Submillimetre astronomy

Submillimetre astronomy is a specialized branch of observational astronomy. It focuses on studying the electromagnetic spectrum at submillimetre wavelengths. These specific wavelengths are also known as terahertz radiation. Astronomers place this waveband between far-infrared and microwave bands. The wavelengths typically range from a few hundred micrometres to one millimetre. Scientists sometimes use the term "microns" to describe these measurements. This field is vital because interstellar dust absorbs much of the universe's light. Astronomers estimate that dust absorbs up to half of all radiation from stars and galaxies. This absorbed energy is re-emitted in the far-infrared and submillimetre bands. By observing these waves, we can see parts of the cosmos that are otherwise hidden.

To understand how this works, we must look at how light interacts with matter. Submillimetre observations trace emission from various gases and dust. Astronomers look for specific emission lines, such as CI, CO, and CII. These lines come from elements like neutral carbon and carbon monoxide. These signals allow us to study molecular clouds and dark cloud cores. We can observe the process of star formation from the earliest collapse to stellar birth. This is done by determining chemical abundances in dark clouds. We also study the cooling mechanisms for the molecules within them. These observations reveal protoplanetary discs and dusty starburst galaxies from the early Universe. They also show the environments surrounding active galactic nuclei (AGN).

Observing these waves from Earth is difficult due to the atmosphere. Water vapour in our air creates absorption bands that block submillimetre radiation. This causes noise and attenuation, which limits our view. To fix this, astronomers look for "windows" of clear visibility between these bands. The ideal observing site must be very dry and cool. It also needs stable weather and must be far from cities. Only a few locations on Earth are suitable for this work. Mauna Kea in Hawaii is a very established and accessible site. The Llano de Chajnantor Observatory on the Atacama Plateau in Chile is another key location. Other excellent sites include the South Pole and Hanle in India. Some scientists even use high-altitude balloons or aircraft to get above the water vapour.

Different types of telescopes are used to capture these signals. Some observatories use single large dishes, like the James Clerk Maxwell Telescope on Mauna Kea. Others use interferometry, which combines many telescopes to work as one. The Atacama Large Millimeter Array (ALMA) is a massive project in Chile. It is an interferometer made of 54 radio telescopes. These include 12 telescopes that are 12 metres wide and 42 that are 7 metres wide. The Submillimeter Array (SMA) at Mauna Kea uses eight 6-metre telescopes. In Chile, the Atacama Pathfinder Experiment (APEX) is the largest submillimetre telescope in the southern hemisphere. Looking forward, the CCAT-prime telescope is scheduled to begin its work in 2026.

Space-based telescopes offer a way to avoid atmospheric interference entirely. The first submillimetre telescope in space was the Soviet BST-1M. It was located inside the Salyut-6 orbital station and used a 1.5-metre mirror. NASA later launched the Submillimeter Wave Astronomy Satellite (SWAS) in 1998. SWAS focused on five specific spectral lines: water, isotopic water, isotopic carbon monoxide, molecular oxygen, and neutral carbon. This helped scientists understand cloud cooling and chemical compositions. In 2009, the European Space Agency launched the Herschel Space Observatory. Herschel used a very large mirror to study the origins of galaxies. It traveled to a Lissajous orbit around the second Lagrangian point of the Earth-Sun system. This position, 1.5 million km from Earth, reduced interference from the Sun and Earth.

Submillimetre astronomy has led to many significant scientific discoveries. It helped measure the Sunyaev–Zeldovich effect in galaxy clusters. This proved that a hot intracluster medium exists. The field also helps scientists constrain models of how planets, stars, and galaxies evolve. By studying the cosmic microwave background (CMB), researchers can investigate the early Universe. This includes studying the role of gravitational waves and relativistic neutrinos. Even the study of black holes relies on this technology. In 2020, the Event Horizon Telescope produced the first image of a black hole. It used radio and far-infrared observations, including Very Long Baseline Interferometry (VLBI) at 870μm. This work connects submillimetre waves to the fundamental laws of quantum gravity.