Hungry galaxy observed while eating

Light of a quasar reveals gas movements around a distant galaxy

This illustration shows a galaxy in the distant universe, just two billion years after the Big Bang, sucking in cold gas (orange) from its surroundings. © ESO / L. Calçada / ESA / AOES Medialab
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What does a galaxy do when it has used up its raw material for star formation? She takes new "food" to herself. And that's exactly where astronomers have now caught a distant galaxy: watching as it absorbs gas from its environment and gradually depletes it to create new stars. This observation was made possible by a rare cosmic event: The galaxy passed in front of a bright quasar, whose light made the processes in the gas visible for the first time, as the researchers in the journal "Science" report.

Gases are the raw material for star production. These gas reservoirs quickly consume galaxies as they form new stars. They therefore need to be continually supplied with new gas in some way in order to sustain this process. Astronomers have long suspected that the galaxies suck in cold gas from the environment through their gravitational attraction. In this scenario, it first pulls the gas to itself, which then circles around the galaxy and rotates with it for a while before it falls into the interior of the galaxy. Although some signs of such accretion have been observed, it has been very difficult to directly observe this process.

Quasar shines through galaxy

Now, astronomers have come to the aid of chance and the Very Large Telescope of the European Southern Observatory (VLT) at the Paranal in Chile. Because this was watching a distant galaxy that was passing between Earth and an even more distant quasar - the extremely bright center of an active galaxy. "This type of arrangement is very rare and has allowed us to make unique observations, " says first author Nicolas Bouché of the Astrophysics and Planetology Research Institute (IRAP) in Toulouse.

The light of the quasar shines through the material around the galaxy in the foreground during this passage. Because the elements in the gas swallow and scatter parts of the light, the light spectrum provides valuable information about the properties of the gas and the galaxy. "That's what ultimately led us to address an important issue in galaxy formation: how do galaxies grow and how do they nourish star formation?"

Gas actually moves into the galaxy

The new observations therefore provide the best view of a galaxy during feeding. The researchers were able to observe how the galaxy rotates and how the gas moves and changes in its environment. "The properties of this enormous amount of surrounding gas were exactly what we would have expected for cold gas being drawn into the galaxy, " says co-author Michael Murphy of Swinburne University of Technology, Melbourne. "It's like the feeding time for lions in the zoo this particular galaxy has a tremendous appetite and we've found out how it feeds itself to grow so fast." Ad

For the first time astronomers could clearly see that the material moves inward - into the galaxy - and not outward. They also determined the composition of this new fuel for future generations of stars. "In this case, we were fortunate that the Quasar is in the right place so its light has to cross the incoming gas, " concludes co-author Crystal Martin of the University of California Santa Barbara in the USA. (Science, 2013; doi: 10.1126 / science.1234209)

(European Southern Observatory - ESO, 05.07.2013 - NPO)