5 December 2016 | Galactic Collision!


Image credit: ESA/Hubble & NASA | Click image to enlarge or click here for full size (9MB)


Once upon a time, the general view of galaxies was that they were static and unchanging. This view however changed as more detailed images of a greater number of galaxies were taken as part of photographic all sky surveys. This led to dedicated astronomers studying galaxy evolution to compile catalogues of galaxies that were termed ‘peculiar’. The first major collection of peculiar galaxies to be published was by the Russian astronomer Boris Vorontsov Velyaminov. His work was the foundation for the Arp Atlas published by the late astronomer Halton Arp in 1966. Their images revealed that a large number of galaxies appeared to exhibit structural and morphological peculiarities such as tidal tails and asymmetric spiral structure.

The cause of distorted structure observed in galaxies was found to be the result of gravitational interaction between two or more galaxies. It was later realised that this was the precursor to galaxies colliding together and eventually two galaxies undergoing a merger and becoming one. It is now generally accepted that this is a common way for galaxies to grow and develop. In fact, our Milky Way galaxy is scheduled for a collision with the nearby Andromeda Galaxy in 4 billion years time!

This exquisite Hubble Space Telescope portrait depicts an example of a peculiar galaxy called NGC 3921, which is located at a distance of 390 million light years in space and in the galaxy rich constellation of Ursa Major in the sky. Most images taken with professional observatories are the result of an astronomer submitting a proposal to take an image in order to support a study of a particular deep sky object. Any observations that are undertaken are used in two ways, firstly the data is analysed by astronomers for a variety of reasons including a greater understanding of their chosen target. Secondly, the raw data can be processed into an aesthetic colour image that has no intrinsic scientific value that can be disseminated to the world! Peculiar galaxies such as NGC 3921 provide awe inspiring views of the cosmos as well as helping astronomers understand topics such as galaxy formation and evolution.

The chaotic and strange appearance of NGC 3921 is the product of a merger between two galaxies with equal mass that took place 700 million years ago. Unsurprisingly, this galaxy was included in both the Vorontsov Velyaminov catalogue and the Arp Atlas as VV 31 and Arp 224 respectively. As opposed to the naming of terrestrial features, nearly all deep sky objects are included in many different catalogues by many different astronomers over a period of many decades with multiple identifiers attributed to a single object. This has led to the management of catalogues and naming being a mostly simple and easy task with no confusion whatsoever!

The looped ring structure has an impressive size of 228,000 light years making it slightly more than twice as large as the Milky Way. Studies of the distribution of neutral hydrogen gas has shown one of the original galaxies to have been gas-rich and the other one to be gas-poor. Neutral gas is a vital ingredient for the formation of stars and a major consequence of interacting and merging galaxies is the triggering of intense star formation and in some cases starburst activity. The detected neutral hydrogen is concentrated in the southern part of the ring while the northern part is devoid of it. A large population of clusters and OB associations have been uncovered and can be seen as small blue knots and patches along the tidal loop and tail.

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