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Astrophysics

Sentinel in the cosmic darkness

Super radio telescope inaugurated in Chile is looking for the earliest stars in the cold, dark and distant Universe

In San Pedro de Atacama

ALMA’s antennas on the Chajnantor Plateau operate in synchronized fashion as if they were a 16-kilometer super radio telescope

C. PADILLA / ALMAALMA’s antennas on the Chajnantor Plateau operate in synchronized fashion as if they were a 16-kilometer super radio telescopeC. PADILLA / ALMA

Located at a little over 5,000 meters above sea level and about an hour from the northern Chilean tourist site of San Pedro de Atacama, the Chajnantor Plateau has become the stage for the largest Earth-based astronomical observatory project that man has ever built. It was at this high elevation in the driest desert on the planet, where the sky is nearly cloudless and the average annual rainfall amounts to less than 100 millimeters (mm), that the Atacama Large Millimeter/submillimeter Array (ALMA) was officially inaugurated on March 13. “ALMA is a radio telescope that will enable us to zoom in on objects in the cold, distant Universe, at a sensitivity 10 to 100 times greater than what is available today,” says Dutch astronomer Thijs de Graauw, director of the observatory, who leaves that post in April. ALMA’s maximum angular resolution is 0.004 arc-second, which means that it is capable of distinguishing a truck on the moon from Earth.

The main objective of the observatory—which consists of an array of 66 giant radio antennas that can operate in synchronized fashion as if they were a single super radio telescope 16 kilometers in diameter—is to reveal the origins of the Universe between one and two billion years after the so-called Big Bang, the explosion thought to have started it all. After that enormous release of energy, the Universe cooled and went dark. It entered into a temporary Dark Age, from which it began to emerge with the birth of the earliest stars, galaxies and planets.  That cold and distant Universe is the primary, though not sole, target of ALMA. The observatory will also look for the presence in space of molecules, such as sugar and water, that may be related to life forms.  The solar cycles, which periodically cause our parent star to eject large quantities of mass, will be the target of still other observations.

The planning and construction of this scientific undertaking in the Chilean Andes, about 1,600 kilometers from the country’s capital city of Santiago, took 15 years and cost $1.4 billion. Three major partners cooperated with the government of Chile to finance the construction of the observatory. Europe contributed 37.5% of the cost of the project through the European Southern Observatory (ESO), of which the member states are 14 Old World countries plus Brazil, whose ratification is pending (see interview with astrophysicist Beatriz Barbuy of the University of São Paulo, on page).  The United States’ contribution matched that of the Europeans, and its participation is coordinated by the National Radio Astronomy Observatory (NRAO). Japan and Taiwan provided 25% of the financing for ALMA, and the National Astronomical Observatory of Japan (NAOJ) coordinates their involvement in the venture.

There are two sizes of radio telescopes at ALMA. The larger array consists of 54 parabolic antennas, each having a 12-meter diameter and weighing about 100 metric tons.  The second grouping has 12 seven-meter antennas. Using interferometry techniques, the signals from all the radio telescopes—or some of them in the case of observations that do not require data produced by the full array of antennas—are combined and converted into astronomical data by a supercomputer installed at the Array Operations Site (AOS), a support unit also located on the plateau. The processed data are transmitted from that point on the plateau to the Operations Support Facility (OSF), an operations center 25 kilometers from Chajnantor, at an elevation of approximately 2,900 meters. Of the total number of antennas involved in the project, 57 are now in operation on the plateau, and another nine are at the OSF being readied for likely operational start-up before the end of this year.

Situated in the unusually arid landscape of the Atacama Desert, which is frequently used as a location for science fiction films to mimic the surface of Mars, the Chajnantor Plateau was chosen as the observatory site because of its stable, transparent sky. At 5,000 meters above sea level, the air is thin, and 40% of the Earth’s atmosphere lies below that altitude. The water vapor content in the air, which distorts and interferes with recording radio frequency emissions, is just 5% of the amount present at sea level.  These characteristics make the area around San Pedro de Atacama a highly favorable location for the type of observation performed by ALMA.

The radio telescope array captures the portion of the electromagnetic spectrum with wavelengths of .32 to 3.6 millimeters, which is invisible to the naked eye. Light at these wavelengths comes from large, cold clouds of interstellar space, where the temperature is just a few degrees above absolute zero, and from some of the oldest galaxies in the Universe. It can be used to study the chemical and physical composition of regions that are dense with gas and dust where new stars are being formed.

These regions are often dark and cannot be observed at visible-light frequencies. They can, however, be “seen” clearly in the part of the light spectrum in which ALMA operates. “The initial results obtained by ALMA are spectacular,” says Pierre Cox, who is assuming the directorship of the observatory as Thijs de Graauw steps down. Cox thinks that in the future, the observatory may be able to detect dark matter, a mysterious component that accounts for about a quarter of the Universe.

Initial results
Although it was officially inaugurated just this year, ALMA has been producing data for scientific studies since September 2011, when it began operating with a small number of antennas, generally 16. The initial studies using data collected by the super radio telescope were first published in 2012. The most interesting results made it to the pages of the journal Nature on March 14 of this year.

A team lead by researchers from the California Institute of Technology (Caltech) in the United States used ALMA, at a wavelength of about 3 mm, to measure the distance of 26 far-off, dust-filled galaxies where large numbers of new stars were forming, and discovered that they were more distant, and therefore older, than had been thought. “The more distant a galaxy is, the longer ago in time we are seeing it. Consequently, when we measure distances, we can reconstruct a timeline showing how vigorous the star formation in the Universe has been at different times in its 13.7-billion-year history,” says Caltech’s Joaquin Vieira, principal author of the article.

The researchers saw that on average, the star formation peaks occurred 12 billion years ago—one billion years earlier than had been presumed. Two of these galaxies are the most distant of this type ever observed: they were 12.7 billion years old. In another galaxy the astrophysicists detected molecules of water. According to the paper’s authors, that is the earliest evidence of water ever identified in the Universe.

Projects led by Brazilians
Zulema Abraham of the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at the University of São Paulo (USP) was the first Brazilian astrophysicist to use ALMA to study a celestial object.  Her project competed for observation time with about a thousand proposals from around the world and was one of the 112 initiatives awarded access to data produced by the observatory. In November of 2012, 23 of ALMA’s radio telescopes were pointed for about 20 minutes in the direction of the enigmatic Eta Carinae, a system consisting of two giant, highly luminous stars, the larger having about 90 solar masses and the smaller having 30 solar masses.

ALMA’s radio telescopes: the stable, transparent sky in the Andes makes it a favorable site for observations

ESO/C. MALINALMA’s radio telescopes: the stable, transparent sky in the Andes makes it a favorable site for observationsESO/C. MALIN

At a distance of 7,500 light-years from Earth, Eta Carinae displays a kind of periodic blackout. Every five and a half years, it stops shining for approximately 90 consecutive days in certain bands of the electromagnetic spectrum. Using ALMA, Abraham measured the radio emissions of the binary star system at four wavelengths: 3 mm, 1.3 mm, 1 mm and 454 micrometers.  Some of these wavelengths had never been used to observe the star. “There are few data on Eta Carinae’s cycle in the radio frequencies,” notes the researcher, who is trying to identify the exact location in the binary system where this type of emission originates. In January and February of this year, Abraham received 15 gigabytes of data produced by the observatory in the Chilean Andes—equivalent to about three full DVDs of information. The angular resolution of the data, 0.4 arc-second, is impressive. When using the Itapetinga radio telescope in Atibaia, 60 kilometers from the city of São Paulo, Abraham has only been able to observe Eta Carinae with a maximum resolution of two arc-minutes, hundreds of times lower than the resolution obtainable with ALMA.

Astrophysicist Thaisa Storchi-Bergmann of the Federal University of Rio Grande do Sul (UFRGS) also obtained observation time at ALMA for collaborative research with Neil Nagar of the University of Concepción, Chile. The project, for which observations have not yet been done, consists of mapping the distribution and kinematics of molecular gas in a region of 100 parsecs—a distance equivalent to 326 light-years—around the nucleus of active galaxies where a supermassive black hole is devouring matter around it. Research conducted by Storchi-Bergmann on wavelengths of optical and infrared light revealed the presence of spiral structures in that region, which appear to be channels for feeding the supermassive black hole. “Where there is dust, there is molecular gas, so we are looking for cold molecular gas, which emits in the spectral bands covered by ALMA, to see whether it is in fact moving towards the nucleus,” says Storchi-Bergmann.

In addition to competing for the use of ALMA, a group of Brazilian astrophysicists is negotiating the installation of a 12-meter antenna like the larger ones purchased by the recently-inaugurated observatory, at a site in the Argentine Andes. The project, known as the Long Latin American Millimeter Array (LLAMA), calls for building a small observatory in the town of San Antonio de Los Cobres, 200 kilometers from the Chajnantor Plateau. The initiative would be a partnership between Brazilians and Argentines. “We would purchase the antenna at a cost of €6 million, and they would build and operate the observatory,” says Jacques Lépine, an astrophysicist at IAG-USP and coordinator of its Research Group on Radio Astronomy (NARA), who is LLAMA’s principal coordinator. If the binational project comes to fruition, the antenna could operate independently or in combination with the observatory in San Pedro de Atacama. “With LLAMA, we could improve ALMA’s angular resolution up to 10 times,” Lépine says.

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