If confirmed, the finding highlights the importance to many galaxies of collisions and mergers in the recent past. It also provides clues for the unique status of our own galaxy, the Milky Way.
Hubble’s scheme divides regular galaxies into three broad classes — ellipticals, lenticulars and spirals — based on their visual appearance. A fourth class contains galaxies with an irregular appearance.
The astronomers think that these peculiar galaxies did indeed become spirals through collisions and merging. Although it was commonly believed that galaxy mergers decreased significantly eight billion years ago, the new result implies that mergers were still occurring frequently after that time — up to as recently as four billion years ago. “Our aim was to find a scenario that would connect the current picture of the Universe with the morphologies of distant, older galaxies — to find the right fit for this puzzling view of galaxy evolution,” says Hammer.
Also contrary to the widely held opinion that galaxy mergers result in the formation of elliptical galaxies, Hammer and his team support a scenario in which these cosmic clashes result in spiral galaxies. In a parallel paper published in Astronomy & Astrophysics, they delve further into their ‘spiral rebuilding’ hypothesis, which proposes that peculiar galaxies affected by gas-rich mergers are slowly reborn as giant spirals with discs and central bulges.
Hammer and his team used data from the Sloan Digital Sky Survey undertaken by Apache Point Observatory, New Mexico, USA, and from the GOODS field and Hubble Ultra Deep Field taken by the Advanced Camera for Surveys aboard Hubble.
R. Delgado-Serrano, et al., 2010, How was the Hubble Sequence 6 Giga-years ago? Astronomy & Astrophysics, 509, A78.F. Hammer et al., 2009, The Hubble Sequence: just a vestige of merger events? Astronomy & Astrophysics, 507, 1313.
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