Mysterious infrared light from space detected by ALMA

Astronomers have detected the faintest millimetre-wave source ever observed using Atacama Large Millimetre/submillimetre Array (ALMA), the world's largest ground-based facility for observations in the millimetre and submillimetre regime in Chile.

By accumulating millimetre-waves from faint objects like this throughout the Universe, researchers from University of Tokyo determined that such objects are fully responsible for the enigmatic infrared background light filling the Universe.

By comparing these to optical and infrared images, they found that 60% of them are faint galaxies, whereas the rest have no corresponding objects in optical or infrared wavelengths and their nature is still unknown.

The Universe looks dark in the parts between stars and galaxies. However, astronomers have found that there is faint but uniform light, called the "cosmic background emission," coming from all directions.

This background emission consists of three main components; Cosmic Optical Background (COB), Cosmic Microwave Background (CMB), and Cosmic Infrared Background (CIB).

The origins of the first two have already been found. The COB comes from a huge number of stars, and the CMB comes from hot gas just after the Big Bang.

Researchers went through the vast amount of ALMA data taken during about 900 days in total looking for faint objects. They also searched the datasets extensively for lensed sources, where huge gravity has magnified the source making even fainter objects visible.

"The origin of the CIB is a long-standing missing piece in the energy coming from the Universe. We devoted ourselves to analysing the gigantic ALMA data in order to find the missing piece," said Seiji Fujimoto from University of Tokyo.

Researchers discovered 133 faint objects, including an object five times fainter than any other ever detected. They found that the entire CIB can be explained by summing up the emissions from such objects.

Dust in galaxies absorbs optical and infrared light and re-emits the energy in longer millimetre waves which can be detected with ALMA.

"However, we have no idea what the rest of them are. I speculate that they are galaxies obscured by dust. Considering their darkness, they would be very low-mass galaxies," said Masami Ouchi from University of Tokyo.

The findings were published in The Astrophysical Journal Supplement Series.