Astronomers have claimed that mature looking galaxies existed much earlier than previously known, when the universe was just about 2.5 billion years old.
Studying the evolution and anatomy of galaxies using the Hubble Space Telescope, an international team of astronomers led by doctoral candidate BoMee Lee and her advisor Mauro Giavalisco at the University of Massachusetts Amherst used two cameras, Wide Field Camera 3 (WFC3), and Advanced Camera for Surveys (ACS), plus observations from the Hubble’s Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) to explore the shapes and colors of distant galaxies over the last 80 percent of the universe’s history.
Lee points out that the huge CANDELS dataset allowed her team to analyze a larger number of these galaxies, a total 1,671, than ever before, consistently and in detail.
Lee said that 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.
She and colleagues confirm for an earlier period than ever before that the shapes and colors 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 colors, shape and size.
Giavalisco said that why modern galaxies are divided into these two main types and what caused this difference is a key question of cosmology.
He said that 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.
Giavalisco asserted that this is not just a classification scheme, it corresponds to a profound difference in the galaxies’ physical properties and how they were formed.
Lee explained that this was a key question: When, and over what timescale did the Hubble Sequence form? To answer 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.
She said that 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.
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. Galaxies at these early times appear to be mostly irregular systems with no clearly defined morphology.
There are blue star-forming galaxies that sometimes show structures such as discs, bulges and messy clumps, as well as red galaxies with little or no star formation. Until now, nobody knew if the red and blue colors were related to galaxy morphology, the UMass Amherst authors note.
There was previous evidence that the Hubble Sequence holds true as far back as around 8 billion years ago, the authors point out, but their new observations push a further 2.5 billion years back in cosmic time, covering 80% of the history of the universe.
The results have been published in the current online issue of The Astrophysical Journal.