"Six billion years ago, there were many more peculiar galaxies than now — a very surprising result," says Rodney Delgado-Serrano, lead author of the paper recently published in Astronomy and Astrophysics. "This means that in the last six billion years, these peculiar galaxies must have become normal spirals, giving us a more dramatic picture of the recent Universe than we had before."
The astronomers think that these peculiar galaxies did indeed become spirals through collisions and merging. Tracing the history of galaxy formation leads us to the way our Universe presently looks. Like any review of a life, there are chaotic, tumultuous times and more dormant periods and, like many teenagers, developing galaxies often collide with those in their way. Crashes between galaxies give rise to enormous new galaxies and, 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.
This image created from data taken from both the NASA/ESA Hubble Space Telescope and the Sloan Digital Sky Survey demonstrates that the Hubble sequence six billion years ago was very different from the one that astronomers see today. The two sections show how many more peculiar shaped galaxies (marked Pec) are seen among distant galaxies, as opposed to among local galaxies. The data organization follows the Hubble tuning-fork classification scheme invented in 1926 by the same Edwin Hubble in whose honor the space telescope is named.
(Photo Credit: NASA, ESA, Sloan Digital Sky Survey, R. Delgado-Serrano and F. Hammer (Observatoire de Paris))
"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, astronomers 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.
Although our own Milky Way galaxy is a spiral galaxy, it seems to have been spared much of the teenage drama; its formation history has been rather quiet and it has avoided violent collisions in astronomically recent times. However, the large Andromeda galaxy from our neighborhood has not been so lucky and fits well into the "spiral rebuilding" scenario. Researchers continue to seek out explanations for this.
Citation: Hammer et al., 'The Hubble sequence: just a vestige of merger events?', Astronomy and Astrophysics, 2010; doi: arXiv:0903.3962v4
Delgado-Serrano et al., 'How was the Hubble sequence 6 Gyrs ago?', Astronomy and Astrophysics, 2009; doi: arXiv:0906.2805v3
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