A multinational group of astronomers has shown that a large proportion of galaxies have undergone a major ‘metamorphosis’ since they were initially formed after the Big Bang.

In their study, the astronomers observed around 10,000 galaxies currently present in the Universe using a survey of the sky created by the Herschel ATLAS and GAMA projects.

They then classified the galaxies into the two main types: flat, rotating, disc-shaped galaxies; and large, oval-shaped galaxies with a swarm of disordered stars.

Using the NASA/ESA Hubble and NASA’s Herschel telescopes, the scientists then looked further out into the Universe, and thus further back in time, to observe the early galaxies.

They showed that 83% of all the stars formed since the Big Bang were initially located in disc-shaped galaxies.

However, only 49% of stars that exist in the Universe today are located in these disc-shaped galaxies – the remainder are located in oval-shaped galaxies.

The results, published online in the Monthly Notices of the Royal Astronomical Society, suggest a massive transformation in which disc-shaped galaxies became oval-shaped galaxies.

A popular theory is that this transformation was caused by many cosmic catastrophes, in which two disk-dominated galaxies, straying too close to each other, were forced by gravity to merge into a single galaxy, with the merger destroying the disks and producing a huge pileup of stars.

An opposing theory is that the transformation was a more gentle process, with stars formed in a disk gradually moving to the center of a disk and producing a central pile-up of stars.

“Many people have claimed before that this metamorphosis has occurred, but by combining Herschel and Hubble, we have for the first time been able to accurately measure the extent of this transformation,” said Prof Steve Eales of Cardiff University, UK, lead author on the study.

“Galaxies are the basic building blocks of the Universe, so this metamorphosis really does represent one of the most significant changes in its appearance and properties in the last 8 billion years.”

“Up to now we have seen individual cases in the local universe where galaxy collisions convert spirals into ellipticals,” said co-author Dr David Clements of Imperial College London, UK.

“This study shows that this kind of transformation is not exceptional, but is part of the normal history of galaxy evolution.”

“This study is important as it establishes statistics showing that almost all stars formed in spiral galaxies in the past, but a large fraction of these now appear as large, dead, elliptical galaxies today,” said co-authors Prof Asantha Cooray from the University of California.

“The study will require us to refine the models and computer simulations that attempt to explain how galaxies formed and behaved over the last 13 billion years.”

_____

Stephen Eales et al. 2015. H-ATLAS/GAMA: quantifying the morphological evolution of the galaxy population using cosmic calorimetry. MNRAS 452 (4): 3489 – 3507; doi: 10.1093/mnras/stv1300