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, a classification scheme invented in 1926 by the same Edwin Hubble in whose honour the space telescope is named.

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.

"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 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. (Credit: NASA, ESA, Sloan Digital Sky Survey, R. Delgado-Serrano and F. Hammer (Observatoire de Paris))"

Source: ESA