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Durham University News

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Wild early lives of today's most massive galaxies revealed

(25 January 2012)

Distant star forming galaxies

Astronomers from Durham University have helped find the strongest link so far between the most powerful bursts of star formation in the early Universe and the most massive galaxies found today.

The galaxies, flowering with dramatic starbursts in the early Universe, saw the birth of new stars abruptly cut short, leaving them as massive - but passive - galaxies of aging stars in the present day.

The astronomers also have a likely culprit for the sudden end to the starbursts - the emergence of supermassive black holes.

The Durham scientists were part of an international team of astronomers who combined observations from the LABOCA camera on the ESO-operated 12-metre Atacama Pathfinder Experiment (APEX) telescope with measurements made with ESO's Very Large Telescope, NASA's Spitzer Space Telescope, and others, to look at the way that bright, distant galaxies are gathered together in groups or clusters.

The more closely the galaxies are clustered, the more massive are their halos of dark matter - the invisible material that makes up the vast majority of a galaxy's mass. The new results are the most accurate clustering measurements ever made for this type of galaxy.

The galaxies are so distant that their light has taken around ten billion years to reach us, so we see them as they were about ten billion years ago. In these snapshots from the early Universe, the galaxies are undergoing the most intense type of star formation activity known, called a starburst.

By measuring the masses of the dark matter halos around the galaxies, and using computer simulations to study how these halos grow over time, the astronomers found that these distant starburst galaxies from the early cosmos eventually become giant elliptical galaxies - the most massive galaxies in today's Universe.

Lead scientist Ryan Hickox (Dartmouth College, USA, and Durham University, UK), said: "This is the first time that we've been able to show this clear link between the most energetic starbursting galaxies in the early Universe, and the most massive galaxies in the present day."

Furthermore, the new observations indicate that the bright starbursts in these distant galaxies last for a mere 100 million years - a very short time in cosmological terms - yet in this brief time they are able to double the quantity of stars in the galaxies. The sudden end to this rapid growth is another episode in the history of galaxies that astronomers do not yet fully understand.

"We know that massive elliptical galaxies stopped producing stars rather suddenly a long time ago, and are now passive. And scientists are wondering what could possibly be powerful enough to shut down an entire galaxy's starburst," said Julie Wardlow (University of California at Irvine, USA, and Durham University, UK), a member of the team.

The team's results provide a possible explanation. At that stage in the history of the cosmos, the starburst galaxies are clustered in a very similar way to quasars, indicating that they are found in the same dark matter halos. Quasars are among the most energetic objects in the Universe - galactic beacons that emit intense radiation, powered by a supermassive black hole at their centre.

There is mounting evidence to suggest the intense starburst also powers the quasar by feeding enormous quantities of material into the black hole. The quasar in turn emits powerful bursts of energy that are believed to blow away the galaxy's remaining gas - the raw material for new stars - and this effectively shuts down the star formation phase.

David Alexander, from the Department of Physics, Durham University, said: "In short, the galaxies' glory days of intense star formation also doom them by feeding the giant black hole at their centre, which then rapidly blows away or destroys the star-forming clouds."

The Science and Technology Facilities Council is the UK sponsor of astronomy and manages the UK subscription to ESO.

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