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Saturday, February 6, 2010

Collisions and mergers shaped spiral galaxies

Galaxies are classified into the few general shapes of the ‘Hubble sequence’, named after the astronomer Edwin Hubble. To see a diagram of this sequence, check my earlier post on the subject.

Now it appears that galaxies can change their form over time.

A team of astronomers compared 116 local galaxies with 148 distant ones. Because of the amount of time it takes for light to reach us from those distant galaxies, they were affectively comparing modern galaxies with ancient one.

They found that 6 billion years ago there was a much greater percentage of ‘peculiar’ galaxies that did not fit the standard galactic taxonomy. This indicates that the oddly shaped galaxies must have become normal spirals over time. The astronomers speculate that collisions and mergers with other galaxies were responsible for this reshaping.

Top panel: local (modern) galaxies
Bottom panel: distant (ancient) galaxies
Left to Right on both panels: dark red = ellipitcal; light red = lenticular (lens-shaped); purple =spiral; green = peculiar.
Credits: NASA, ESA, Sloan Digital Sky Survey, R. Delgado-Serrano and F. Hammer (Observatoire de Paris)

Prior to this study, it was understood that collisions and mergers between galaxies happened, but that those events had mostly ended by 8 billions years ago. This new data suggests that the clashes continued until at least 4 billion years ago.