(Sen) - Astronomers have used the Hubble Space Telescope to build up a detailed picture of how galaxies have evolved over most of the history of the Universe. And they find not much has changed.
The scientists have explored the sizes, shapes and colours of distant galaxies over the last 11 billion years - the Universe formed in the Big Bang 13.4 billion years ago.
Today, professional astronomers classify galaxies according to something called the Hubble Sequence which, like the telescope, is named after the legendary cosmologist Edwin Hubble who devised it in 1926.
The sequence, which has been likened to a tuning fork because of its shape, classifies galaxies according to their shape and levels of star-forming activity.
Though they vary in shape, haloes and central bulges, they come in two main types - elliptical and spiral. A third type, called lenticular, falls somewhere between the two.
The latest results come from data collected by the telescope Hubble’s CANDELS observations - it stands for Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey.
This is the largest project ever carried out by Hubble, which is operated jointly by NASA and the European Space Agency, and it involves two of its cameras, the WFC3 and the ACS.
What surprised scientists when they looked billions of years back in time at more distant galaxies was that the sequence that we know today was much the same as far back as 11 billion years ago.
It had been realised that the Hubble Sequence was true as long ago as eight billion years, but the new findings extend that a further 2.5 billion years back in time. The astronomers wanted to know if galaxies’ shapes were different earlier.
BoMee Lee of the University of Massachusetts, USA, who is lead author of a new paper exploring the sequence, said: “This is a key question: when and over what timescale did the Hubble Sequence form?”
“To do this you need to peer at distant galaxies and compare them to their closer relatives, to see if they too can be described in the same way.”
Though there have been previous studies of this cosmic epoch, they looked at low-mass galaxies and not the large, more mature ones similar to our own Milky Way.
How galaxies in the sequence looked when the Universe was 20 per cent of its present age. Credit: NASA, ESA, M. Kornmesser
Galaxies at these earlier times appear to be split between blue star-forming galaxies with a complex structure (including discs, bulges, and messy clumps), and massive red galaxies that are no longer forming stars, as seen in the nearby Universe
More massive galaxies like the Milky Way were rarer in the early Universe so that not enough could be sampled to describe properly their characteristics.
Previous studies were aso inconclusive because they looked at the galaxies in visible light only. Because of their vast distances, the red shift in this light highlighted regions of star formation. Looking further into the infrared part of the spectrum shows how the galaxies would normally appear in visible light, without this red shift.
What CANDELS has shown is that all galaxies that far back, whatever their size, fit the sequence’s different classifications.
Lee added: “The huge CANDELS dataset was a great resource for us to use in order to consistently study ancient galaxies in the early Universe. The Hubble Sequence underpins a lot of what we know about how galaxies form and evolve — finding it to be in place this far back is a significant discovery.”
The study’s co-author Arjen van der Wel, of the Max Planck Institute for Astronomy in Heidelberg, Germany, said: “This is the only comprehensive study to date of the visual appearance of the large, massive galaxies that existed so far back in time.
“The galaxies look remarkably mature, which is not predicted by galaxy formation models to be the case that early on in the history of the Universe.”