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 ten 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 twenty times faster than nowadays.
"Massive dead spheroids contain about half of all the stars that the universe has produced during its entire life," said lead author Sandro Tacchella of ETH Zurich in Switzerland in a joint press release from European Southern Observatory (ESO) and Hubble European Space Agency. "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 and colleagues observed a total of 22 galaxies, spanning a range of masses, from an era about three billion years after the Big Bang. The SINFONI instrument on ESO's Very Large Telescope (VLT) collected light from this sample of galaxies, showing precisely where they were churning out new stars. SINFONI could make these detailed measurements of distant galaxies thanks to its adaptive optics system, which largely cancels out the blurring effects of Earth's atmosphere.
"What is amazing is that SINFONI's adaptive optics system can largely beat down atmospheric effects and gather information on where the new stars are being born, and do so with precisely the same accuracy as Hubble allows for the stellar mass distributions," said co-author Marcella Carollo, also of ETH Zurich, according to the press release.
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 centers, however, star formation had already stopped.
"The newly demonstrated inside-out nature of star formation shutdown in massive galaxies should shed light on the underlying mechanisms involved, which astronomers have long debated," said Alvio Renzini, Padova Observatory, of the Italian National Institute of Astrophysics, according to the press release.
A leading theory is that star-making materials are scattered by torrents of energy released by a galaxy's central supermassive black hole as it sloppily devours matter. Another idea is that fresh gas stops flowing into a galaxy, starving it of fuel for new stars and transforming it into a red and dead spheroid.
"There are many different theoretical suggestions for the physical mechanisms that led to the death of the massive spheroids," said co-author Natascha Förster Schreiber, at the Max-Planck-Institut für Extraterrestrische Physik in Garching, Germany, according to the press release. "Discovering that the quenching of star formation started from the centers and marched its way outwards is a very important step towards understanding how the universe came to look like it does now."
"Evidence for Mature Bulges and an Inside-Out Quenching Phase 3 Billion Years After the Big Bang," S. Tacchella et al., to appear in the journal Science [http://www.sciencemag.org] on 17 April 2015. Preprint (PDF): http://www.spacetelescope.org/static/archives/releases/science_papers/heic1508a.pdf