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The feeding habits of teenage galaxies

Smooth gas flow seems to have been a big factor in the building of galaxies in the very young universe, whereas mergers became more important later.
Galaxies in Cetus
This deep view of a tiny patch of sky in the constellation Cetus the Sea Monster shows a selection of galaxies, marked with red crosses, that were used in a new survey of the feeding habits of young galaxies as they grew through cosmic time. Each of the tiny blobs, galaxies seen as they were between three and five billion years after the Big Bang, has been studied in detail using ESO’s VLT and the SINFONI instrument. Credit: ESO/CFHT
Astronomers have known for some time that the earliest galaxies were much smaller than the impressive spiral and elliptical galaxies that now fill the universe. Over the lifetime of the cosmos, galaxies have put on a great deal of weight, but their food and eating habits are still mysterious. A new survey of carefully selected galaxies has focused on their teenage years — roughly the period from about 3 to 5 billion years after the Big Bang.

By employing the state-of-the-art instruments on the European Southern Observatory’s (ESO) Very Large Telescope (VLT), an international team is unraveling what really happened. In more than 100 hours of observations, the team has collected the biggest set ever of detailed observations of gas-rich galaxies at this early stage of their development.

“Two different ways of growing galaxies are competing: violent merging events when larger galaxies eat smaller ones, or a smoother and continuous flow of gas onto galaxies. Both can lead to lots of new stars being created,” said Thierry Contini from the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse, France.

The new results point toward a big change in the cosmic evolution of galaxies when the universe was between 3 and 5 billion years old. Smooth gas flow seems to have been a big factor in the building of galaxies in the very young universe, whereas mergers became more important later.

“To understand how galaxies grew and evolved, we need to look at them in the greatest possible detail,” said Contini. “The SINFONI instrument on ESO’s VLT is one of the most powerful tools in the world to dissect young and distant galaxies. It plays the same role that a microscope does for a biologist.”

Distant galaxies, like the ones in the survey, are just tiny, faint blobs in the sky, but the high image quality from the VLT used with the SINFONI instrument means that the astronomers can make maps of how different parts of the galaxies are moving and what they are made of.
There were some surprises.

“For me, the biggest surprise was the discovery of many galaxies with no rotation of their gas,” said Benoit Epinat from the Astronomy Observatory of Marseilles, France. “Such galaxies are not observed in the nearby universe. None of the current theories predict these objects.”

“We also didn’t expect that so many of the young galaxies in the survey would have heavier elements concentrated in their outer parts. This is the exact opposite of what we see in galaxies today,” said Contini.

The team is only just starting to explore the rich set of observations. They plan to also observe the galaxies with future instruments on the VLT as well as using ALMA to study the cold gas in these galaxies. Looking further into the future, the European Extremely Large Telescope will be ideally equipped to extend this type of study deeper into the early universe.

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