The first supermassive black holes were formed shortly after the "Big Bang". That is the conclusion reached by an international research group led in 2011 by Prof. Lucio Mayer from the University of Zurich. The supermassive black holes were formed through the collision of galaxies 13 billion years ago. The new findings are important in order to understand the origin of gravitation and cosmological structures.
For more than two decades, science has assumed that galaxies grow hierarchically, i.e. that initially, small masses are pulled together by gravitation, and from them, larger structures form step by step. The researchers at the University of Zurich have now turned that assumption upside down.
"Our result shows that large structures such as galaxies and massive black holes formed quickly in the history of the universe," said Mayer. "At first glance, this seems to contradict the standard theory with cold dark material which describes the hierarchical building of galaxies." The apparent paradox is explicable according to Lucio Mayer: "Normal matter from which the visible parts of the galaxies and supermassive black holes are formed collapse more strongly than dark material forming quickly the most massive galaxies in the densest regions of the Universe, where gravity begins to form structures earlier than elsewhere. This enables the apparent non-hierarchical formation of galaxies and black holes."
Huge galaxies and supermassive black holes form quickly. Small galaxies - on the other hand, such as the Milky Way and its comparatively small black hole in the centre weighting only 1 million solar masses instead of the 1 billion solar masses of the black holes simulated by Mayer and colleagues - have formed more slowly.
The galaxies in their simulation would count among the biggest known today in reality - they were around a hundred times larger than the Milky Way. A galaxy that probably arose from a collision in that way is our neighbouring galaxy M87 in the Virgo cluster, located at 54 million light years from us.
The scientists began their simulation with two large, primary galaxies comprised of stars and characteristic for the beginning of the universe. They then simulated the collision and the merging of galaxies. Thanks to the super-computer "Zbox3" at the University of Zurich and the "Brutus Cluster" from the ETHZ, the researchers were able to observe, at a resolution higher than ever before, what happened next: Initially, dust and condensed gases collected in the center of the new galaxy and formed a dense disk there. The disk became unstable, so that the gases and the dust contracted again and formed an even more dense region. From that, a supermassive black hole eventually came into existence without forming a star first.
The assumption that the characteristics of galaxies and the mass of the black hole are related to each other because they grow in parallel will have to be revised. In Mayer's model, the black hole grows much more quickly than the galaxy. It is therefore possible that the black hole is not regulated by the growth of the galaxy. It is far more possible that the galaxy is regulated by the growth of the black hole.
Mayer and his colleagues believe that their research will also be useful for physicists who search for gravitational waves and thus want to supply direct proof of Einstein's theory of relativity. According to Einstein, who received his doctorate in 1906 at the University of Zurich, the merging of supermassive black holes must have caused massive gravitational waves - waves in a space-time continuum whose remains should still be measurable today.
More recently, using the deepest X-ray image ever taken, astronomers found the first direct evidence that massive black holes were common in the early universe. This discovery from NASA's Chandra X-ray Observatory shows that very young black holes grew more aggressively than previously thought, in tandem with the growth of their host galaxies.
By pointing Chandra at a patch of sky for more than six weeks, astronomers obtained what is known as the Chandra Deep Field South (CDFS). When combined with very deep optical and infrared images from NASA's Hubble Space Telescope, the new Chandra data allowed astronomers to search for black holes in 200 distant galaxies, from when the universe was between about 800 million to 950 million years old.
"Until now, we had no idea what the black holes in these early galaxies were doing, or if they even existed,” said Ezequiel Treister of the University of Hawaii, lead author of the study appearing in the June 16 issue of the journal Nature. “Now we know they are there, and they are growing like gangbusters."
The super-sized growth means that the black holes in the CDFS are less extreme versions of quasars -- very luminous, rare objects powered by material falling onto supermassive black holes. However, the sources in the CDFS are about a hundred times fainter and the black holes are about a thousand times less massive than the ones in quasars.
The observations found that between 30 and 100 percent of the distant galaxies contain growing supermassive black holes. Extrapolating these results from the small observed field to the full sky, there are at least 30 million supermassive black holes in the early universe. This is a factor of 10,000 larger than the estimated number of quasars in the early universe.
“It appears we've found a whole new population of baby black holes,” said co-author Kevin Schawinski of Yale University. “We think these babies will grow by a factor of about a hundred or a thousand, eventually becoming like the giant black holes we see today almost 13 billion years later.”
A population of young black holes in the early universe had been predicted, but not yet observed. Detailed calculations show that the total amount of black hole growth observed by this team is about a hundred times higher than recent estimates.
Because these black holes are nearly all enshrouded in thick clouds of gas and dust, optical telescopes frequently cannot detect them. However, the high energies of X-ray light can penetrate these veils, allowing the black holes inside to be studied.
Physicists studying black holes want to know more how the first supermassive black holes were formed and how they grow. Although evidence for parallel growth of black holes and galaxies has been established at closer distances, the new Chandra results show that this connection starts earlier than previously thought, perhaps right from the origin of both.
“Most astronomers think in the present-day universe, black holes and galaxies are somehow symbiotic in how they grow,” said Priya Natarajan, a co-author from Yale University. “We have shown that this codependent relationship has existed from very early times.”
The image below shows the evolution of a gas disk created by the collision of two identical protogalaxies, from the disk's formation (upper left) until the onset of central collapse (lower right). Brighter colors indicate regions of higher density. (Image courtesy L. Mayer et al.)