Posted: October 17, 2008
Observations of a spectacular collision of galaxies in the distant Universe have revealed that colossal black holes were present when galaxies were just beginning to form.
The observations were made with the Submillimeter Array (SMA) in Hawaii, which focuses on wavelengths of 200 microns to 1 millimetre that are most sensitive to the very cold gas and dust associated with the earliest evolutionary stages of stars, galaxies and planets. The SMA can peer into these primordial interstellar clouds and witness the birth of stars.
This image captures the moment, approximately 12 billion years ago, when the observed galaxy's black hole ripped a stream of dusty gas from a neighbour. Image: STFC.
In the new observations galaxy 4C60.07 attracted the attention of astronomers because of its bright radio emission that betrayed the presence of a quasar – a black hole spinning rapidly as it feeds on its parent galaxy. At first, astronomers thought that the gas surrounding the black hole was undergoing a burst of star formation, turning fresh new gas into stars at a rate of about 5,000 of our Suns every year. But the latest research revealed that the stars of 4C60.07 are actually much older than originally perceived. Instead, the star forming factory is located in a previously unknown companion galaxy, which is rich in gas and clouded with dust, and has another colossal black hole residing at its centre.
"This new image reveals two galaxies where we only expected to find one," says Professor Rob Ivison, lead author of the study that will be published in the Monthly Notices of the Royal Astronomical Society. "Remarkably, both galaxies contain super-massive black holes. Such monsters are rare and we now suspect that the radio galaxy had only ‘turned on’ because it is involved in a collision with a previously unknown galaxy, which helps feed its black hole and thus power the radio jets.”
The two galaxies – and their central black holes – smashed together in a dramatic collision, but because this cataclysmic event occurred less than two billion years after the big bang, the galaxies will have long since merged, creating a single monstrous black hole. The implications are wide reaching, and the astronomers can’t help but wonder how many other colossal black holes may be lurking unseen in the distant Universe.
"We know the black holes we have found are massive because one is powering a pair of staggeringly luminous radio lobes and the other is modifying the whole optical/infrared spectral energy distribution of its parent galaxy," Ivison tells Astronomy Now. "Massive black holes were already known to exist in the early Universe, but these objects are amazingly rare – we can count them on our fingers. Until now, they were thought to be freaks – completely unrepresentative of the general galaxy population. The observations we've presented in this paper show that such massive black holes may be ten-a-penny."
The galaxies hosting the black holes are both about the size of the Milky Way, but each one is unique. "The radio galaxy looks to be more evolved than its neighbour," says Ivison. "It may have used up most of its gas by forming stars and feeding its black hole in the run-up to the collision, possibly hundreds of millions of years earlier. The neighbour is forming thousands of stars in an intense starburst, triggered by the collision, so it is less evolved and clearly has plenty of gas."
The presence of these two giant black holes in both of the galaxies has big implications for their genesis, since supermassive black holes take time to grow and can have a profound influence on the evolution of their parent galaxy, by triggering star formation or even shutting it down completely. Understanding the prevalence of black holes in the early years of the Universe will rely on combining information from across the whole electromagnetic spectrum. The UK's revolutionary new submillimetre camera, SCUBA 2, should play a key role in black hole characterisation and help astronomers to monitor their growth and their host galaxies in much more detail.