Evolution of the Universe

The birth of supermassive black holes

New model explains the birth of these enormous black holes that thrive at the center of galaxies

068-069_Galaxias_194Imagem: Infographic: Tiago Cirillo / Illustration: DrümMost, and possibly all, galaxies contain gigantic black holes at their centers; their mass is millions and billions times denser than that of the Sun. There is no known astrophysical object that can generate such an aberration, and, as such, the secret of its origin has been lost in the dawn of the Universe. Now, a new model devised by Brazilian researchers might help explain the origin and evolution of these mysterious yet important creatures of the cosmic zoo.

It is not difficult for a black hole to be created. Each star with enough mass, when it runs out of fuel, implodes under its own weight and becomes a black hole. The gravitational field of the star is so strong that nothing – not even light – can escape from its surface.

The stars with the densest known mass are those whose stellar mass is 150 times that of the Sun. Before they become black holes, these stars – referred to as blue giants – explode in the form of a supernova and lose most of their original stellar mass. In the best of hypotheses, a black hole with dozens of solar masses remains. How can millions of suns in a black hole at the center of a galaxy be observed?

In the opinion of astrophysicists Eduardo dos Santos Pereira and Oswaldo Miranda, from the National Institute of Space Research (Inpe),in the city of São José dos Campos, State of São Paulo, special circumstances in the cosmic past resulted in the birth of these giants. To begin with, the formation of much bigger stars than those that exist nowadays was possible in the Universe’s very distant past. These ancient supermassive stars might have been able to generate the seeds of the current galactic gluttons that, over billions of years , might have increased their stellar mass by swallowing objects falling into their ever-growing gravitational field.

Astrophysicists have come to a kind of consensus on this process, referred to as accretion. However, accretion has always been viewed with some reluctance. “The growth of these black holes through accretion has always been handled in a rather ad hoc way,” says Miranda. “The researchers establish a mass accretion rate and adjust it to obtain the mass that the black holes supposedly have at present.”

The innovative aspect of this research work, published in late 2011, was to show that it is possible to explain the birth of supermassive black holes based on the cosmic star formation rate – a number that describes how many stars are born on average in the Universe, at every moment. “Many people have been looking for this link and we found it,” Miranda states.

An intriguing aspect of the supermassive black holes is their relationship with the formation of the galaxies they inhabit. Are they the seeds around which stars group up? Or could the formation of galaxies induce the appearance of a black hole at the center?

Coevolution
Apparently, the answer is the coevolution of two phenomena, driven by a third element: dark matter. Halos of this mysterious component – it accounts for most of the matter of the Universe and only interacts with conventional particles by means of gravitational force – might have induced the birth of gigantic stars in the beginning of the Cosmos and, later, might have agglomerated the surrounding matter inside it, thus providing the “bricks” for the construction of galaxies. In this context, black holes preceded the formation of galaxies, but both elements evolved under the influence of the dark matter.

This recent research study also indicates that the growth of gigantic black holes at the center of galaxies might have occurred gradually, over the course of the 13.5 billion years that followed the birth of the first stars. Most of the previous models suggested the need for hyper accelerated growth that did not match what was understood as being the accretion mechanisms involved in this growth.

Another important consequence is that, once the relationship between the star formation rate and the growth of the gigantic black holes was established, it was possible to estimate the behavior of these black holes in the distant past. Perhaps these hypotheses will be confirmed by the next generation of telescopes, such as the James Webb one, designed by Nasa to substitute the Hubble in the next decade.

“The model explains the elements that can be observed, provided the primordial black holes have a thousand solar masses. This is the problem,” says João Steiner, an astronomer from the University of São Paulo. In his opinion, it is unclear whether the primordial Universe could have generated such gigantic black holes, even under conditions favorable to the birth of bigger stars.

Bigger stars may have appeared in the distant past as a result of the primordial Universe’s simpler composition. Soon after the Big Bang, when the first stars were supposedly formed, the only chemical elements available were hydrogen and helium. Heavier atoms – such as oxygen and carbon, which are essential for life – may have appeared later, after the first stars began to explode into supernovas. Lighter elements, which fragment the clouds of gas and thus reduce the change of forming objects with a denser mass, lead to the possibility that much bigger stars than the current ones existed then.

But were these stars really that much bigger? “The hope is that this is where the answer might be found,” says Steiner. “But perhaps this is merely a wish of the researchers. Why are there no supermassive stars in the Small Magellanic Cloud, for example? This cloud displays metallicity [the presence of heavy elements] that is almost primordial.” In Miranda’s opinion, the lack of observable examples leads to the need for theoretical creations. “Computer simulations,” he says, “show that stars with 500 to one thousand solar masses might have been common in the primordial Universe.”