Astronomers using NASA’s Hubble Space Telescope have established that mature-looking galaxies existed much earlier than previously known, when the Universe was only 2.5 billion years old.
They used Hubble’s Wide Field Camera 3, Advanced Camera for Surveys and the Hubble’s Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) to explore the shapes and colors of 1,671 extremely distant young galaxies over the last 80 percent of the Universe’s history.
“Finding them this far back in time is a significant discovery,” said Dr Bomee Lee of the University of Massachusetts Amherst, lead author of the study reported in the Astrophysical Journal (preprint at arXiv.org).
Dr Lee’s team confirms for an earlier period than ever before that the shapes and colors of the 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: elliptical and spiral galaxies, and lenticular galaxies as a transitional group.
Study second author Dr Mauro Giavalisco, also from the University of Massachusetts Amherst, said: “why modern galaxies are divided into these two main types and what caused this difference is a key question of cosmology.”
“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.”
“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. 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,” Dr Lee added.
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.
There was previous evidence that the Hubble Sequence holds true as far back as around 8 billion years ago, but their new observations push a further 2.5 billion years back in cosmic time, covering 80 percent of the history of the Universe.
Previous studies had also reached into this epoch to study lower-mass galaxies, but none had conclusively looked at large, mature galaxies like the Milky Way. The new study confirms that all galaxies this far back, big and small, already fit into the sequence a mere 2.5 billion years after the Big Bang.
Bibliographic information: Bomee Lee et al. 2013. CANDELS: The Correlation between Galaxy Morphology and Star Formation Activity at z ~ 2. ApJ 774, 47; doi: 10.1088/0004-637X/774/1/47