The galaxy clusters, like MACSJ0138.0-2155 here, magnifies and distorts the image of a galaxy, enabling astronomers to study it in great detail. (Illustration: ESA/NASA/Hubble, A. Newman/M. Akhshik/K. Whitaker)

Astronomers discover six distant galaxies that ran out of fuel

Space research

Researchers from DTU and the University of Copenhagen have discovered six distant galaxies in the universe, which have run out of fuel to form new stars. How they ran dry is a bit of a mystery that researchers are still looking for answers to.

Using the Hubble Space Telescope, the ALMA radio telescope, and via a massive cluster of galaxies, astronomers from the Cosmic Dawn Center, a collaboration between DTU Space at the Technical University of Denmark and the Niels Bohr Institute at the University of Copenhagen, have found six galaxies in the early Universe that have run out of gas to make new stars. 

The observations challenge our understanding of star formation and galaxy evolution.

Galaxies are full of stars, and stars are made from gas. But some galaxies have run out of gas, and hence stopped forming new stars, while others, by reusing gas from old stars or by accreting new gas from intergalactic space, are able to continue to form new stars. Most stars were created in the early Universe about 2–4 billion years after the Big Bang. Because light from distant galaxies takes time to reach us, we look further back in time the farther we look into space. If we look 10 to12 billion years back in time, we then see predominantly strongly star-forming galaxies.

But now a team of astronomers, led by assistant professor Kate Whitaker from University of Massachusetts Amherst and associate faculty at the Danish Cosmic Dawn Center at DTU Space and the Niels Bohr Institute have, observed six massive galaxies in exactly this period which have already used up their gas and stopped forming stars.

The new discovery has just been published in the scientific journal Nature with Whitaker as lead author. 

Clusters of hundreds of galaxies works as a 'cosmic telescope' 

The observations that show these galaxies running on empty have been carried out with the radio telescope ALMA in Chile In order to find the galaxies in the first place, astronomers have taken advantage of the Hubble Space Telescope's high resolution.

Moreover, with a phenomenon known as 'gravitational lensing', the astronomers were able to study this type of galaxies in unprecedented detail. The gravitational lenses in these six cases are intervening clusters of hundreds of galaxies, which with their enormous mass curve space itself so much that it focuses the light from the distant galaxies towards us.

This 'cosmic telescope' magnifies the six galaxies and amplifies their light up to 30 times, allowing Kate Whitaker and her group to constrain the amount of gas to much lower limits than has previously been possible.

Why some galaxies run out of gas and stop forming stars is an open question

What makes some galaxies stop forming stars and become inactive, or 'quiescent', is an open question in astronomy. A criterion for star formation is that the galaxy must contain enough gas, and that this gas must not be too hot, because hot gas clouds are unable to clump and collapse into stars.

So while the discovery is long-yearned for, it is not a huge surprise to the astronomers.

"For a long time, we have suspected that running out of gas is one of the main reasons that some galaxies cease to form stars. But now we have the evidence," said Georgios Magdis, associate professor at DTU Space and the Cosmic Dawn Center, co-author to the study in Nature and part of the team that discovered the six galaxies.

Detecting gas in distant dead galaxies is usually close to impossible because the galaxies are both too faint and too small to see sufficiently small details. But in an observational program, the REsolving QUIEscent Magnified (REQUIEM) Galaxies, Kate Whitaker and colleagues have found some of these peculiar galaxies.

"By using strong gravitational lensing as a natural telescope, we can find the distant, most massive and first galaxies to shut down their star formation," she explains.

Why the six galaxies shut down is still not clear.

"Did a supermassive black hole in the galaxy's center turn on and heat up all the gas. If so, the gas could still be there, but now it's hot. Was the gas blown out of the galaxy. Or did the galaxy simply use it all up. These are some of the open questions that we'll continue to explore with new observations down the road," said Kate Whitaker.