Astrophile: Square galaxy is a rebel

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 700 million light years from Earth is the boxiest galaxy known? ? and could bring a new understanding of how galaxies form and evolve.

Galaxies take on one of three shapes: a flattened circular disc typically hosting a spiral pattern of stars like our Milky Way, an ellipsoid? ? like a rugby ball or American football? ? or an irregular shape without clear symmetry. Box-like galaxies are virtually unheard of, says Alister Graham at the Swinburne University of Technology in Hawthorn, Victoria, Australia.

Graham is one of the researchers who discovered the rectangular galaxy? ? named LEDA 074886 ? while using the Subaru Prime Focus Camera, mounted on the Subaru telescope in Japan, to look for globular clusters of stars swarming around NGC 1407, a bright giant galaxy in the constellation Eridanus.

"It's one of those things that just makes you smile because it shouldn't exist, or rather, you don't expect it to exist," says Graham.

Side-on cylinder

LEDA 074886 is a dwarf galaxy, a type much smaller than our Milky Way. His team aren't 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.

"The alternative shape, a cube, seems too bizarre to contemplate," says Alister, although he points out that stars with "box orbits" 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't 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's 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't involve the production of new stars because these galaxies don't contain much gas. The disc, on the other hand, is more similar to simulations of mergers of gassy galaxies, which involve star formation. "The hybrid nature of LEDA 074886 suggests that both types of event have occurred," says Graham.

"We can now combine lessons learned from both types of simulations," 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 "precessing" as well as spinning ? rather like the way a spinning top both spins on its own axis and turns around the main vertical axis of rotation, says Graham.

Reference: arxiv.org/abs/1203.3608v1

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