Atacama Large Millimetre/Submillimetre Array (ALMA)

The Atacama Large Millimetre/Submillimetre Array (ALMA) is one of the world’s most advanced astronomical observatories, designed for observing the universe in millimetre and submillimetre wavelengths. Located high in the Atacama Desert of northern Chile, ALMA allows astronomers to study celestial objects such as stars, galaxies, and planetary systems with unprecedented clarity. Its ability to detect cool gas and dust in space makes it indispensable for understanding the origin and evolution of the cosmos.

Background and Development

ALMA is an international partnership involving Europe (ESO – European Southern Observatory), North America (NRAO – National Radio Astronomy Observatory), and East Asia (National Astronomical Observatory of Japan), in cooperation with the Republic of Chile. The project represents one of the largest scientific collaborations in the field of ground-based astronomy.
The concept of a large array telescope operating at millimetre wavelengths emerged in the 1980s, when advances in radio astronomy revealed that cold regions of the universe could be studied more effectively through these wavelengths than through optical observation. Construction of ALMA began in 2003, and the array became fully operational in 2013.
Its location on the Chajnantor Plateau at an altitude of 5,000 metres above sea level was chosen because the region has one of the driest climates on Earth. The thin, dry atmosphere at this altitude is ideal for millimetre and submillimetre observations, as water vapour in the atmosphere absorbs these wavelengths.

Structure and Design

ALMA consists of a total of 66 high-precision antennas, which work together as an interferometer—a system that combines the signals from multiple antennas to simulate a single telescope with a vast collecting area. The array comprises:

• Fifty-four 12-metre antennas, which form the main array.
• Twelve 7-metre antennas, forming the Atacama Compact Array (ACA), used for observing larger celestial structures with higher sensitivity.

The antennas can be arranged over distances ranging from 150 metres to 16 kilometres, allowing ALMA to adjust its resolution according to the observation requirements. The farther apart the antennas are, the higher the resolution achieved.
Each antenna is equipped with cryogenically cooled receivers, which detect faint cosmic signals in millimetre and submillimetre wavelengths. These signals are then transmitted to a central correlator, a powerful supercomputer capable of processing up to 17 quadrillion operations per second, combining data from all antennas to create detailed astronomical images.

Operating Principles

ALMA operates in the millimetre and submillimetre range (0.3 to 9.6 millimetres) of the electromagnetic spectrum. These wavelengths occupy the region between infrared light and radio waves. Observations at these wavelengths are essential for studying cold and dusty objects that are invisible to optical or infrared telescopes.
The observatory uses the principle of radio interferometry, where multiple antennas observe the same object simultaneously. The combined data provide high spatial resolution equivalent to that of a single telescope with a diameter equal to the maximum distance between antennas. This technique allows ALMA to achieve resolution levels finer than even the Hubble Space Telescope in certain wavelength ranges.

Scientific Objectives

The main scientific objectives of ALMA include:

• Studying the formation of stars and planets: By observing cold molecular clouds and protoplanetary disks, ALMA provides insights into how stars and planets form from cosmic dust and gas.
• Investigating early galaxies: ALMA detects faint emissions from galaxies that formed shortly after the Big Bang, helping astronomers trace the evolution of cosmic structure.
• Understanding the chemistry of the universe: The array can detect molecular signatures, allowing scientists to identify complex organic molecules in interstellar space, potentially linked to the origins of life.
• Observing black holes and active galactic nuclei: ALMA contributes to high-resolution imaging of galactic cores, including studies of material swirling around supermassive black holes.
• Exploring the Sun and Solar System: The array helps observe solar flares, comets, and the atmospheres of planets and moons with exceptional detail.

Key Discoveries

Since becoming operational, ALMA has made numerous groundbreaking discoveries that have transformed modern astronomy. Some of its notable achievements include:

• Imaging of Protoplanetary Disks: In 2014, ALMA captured a detailed image of the protoplanetary disk around the young star HL Tauri, revealing concentric rings that showed the early stages of planet formation.
• Detection of Complex Organic Molecules: ALMA has detected molecules such as methanol and methyl cyanide in star-forming regions, suggesting that prebiotic chemistry occurs even before star and planet formation.
• Observation of Distant Galaxies: The array has observed galaxies over 13 billion light years away, providing evidence of star formation during the early universe.
• Support for the Event Horizon Telescope (EHT): ALMA played a crucial role as a key component in the EHT network, which produced the first-ever image of a black hole’s event horizon in 2019.

Technological Innovations

ALMA represents a triumph of modern engineering and computing. Its advanced design includes:

• Cryogenic Receivers: These operate at temperatures close to absolute zero to minimise noise and enhance signal sensitivity.
• High-Speed Data Processing: The central correlator processes enormous data volumes in real time, ensuring accurate signal combination from all antennas.
• Flexible Configuration: The antennas can be relocated across the plateau using specially designed transporters, enabling different observational setups.
• Precision Engineering: Each dish surface maintains a precision of within 25 micrometres, critical for accurate millimetre-wave reflection.

Environmental and Logistical Challenges

Operating at 5,000 metres above sea level poses severe environmental and physiological challenges. The thin air reduces oxygen availability, affecting both human workers and electronic systems. To mitigate these difficulties, most human operations and control functions occur at a lower-altitude support facility (Operations Support Facility, OSF) located at 2,900 metres.
Moreover, maintaining the antennas in the harsh, arid environment of the Atacama Desert requires continuous calibration and advanced thermal control to protect sensitive instruments from extreme temperature variations.

International Collaboration and Management

ALMA exemplifies large-scale international scientific cooperation. It is jointly operated by:

• ESO (European Southern Observatory) on behalf of its member countries.
• NRAO (National Radio Astronomy Observatory) on behalf of the United States and Canada.
• NAOJ (National Astronomical Observatory of Japan) representing East Asia.The Joint ALMA Observatory (JAO) coordinates overall scientific and technical operations from Chile.

Such collaboration ensures shared access to data, equal participation in research opportunities, and global advancement of astronomical science.

Future Prospects and Scientific Importance

The future of ALMA includes plans for expanded bandwidth, improved receiver sensitivity, and advanced data processing systems. Upgrades to the correlator and addition of new frequency bands will enhance ALMA’s capability to study fainter and more distant cosmic objects.