Forming the present-day spiral galaxies (2/6/2010)

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. The top image represents the current -- or local -- universe. Using their sample, researchers found that 3 percent of galaxies were elliptical (marked E), 15 percent lenticular (marked S0), 72 percent spiral (marked Sa to Sd, or SBb to SBd) and 10 percent peculiar (marked Pec). The bottom image represents the make up of the distant galaxies (six billion years ago), showing a much larger fraction of peculiar galaxies. The census found 4 percent of distant galaxies were elliptical, 13 percent lenticular (S0), 31 percent spiral and 52 percent peculiar. This implies that many of the peculiar galaxies ultimately become large spirals. According to the 'spiral rebuilding' hypothesis, devised by the astronomers François Hammer, Rodney Delgado-Serrano and their group, this is due to the large number of major, gas-rich galaxy mergers between galaxies that were previously labeled 'peculiar' in the distant Universe. It is thought that the large Andromeda galaxy from our neighbourhood formed in this manner. In total, 116 local galaxies and 148 distant galaxies were sampled. Spiral galaxies are further classified by labels that characterize their appearance; for example, an SBd galaxy means that it is a spiral galaxy that has slightly looser 'arms' than an SBa galaxy and a less prominent bulge. These images were created from data that are part of large sky surveys undertaken by the NASA/ESA Hubble Space Telescope and the 2.5-metre telescope at Apache Point Observatory, New Mexico, USA (Sloan Digital Sky Survey). - NASA, ESA, Sloan Digital Sky Survey, R. Delgado-Serrano and F. Hammer (Observatoire de Paris)

Galaxy morphology, or the study of the shapes and formation of galaxies, is a critical and much-debated topic in astronomy. An important tool for this is the Hubble sequence or Hubble tuning-fork diagram [1], a classification scheme invented in 1926 by the same Edwin Hubble in whose honour the space telescope is named.

A team of European astronomers led by François Hammer of the Observatoire de Paris has, for the first time, completed a demographic census of galaxy types at two different points in the Universe's history - in effect, creating two Hubble sequences - that help explain how galaxies form [2]. In this survey, researchers sampled 116 local galaxies and 148 distant galaxies.

Contrary to previous thought, the astronomers showed that the Hubble sequence six billion years ago was very different from the one that astronomers see today.

"Six billion years ago, there were many more peculiar galaxies than now - a very surprising result," says Rodney Delgado-Serrano, lead author of the related paper recently published in and highlighted on the cover of Astronomy & 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.

"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 [3], Hammer and his team 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 neighbourhood has not been so lucky and fits well into the "spiral rebuilding" scenario. Researchers continue to seek out explanations for this.

Note: This story has been adapted from a news release issued by the ESA/Hubble Information Centre

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