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How black holes and galaxies grew up together
KEITH COOPER
ASTRONOMY NOW
Posted: 15 June 2011


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The most distant and ancient black holes ever uncovered have been found in the centres of faraway galaxies, revealing that black holes and galaxies have been growing in tandem since the very earliest epoch of galaxy-building in the Universe.


An artist’s impression of a galaxy containing an active supermassive black hole at its centre, guzzling gas. Image: NASA/JPL–Caltech.

In the modern Universe all large galaxies appear to possess a supermassive black hole at their centre that weigh in with masses millions of times greater than the Sun. In most galaxies these monstrous black holes are relatively quiescent, but in past times they were highly active, attracting large amounts of gas into encircling accretion discs that grow so hot they emit X-rays. Exactly how these immense black holes formed is one of cosmology’s greatest mysteries, however. To get closer to a solution astronomers directed NASA’s Chandra X-ray Observatory towards over 250 distant galaxies identified by the Hubble Space Telescope’s Wide Field Camera 3, which can delve into near infrared wavelengths to see the light of highly redshifted galaxies whose light first set out 13 billion years ago. By ‘stacking’ multiple Chandra X-ray images of each galaxy in the same way webcam images of the planets can be stacked by amateur astrophotographers, astronomers at Yale University, the University of Hawaii, the University of Michigan and Rutgers University in New Jersey were able to multiply the weak X-ray signals emitted by the black holes.

Despite their faintness, the X-rays detected are of the highest energies only, meaning that weaker X-rays are being blocked by dense swathes of gas that shroud these early galaxies. This is the very gas from which these early galaxies – that our Milky Way Galaxy was once like – are still growing, forming new stars, building their mass and feeding their black hole that hold the whole galaxy together.

“Our observations suggest that this symbiotic connection between black holes and galaxies goes back all the way to the dawn of the Universe,” Dr Kevin Schawinski of Yale University, who worked on the observations, tells Astronomy Now. “They have been co-evolving since the first billion years.”

Although the stacking technique averages the observations that makes it difficult to measure the masses of the black holes, its not out of the question that these black holes, which existed less than 700–800 million years after the big bang, may have already had masses of the order of millions of times greater than the Sun’s mass. One of the biggest questions about galaxy formation has been what came first – the black holes whose gravity can dominate a galaxy, or the galaxies themselves that feed the black holes. Were the black holes born very massive, or did they experience a rapid growth phase even earlier in the Universe? It’s a chicken and egg scenario that, although left unanswered by these observations, has been pushed right back to the very first epoch of growth says Schawinski. “Now we have started to peek at galaxies and black holes right after those ‘seed’ black holes formed and started growing.”

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
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3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
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