Feeding galaxy caught by the light of a quasar
Sen— It has long been suspected that galaxies grow by pulling in material from their surroundings, but this process has proved very difficult to observe directly. Now astronomers have found the best direct observational evidence so far to support the theory.
The European Southern Observatory's (ESO) Very Large Telescope was used to study a rare alignment between a distant galaxy and an even more distant quasar, the extremely bright centre of a galaxy powered by a supermassive black hole.
Galaxies deplete their reservoirs of gas as they create new stars, and must somehow be continuously replenished with fresh gas. Astronomers suspected that a galaxy drags gas inwards, which then circles around the galaxy, rotating with it before falling in. Although some evidence of such accretion had been observed in galaxies before, the motion of the gas and its other properties had not been fully explored up to now. The new observations revealed how the galaxy was rotating, and the composition and motion of the gas outside the galaxy.
The gas clouds surrounding the galaxy do not shine and are not visible in direct images. Without the quasar in the background far less information would have been be obtained. They observed gas falling inwards towards the galaxy, creating a flow that both fuels star formation and drives the galaxy's rotation. The galaxy itself lies at a redshift of 2.3285, meaning that we are seeing it when the Universe was just about two billion years old.
"This kind of alignment is very rare and it has allowed us to make unique observations," explains Nicolas Bouche of the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse, France, lead author of the new paper. "We were able to use ESO's Very Large Telescope to peer at both the galaxy itself and its surrounding gas. This meant we could attack an important problem in galaxy formation: how do galaxies grow and feed star formation?"
The light from the quasar, called QSO J2246-6015, or HE 2243-60, passes through the gas clouds around the foreground galaxy and some wavelengths are absorbed. These absorption fingerprints tell astronomers about the motions and chemical composition of the gas around the galaxy.
"The properties of this vast volume of surrounding gas were exactly what we would expect to find if the cold gas was being pulled in by the galaxy," says co-author Michael Murphy from Swinburne University of Technology, Melbourne, Australia. "The gas is moving as expected, there is about the expected amount and it also has the right composition to fit the models perfectly. It's like feeding time for lions at the zoo, this particular galaxy has a voracious appetite, and we've discovered how it feeds itself to grow so quickly."
This is the first time that astronomers have been able to show clearly that the material around galaxies is moving inwards rather than outwards, and to determine the composition of this fresh fuel for future generations of stars.
"In this case we were lucky that the quasar happened to be in just the right place for its light to pass through the infalling gas. The next generation of extremely large telescopes will enable studies with multiple sightlines per galaxy and provide a much more complete view," concludes co-author Crystal Martin of the University of California Santa Barbara, USA.