Mature-looking galaxies existed much earlier than previously thought, when the universe was only about 2.5 billion years old, or 11.5 billion years ago, a new study has found.
An international team of astronomers led by doctoral candidate BoMee Lee and Mauro Giavalisco from the University of Massachusetts Amherst studied the evolution and anatomy of galaxies using the Hubble Space Telescope.
"Finding them this far back in time is a significant discovery," said lead author Lee.
The team used two cameras and observations from the Hubble's Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), to explore the shapes and colours of distant galaxies over the last 80 per cent of the Universe's history.
Lee points out that the huge CANDELS dataset allowed her team to analyse a larger number of these galaxies, a total 1,671, than ever before, consistently and in detail.
"The significant resolution and sensitivity of WFC3 was a great resource for us to use in order to consistently study ancient galaxies in the early Universe," said Lee.
Researchers confirm for an earlier period than ever before that the shapes and colours of these extremely distant young galaxies fit the visual classification system introduced in 1926 by Edwin Hubble and known as the Hubble Sequence.
It classifies galaxies into two main groups: Ellipticals and spirals, with lenticular galaxies as a transitional group. The system is based on their ability to form stars, which in turn determines their colours, shape and size.
Why modern galaxies are divided into these two main types and what caused this difference is a key question of cosmology, said Giavalisco.
"Another piece of the puzzle is that we still do not know why today 'red and dead' elliptical galaxies are old and unable to form stars, while spirals, like our own Milky Way, keep forming new stars. This is not just a classification scheme, it corresponds to a profound difference in the galaxies' physical properties and how they were formed," Lee added.
"The Hubble Sequence underpins a lot of what we know about how galaxies form and evolve. It turns out that we could show this sequence was already in place as early as 11.5 billion years ago," said Lee.
Galaxies as massive as the Milky Way are relatively rare in the young Universe. This scarcity prevented previous studies from gathering a large enough sample of mature galaxies to properly describe their characteristics.
The study appears in The Astrophysical Journal.
