Astronomers have used the ESO's Very Large Telescope (VLT) in conjunction with the NASA/ESA Hubble Space Telescope to reveal how star formation shut down in distant galaxies just three billion years after the Big Bang. Focusing on huge, quiescent elliptical galaxies known as spheroids, the findings are expected to improve our understanding of the evolution of the Universe.
The study focused on 22 distant galaxies with a mass around 10 times that of the Milky Way, and with a density of stars in their central regions around 10 times that observed in our home galaxy. Astronomers often refer to the galaxies as being "red and dead" thanks to their lack of bright, young stars and abundance of ancient red stars.
All of the observed spheroid galaxies are from an era some three billion years after the Big Bang, with the estimated ages of the red stars indicating that their host galaxies ceased to produce new stars some 10 billion years ago. The observed shutdown of star formation took place at a time when many galaxies were giving birth to stars at a pace twenty times that of current rates.
Gaining a better understanding of these galaxies is thought to be essential in furthering our knowledge of the wider Universe.
"Massive dead spheroids contain about half of all the stars that the Universe has produced during its entire life," said Sandro Tacchella, lead author of the study. "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."
To make the observations, the team turned to the VLT's Spectrometer for Infrared Faint Field Imaging instrument (SINFONI), which includes an adaptive optics system that's able to largely cancel out the blurring caused by Earth's atmosphere. The instrument was able to collect near infrared light from the galaxies, accurately detailing exactly where new stars were being born.
The Hubble Space Telescope was also used to study the sample of galaxies, with the team using the observatory's WFC3 camera to capture near-infrared images detailing the distribution of the older, red stars in the galaxies still observed to be forming stars. The data from both instruments was recorded with the same high levels of accuracy.
The observations reveal that star formation in spheroid galaxies shuts down from the inside out, with the largest of the observed galaxies continued to produce stars towards their edges long after star formation had ceased in their dense central regions. It's thought that the findings will help shed light on the underlying mechanisms of star formation shutdown in the giant galaxies – a topic of great discussion among astronomers.
One leading theory to explain the findings suggests that the materials necessary for new stars to form were scattered towards the edges of the spheroids by energy released by the supermassive black holes at their hearts, but many other theories exist.
"There are many different theoretical suggestions for the physical mechanisms that led to the death of the massive spheroids," stated Natascha Förster Schreiber of the Max-Planck-Institut für extraterrestrische Physik in Germany, and co-author of the study. "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."