Too old and too big: The dark enigma of supermassive black holes

A research team from Italy is taking on one of the biggest mysteries of cosmology – supermassive black holes. How could it be that they existed so soon after the Big Bang? And what exactly were the progenitors of these dark enigmas? 

One of the mysteries that Cosmology is shedding some light on is related to black holes, the darkest characters in the cosmic theater. These objects are so dense that not even light can escape from their immense gravitational field. Quite ironically, a black hole is characterised by at least two, huge, contradictions.

[caption id="attachment_54920" align="alignnone" width="620"] Supermassive black holes are found in very old galaxies.[/caption]

Firstly, black holes are very simple solutions of Einstein's equations of General Relativity, but they are also the most diverse from their Newtonian analogues: the gravitational field of a black hole cannot be described in terms of classical, 19th century, Physics. Secondly, by definition, black holes do not emit photons from their event horizon, but the environments around them may be the most luminous of the Universe, due the huge energy irradiated by the in-falling material, approaching the event horizon at high speed.

We observe several stars spinning at high velocities around an apparently empty spot at the center of the galaxy: the clear signature of the presence of a dark cosmic giant.

Over recent years, at the centre of almost every galaxy in the neighbouring Universe, we have observed supermassive black holes – those with a mass of millions to billions that of the Sun. For instance, at the center of our own galaxy, the Milky Way, there is a black hole with about 4 million solar masses. We observed several stars spinning at high velocities around an apparently empty spot at the center of the galaxy: the clear signature of the presence of a dark cosmic giant. Some of these black holes are active and devour huge amounts of gas, irradiating their own host galaxy. Others, like the one at the centre of the Milky Way, are quieter, fortunately for us.

Current theories are very successful and precise in describing the formation process of small black holes, up to tens of solar masses. These objects are formed after the collapse of stars more massive than the Sun, at the end of their life, when the nuclear fuel burned at their center is exhausted. From small black holes, of "stellar mass" as they are called, it is possible to form super-massive black holes through a growth process, with consists of accreting gas from the surrounding environment or merging with other black holes. Nonetheless, this process requires a long time, billions of years, to eventually reach the size of supermassive black holes.

Some of these black holes are active and devour huge amounts of gas, irradiating their own host galaxy.

But, as often happens in the study of nature, the coup de theatre is just around the corner. These supermassive black holes are not only commonly observed at the centre of nearby galaxies, but also in extremely far away galaxies. These are very old galaxies, and existed when the Universe was less than 1 billion years old. This is a small fraction of the 13-14 billion years of its current age, less than one tenth. Understanding how it was possible to form black holes of billion solar masses in such a short time is one of the biggest mysteries in modern Cosmology.

[caption id="attachment_54921" align="alignnone" width="620"] The researchers managed to observe a supermassive black hole with the use of photometric filters.[/caption]

One of the theories proposed to solve this puzzle states that, in the environmental conditions of the early Universe, black holes that formed in the first billion years after the Big Bang were much more massive than the ones that form at the end of the life of a ‘modern’ star. Not tens, but up to 100,000 times the mass of the Sun. One pathway to form these massive and ancient objects is through the direct collapse black hole scenario. As the name suggests, they should have been formed from the direct collapse of huge amounts of primordial gas. Direct collapse, in this case, means without fragmentation. Instead of forming 100,000 stars similar to the Sun, they formed just a single, collapsed, object of 100,000 solar masses. In fact, due to a general relativistic instability, such a huge object cannot exist as a star – it would be an object in hydrostatic equilibrium and consequently, it would immediately collapse into a very massive black hole.

The existence of these intermediate-mass objects would help in understanding the observations indicating the presence of super-massive black holes in the early Universe. They would then represent the missing link between small black holes, formed from stars, and super-giant ones, found at the center of galaxies. Direct collapse black holes are objects theorised by astrophysicists, but they were never observed before.

The light emitted from the environment surrounding the central black hole would lose energy crossing it and would become less energetic and therefore appear redder.

In a very recent study of an Italian team led by cosmologists of the Scuola Normale Superiore in Pisa, we believe we have found a simple and effective method to observe the first black holes formed in the Universe. Our study suggests that it is possible to select these objects, among the billions appearing in a small patch of the sky, by looking at their colours¹. Observed through special photometric filters that allow the selection of wavelengths in a similar way to photographic filters, these objects should appear much redder than other objects observable in the early Universe. This reddening of the outgoing light should be caused by the huge amount of gas present in the host galaxy collapsing toward this central object. The light emitted from the environment surrounding the central black hole would lose energy crossing it and would become less energetic and therefore appear redder.

We predict that the reddening should decrease with time, because the gas present in the host galaxy decreases as well, due to gas accretion into the black hole and to its ejection through outflows. Similarly to the daily cycle of the Sun, at the rise of their life these black holes are very red, progressively turning whiter with the approach of noon.

This method was applied to a very deep photograph of a patch of the sky, taken from three NASA telescopes. We found two very ancient objects that could be the first black holes of this intermediate-mass class ever discovered. These objects were observed in the visible light and in the infrared from the NASA space telescopes Hubble and Spitzer, inside the southerner constellation of the Fornax. Furthermore, these objects are clearly detected in the X-ray spectrum from the NASA X-ray observatory Chandra. The emission of high-energy radiation, like the X-rays, strongly suggests that these objects are really black holes. These two objects were formed when the Universe was younger than one billion years.

If this discovery will be confirmed by upcoming deeper observations, our vision of the formation mechanisms of supermassive black holes would dramatically change. Direct collapse black holes may also represent the progenitors of local supermassive black holes, localised at the center of most galaxies. Consequently, our understanding of the formation process of galaxies would be improved. And we know that galaxies are the basic building blocks of the large-scale structure of the Universe.

Author: Fabio Pacucci is a Cosmology PhD student at the Scuola Normale Superiore in Pisa, Italy. His research focuses on understanding the properties of the first black holes formed in the Universe.

References: 1. www.arxiv.org/pdf/1603.08522v2.pdf