Giant galaxies die from the inside out

Half of all mass in stars in the universe is found in giant galaxies that stopped forming new stars several billion years ago. Astronomers refer to these galaxies as “red and dead” because they consist almost exclusively of older stars that emit a reddish light, in contrast to young stars that emit a blueish light. An international team of astronomers led by ETH researchers studied the progenitor galaxies of these giant red and dead galaxies and discovered how star formation came to a halt. First, the star formation at the centre of these galaxies ceases while new stars continue to form on the edges. Over the course of a few billion years, star formation in the outer regions also stops.

The team led by Sandro Tacchella, a doctoral student supervised by Marcella Carollo, a professor of astrophysics at ETH Zurich, came to this conclusion by studying 22 giant galaxies in detail using the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile as well as the Hubble Space Telescope. This allowed them to combine very sharp and highly detailed infrared images from the Hubble Space Telescope with spectroscopic data collected at the same resolution with the VLT, making it possible to unveil the old stars and also discover where the young stars are made in these galaxies.

The progenitors of the massive galaxies studied by the ETH-led team go all the way back to the infancy of the universe – more than ten billion years before our time or “only” three billion years after the Big Bang. This is due to the great length of time required for light from these galaxies to reach the earth. “The fact that we can now study the predecessors of these dead giant galaxies in detail is fascinating,” says Tacchella. “It is important to study what happened in galaxies at those early times to understand how the universe became what it is today.”

The scientists hope their findings will help them understand the exact mechanism of how the birth of new stars in giant galaxies came to a standstill, which has been a topic of debate among astronomers for a long time. Amongst the possible mechanisms that are consistent with the new results there are super-massive black holes at the centre of galaxies, which could inject into the galaxies vast amounts of energy that would heat up or even blow out of the galaxies the cold gaseous material from which new stars are born.