Giant galaxies die inside out

In a major breakthrough towards understanding the formation of the universe, astronomers have figured out that star formation shuts down in the centres of elliptical galaxies first.

The scientists made this observation with the help of ESO's Very Large Telescope (VLT) and the Hubble Space Telescope.

A major astrophysical mystery has centred on how massive elliptical galaxies, common in the modern universe, quenched their once furious rates of star formation.

Such colossal galaxies, also called spheroids, typically pack in stars 10 times as densely in the central regions as in our home galaxy, the Milky Way, and have about 10 times its mass.

"Massive dead spheroids contain about half of all the stars that the universe has produced during its entire life," said lead author Sandro Tacchella from the ETH Zurich in Switzerland.

"We cannot claim to understand how the universe evolved and became as we see it today unless we understand how these galaxies come to be," Tacchella added.

Astronomers refer to these big galaxies as red and dead as they exhibit an ample abundance of ancient red stars, but lack young blue stars and show no evidence of new star formation.

The estimated ages of the red stars suggest that their host galaxies ceased to make new stars about 10 billion years ago.

This shutdown began right at the peak of star formation in the universe, when many galaxies were still giving birth to stars at a pace about 20 times faster than nowadays.

Tacchella and colleagues observed a total of 22 galaxies, spanning a range of masses, from an era about three billion years after the Big Bang.

According to the new data, the most massive galaxies in the sample kept up a steady production of new stars in their peripheries.

In their bulging, densely packed centres, star formation has already stopped.

"There are many different theoretical suggestions for the physical mechanisms that led to the death of the massive spheroids," said co-author Natascha Forster Schreiber from Max-Planck-Institut for extra-terrestrische Physik in Garching, Germany.

"Discovering that the quenching of star formation started from the centres and marched its way outwards is a very important step towards understanding how the universe came to look like it does now," Schreiber added.