Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Object type: Dwarf galaxy
Location: Eridanus constellation
If a person is square they are a bit dull, but for a galaxy, it is the mark of a true rebel. A rectangular galaxy spotted 70 million light years from Earth is the boxiest galaxy known聽 鈥� and could bring a new understanding of how galaxies form and evolve.
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Galaxies take on one of three shapes: a flattened circular disc typically hosting a spiral pattern of stars like our Milky Way, an ellipsoid聽 鈥� like a rugby ball or American football聽 鈥� or an irregular shape without clear symmetry. Box-like galaxies are virtually unheard of, says at the Swinburne University of Technology in Hawthorn, Victoria, Australia.
Graham is one of the researchers who the rectangular galaxy 鈥� named LEDA 074886 鈥� while using the and telescopes in Hawaii to look for globular clusters of stars swarming around NGC 1407, a bright giant galaxy in the constellation Eridanus.
鈥淚t鈥檚 one of those things that just makes you smile because it shouldn鈥檛 exist, or rather, you don鈥檛 expect it to exist,鈥� says Graham.
Side-on cylinder
LEDA 074886 is a dwarf galaxy, a type much smaller than our Milky Way. His team aren鈥檛 sure what its three-dimensional shape is. One possibility is that it is an inflated disc that more closely resembles a cylinder and that looks like a rectangle because we view it side-on from Earth.
鈥淭he alternative shape, a cube, seems too bizarre to contemplate,鈥� says Alister, although he points out that stars with 鈥渂ox orbits鈥� are already documented.
If it is a cylinder, how might it be formed? One option is gravitational torques from its giant neighbour, NGC1407. The trouble is that this wouldn鈥檛 explain another curious thing about LEDA 074886: at its heart is an edge-on inner disc of young stars, 8000 light years across (see the black disc, in picture above).
Another option is that the angular galaxy formed out of the collision of two spiral galaxies. The impact threw the pre-existing stars from these galaxies into large orbits, creating the rectangular outline, while the gas sank to the middle and condensed to form the central disc of new stars.
Hybrid merger
That would suggest that LEDA鈥檚 formation was a hybrid of two known types of galaxy formation. The outer rectangular shape is consistent with simulations of elliptical galaxy mergers, which don鈥檛 involve the production of new stars because these galaxies don鈥檛 contain much gas. The disc, on the other hand, is more similar to simulations of mergers of gassy galaxies, which involve star formation. 鈥淭he hybrid nature of LEDA 074886 suggests that both types of event have occurred,鈥� says Graham.
鈥淲e can now combine lessons learned from both types of simulations,鈥� he adds. That might be useful for modelling other galaxies containing both old and young stars.
The researchers suspect that the inner disc may be 鈥減recessing鈥� as well as spinning 鈥� rather like the way a spinning top both spins on its own axis and turns around the main vertical axis of rotation, says Graham.
An earlier version of the story incorrectly listed the galaxy鈥檚 distance as 700 million light years
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