennae Galaxies are several examples of galaxies that appear to be in the process of cannibalizing and merging, that have been observed with the assistance of the Hubble telescope.
However, this process should not be confused with Galactic Collision which is a similar process where galaxies collide but retain most of their original shape. In these cases, a considerable difference in the size of the two galaxies, and a smaller degree of momentum is responsible. In the former case, the galaxies stop moving after merging because they have no more momentum to spare; in the latter, the larger galaxy’s shape overtakes the smaller one and there appears to be little change due to the vast difference in sizes.
Milky Way – A Cannibal Galaxy
Our own galaxy is a cannibal galaxy and is currently feeding on the Sagittarius Dwarf Spheroidal Galaxy, and on the Large and Small Magellanic clouds as well, and it is quite likely that it had merged with other galaxies in the past as well, as some systems in the Milky Way, like solar systems and stars, are moving in a different path and of different ages compared to others, suggesting that they were from a galaxy of a different shape, and orientation with respect to its movement, that the Milky Way “ate”.
As seen in the diagram, the Sagittarius dwarf galaxy starts to circle the Milky Way and is slowly being consumed by it. Something even more interesting is that the Milky Way galaxy could be consumed by our neighbor galaxy, Andromeda. But this would happen in around 4 billion years. Perhaps eventually every galaxy in the Virgo supercluster would form with Andromeda to form one giant intergalactic blob. One could think that this way every galaxy in the universe would condense into one super-galaxy. But this again is highly unlikely due to the fact that the universe is expanding, and the gravitational force of two bodies decreases with an increase in distance between the bodies.
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Nature of Process
It is important to note that although these processes are violent, they take place over millions and billions of years, and are not instantaneous in any way. This gives us the unique opportunity to capture different phases of interaction at different time stamps, across millions of galaxies because of the frequency with which they occur. These collisions wouldn’t affect the individual components of the galaxies. You can think of a galaxy as a school of fish, and when two schools of fish merge, the overall shape of the school would only change, the fish would remain the same in the sense that they wouldn’t bump into each other.
We can effectively visualize a galaxy by imagining solar systems and stars as sand grains, and each sand grain is separated from another by the length of a football field. Thus it is extremely unlikely that these intergalactic collisions would affect the components of a galaxy. However, when these collisions and mergers do take place, they have a lot of extra gas and energy that prove to be the perfect breeding ground for the formation of new stars, and galactic systems. Another visual aspect of galactic cannibalism is that they result in beautiful collisions as seen in the pictures below.