Still producing the starry goods (Image: Joe Bergeron/Dunlap Institute for Astronomy and Astrophysics)

The universe’s galactic zoo evolved into recognisable species a mere 2.5 billion years after the big bang. An extensive survey of massive galaxies in the early universe shows that the objects had adopted the same distinct colours and shapes we see among mature galaxies today.

The find deepens a mystery surrounding the processes that create our galactic catalogue, says Mauro Giavalisco of the University of Massachusetts in Amherst. In the 1920s astronomer Edwin Hubble sorted the massive galaxies we see today into two general types based on their shapes and colours. Galaxies that are discs or spirals are mostly blue, a sign that they are forming new stars. Blobby spheroidal galaxies, meanwhile, are red and are no longer making stars.

“The two types are equally massive, so they have the same gravity,” says Giavalisco. “Why can one type accrete gas and convert it into stars and keep on evolving, while the other is red and dead and pretty much defunct?” The new survey shows that whatever shuts down star birth in spheroidal galaxies, it must happen very quickly to have caused them to emerge so soon after the big bang.


High noon

To probe galactic structure over time, Giavalisco and his graduate student BoMee Lee used the Hubble Space Telescope’s Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS), which includes the largest sample of very distant galaxies yet collected.

The team focused on the era between 1.5 and 3 billion years after the big bang. Astronomers call this time “cosmic high noon”, because star formation in galaxies was at its peak. Previous studies had peered back into this epoch for lower-mass galaxies, which don’t have distinctive shapes. But none had looked at the galaxies that are Milky Way-sized or bigger.

The team analysed 1671 massive galaxies from cosmic high noon. Even back then, the familiar breakdown of galactic shapes was clearly apparent: the same fraction of galaxies was blue and disc-shaped, while the others were red and globular.

“Already, 2.5 billion years after the big bang, galaxies appeared to have the same correlation of morphological properties and star formation properties as we see in the local, present-day universe,” says Giavalisco. “So this process of galaxy diversification is really fast and effective.”

Flagship finding

Galaxies can change their shapes over time. When two spirals merge, for instance, they often lose much of their cold, star-forming gas, shut down and settle into a spheroidal shape. And spheroids can be rejuvenated and start forming new stars if they manage to snare cold gas from a neighbouring galaxy that gets too close.

The new results will help theorists sort out which of the processes that go into galaxy building are the most important to a mature object’s shape, says Eric Murphy of the California Institute of Technology in Pasadena, who was not involved in the new study. That in turn can tell us a lot about the distribution of matter in the universe, which is tied to how structure emerged from the homogenous soup that existed right after the big bang.

“It’s a very important conclusion that will certainly help in guiding models,” he says. “This is going to be a flagship paper for CANDELS.”

Journal reference: The Astrophysical Journal, DOI: 10.1088/0004-637X/774/1/47