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Astronomy

Remote universe lights itself up

Reionization, a phenomenon that put an end to the opaque universe, is proved for the first time

CALTECHQuasar SDSS 1044-0125, which led to the discoveriesCALTECH

“End of the dark ages” and “cosmic rebirth”: this was how the astronomers from the California Institute of Technology (Caltech), of the United States, classified a crucial moment in the history of the universe, which they were successful in observing in 2001. This is the reionization of the cosmos – that is, the formation of matter in positive atomic nuclei and negative electrons – a phenomenon that occurred about 12 billion years ago, and which was foreseen in theoretical models but has never been proved before.

The group that is the author of this unprecedented observation is coordinated by the Yugoslav naturalized American George Djorgovski and has a leading participation from Brazilian Sandra Castro, who took her doctor’s degree at the Astronomical and Geophysical Institute of the University of São Paulo (IG-USP), did a post-doctorate at the National Observatory of Rio de Janeiro (ON) and embarked for Caltech in 1999. “Before reionization, it was as if the universe were full of a dark, opaque mist”, says Sandra. “Then the fires lit up and burned through the mist, producing light and clarity.” The “fires” to which she refers, in almost biblical language, are the oldest quasars, one of which, the SDSS 1044-0125, which Sandra studied in depth, led the Caltech team to its great discovery.
Quasars are extraordinary cosmic objects: a little larger than the solar system, they shine as much as 100 times more than a galaxy. They are the ones that reionized the primitive mist, making the universe transparent. This mist was made up of neutral hydrogen, which absorbs light easily. With reionization, the hydrogen atoms were dismembered into protons and electrons, which no longer retain the luminous radiation. As a result, the gas lost its opacity and there was light.

Fundamental part
The existence of that dense and neutral medium, later reionized, was a fundamental part in the jigsaw puzzle of the evolution of the cosmos, hitherto not found. The way of finding it was to observe the universe in great depth, penetrating ages more and more remote. “This is what happened with the discovery of the SDSS 1044-0125 quasar. In the spectroscopic analysis of its radiation, it was found that it had undergone the characteristic absorption of a dense and neutral medium. In the range corresponding to the frequencies absorbed, the graph also showed small peaks of emission, typical of primordial objects being reionized”, says Reinaldo Ramos de Carvalho, from the National Observatory, who worked with Djorgovski in the observation of over 100 quasars – the largest collection of these objects yet studied – and advised on Sandra Castro’s post-doctorate studies.

In the long journey from SDSS 1044-0125 to the Earth, the light from the quasar passed through part of the thick mist that filled the cosmos, which left a mark on the luminous spectrum of the object: its graph indicates that electromagnetic radiation decreased in the ultraviolet range, which corresponds to the photons with sufficient energy to ionize the atomic hydrogen. This shows that it was quasars from the generation of the SDSS 1044-0125 that dragged the universe from its age of darkness and brought about cosmic renaissance.

Largest telescope
Considering the distance of the object, the spectroscopic analysis of SDSS 1044-0125 is a feat that called for great resources. To do so, the Caltech team used no less than the most powerful telescope in the world: Keck II, located on top of the Mauna Kea mountain, in Hawaii, which is fitted with a collecting mirror 10 meters in diameter. Sandra says: “We observed this quasar for a period of 1 year, with a total of 5 and a half hours of exposures, and we only succeeded in identifying the effect produced by reionization after combining all the images obtained into just one.”

When discovered, SDSS 1044-0125 was the oldest object ever observed. It is difficult to determine the distance or the age of a quasar like this one. What can be done, with cutting edge equipment and persistence, is to calculate the red-shift of the electromagnetic radiation that it emits – a direct consequence of the expansion of the universe. It so happens that the radiation that travels through space accompanies the metrics of space: if the universe expands, light expands too, so as to reach the observer with a longer wavelength than when it was emitted.

In the case of visible light, this corresponds to a shift towards red – the range of radiation that that has the longest wavelength – hence the name red-shift. Saying that an object has a red-shift larger than another is equivalent to stating that their emissions have traveled for more time in space, undergoing in a more prolonged manner the effects of the expansion of the universe, until hitting the Earth – and, therefore, that it is more distant.

Red-shift (represented by z) is therefore an indirect way of quantifying the distance – and, consequently, the age – of an object. If the light reaches an observer with double the wavelength that it left the object, it has a red-shift equal to 1; if it arrives with a wavelength six times longer, its red-shift is 5 – the value of z is always one unit less than that of the final wavelength. The radiation from SDSS 1044-0125 showed a shift of z = 5.73. There are structures that are even further back, but today’s resources do not make it possible to observe them, due to the barrier imposed by the mist of neutral hydrogen.

A quasar is born
The true Nature of quasars is an issue that has still not been settled, but studies have made much headway since they were discovered in the 60’s. Today, the majority of researchers consider them products of the gravitational collapse of the enormous quantity of gas and stars that build up in the center of galaxies. When all this matter is compressed by the effect of gravity, the result is a supermassive black hole – a place where the force of gravity is so strong that it does not even allow light to escape -, with a mass equivalent to 100 million to 1 billion Suns. This immense entity then attracts gas and stars from its surroundings, creating a structure known as an accretion disk (agglomeration).

Before being swallowed by the black hole, the particles of the disc are violently accelerated in a spiral. The friction between them heats the disc and produces part of the quasar’s extraordinary radiation. The rest of the radiation is due to another phenomenon. From the combined effect of the pressure of radiation and of the magnetic field, a large number of particles are thrown out, in a jet that is perpendicular to the disc, close to its inner rim. This flow is made up of relativistic electrons, with speeds close to the speed of light, and of a powerful electromagnetic emission.

The birth of the quasars is posterior to the emergence of the galaxies, since the gravitational pull needs at least half a billion years to build up, in the center of the galaxy, that critical mass of 100 million to 1 billion Suns. And a significant quantity of quasars was needed for reionization to reach the scale of the entire universe. In decreasing order of red-shifts (from the most distant and oldest events to the closest and most recent), the first galaxies can be attributed red-shift 10; the first quasars, between 10 and 5; and reionization, between 6 and 5 – according to the findings of the Caltech team.

Recombinant universe
But the universe extends far further than that – in distance and in the past. The moment of the formation of the dense mist that filled the cosmos before the reionization corresponds to the fantastic red-shift 1,500! It is estimated that it happened 300,000 years after the Big Bang – the event that, for the predominant cosmological model, originated the universe some 15 billion years ago. The phenomenon of the formation of the mist was called recombination; with this, after the universe had cooled down from the infinite temperature of the Big Bang to the modest level of 3,000 Kelvin (zero on the Kelvin scale, or absolute zero, is equal to -273.16 degrees Celsius), the electrons and photons, which before had been interacting intensely, separated. It was matter and radiation being uncoupled.

Free from the action of the photons, the electrons could then be captured by simple atomic nuclei, making up the first atoms. And these atoms – basically hydrogen – formed the mist that filled the universe in its dark ages. Recombination made matter become electrically neutral, having been totally ionized before, made up of positive atomic nuclei and negative electrons. The mist was to dominate the scene for half a billion years, until matter was reionized.

Set free when uncoupled, the primordial photons make up today’s background cosmic radiation, a sea of microwaves that fills the whole universe at a temperature of 2.7 K. This radiation is a relic of the age of recombination: it provides scholars with a fantastic snapshot of the universe at that crucial moment. Discovered in the 60’s, it provided a powerful argument in favor of the Big Bang theory. Now, the evidence from observation of reionization have come to reinforce even more this model.

United rivals
This evidence is consequence of the involuntary synergy between two rival teams. The first step was taken by the Sloan Group, made up of researchers from Princeton University, from Fermilab (Fermi National Accelerator Laboratory, from the USA) and from other institutions. Reinaldo de Carvalho informs: “With detectors of the CCD kind, 20 times more efficient than the old photographic plates and capable of measuring the flow from the emitting source in each pixel of the image, they are mapping the whole sky of the Northern Hemisphere in various wavelengths. And they have already found two quasars with red-shifts of around 5.7 – the most distant and hence the most ancient discovered until this moment.”

By defining as a target one of the quasars, the SDSS 1044-0125, the Caltech Group went into action. “Caltech has 45% of the observation time of the two best optical telescopes in the world, Keck I and II, in Hawaii, the fruit of a millionaire donation from a private individual to California University”, Carvalho reveals. This exceptional equipment allowed Sandra and her colleagues to carry out their painstaking work of spectroscopy, which clearly suggests reionization.

“And the proof would not have been possible without an observatory like Keck”, Sandra recognizes. “Today’s universe is complicated and full of patterns that are difficult to understand. The big telescopes take us close to an age in which the universe was simple. They give us a vision of how matter started to organize itself, step by step, to form the billions of galaxies and stars that we see today.”

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