Mysterious Infrared Light From Space Puzzles Astronomers

Astronomers have detected the faintest millimeter-wave source ever observed, and determined that such objects are responsible for the enigmatic infrared background light scientists observe throughout the universe.

Using the Atacama Large Millimeter-submillimeter Array (ALMA), astronomers have accumulated several observations of millimeter-waves from faint galaxies throughout the universe. By comparing ALMA observations with optical and infrared data from Hubble and the Subaru Telescope, researchers found that 60% of them originate from faint galaxies, whereas the rest have no corresponding source in optical/infrared wavelengths, and their nature is still unknown. [ALMA Uncovers Planet Formation Around Binary Star]


Messier 101 in visible and infrared light. Image credit: NASA/ JPL-Caltech

In the past decade, astronomers have noticed that in the vast empty space between stars and galaxies, there is a uniform faint light coming from all directions. The mysterious phenomena, dubbed the “cosmic background emission,”consists of three main components: Cosmic Optical Background (COB), Cosmic Microwave Background (CMB), and Cosmic Infrared Background (CIB).

The mechanisms of the first two have already been discovered. The COB originates from a massive number of stars, and the CMB is the most ancient light in the universe, coming from hot gas just after the Big Bang. The cosmic infrared background (CIB) has still yet to be fully explained.

“The origin of the CIB is a long-standing missing piece in the energy coming from the Universe,” said Seiji Fujimoto, who was lead author of a study published in The Astrophysical Journal.

Researchers analyzed data from ALMA taken over the course of 900 days of operation, looking for faint objects. In total, the team discovered 133 faint objects, including an object about five times fainter than any other ever discovered. Evidence suggests that the entire CIB can be explained by summing up the emissions from such objects.

You might ask, what is the nature of these faint objects? By comparing ALMA data with observations from Hubble and Subaru, researchers found that 60% of the faint objects are in-fact galaxies that can be detected in either optical or infrared wavelengths. Scientists suggest dust in the galaxies absorb optical and infrared light and re-emits the energy in longer millimeter 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 team member Masami Ouchi. “This means that such small galaxies contain great amounts of dust. That conflicts with our current understanding: small galaxies should contain small amounts of dust. Our results might indicate the existence of many unexpected objects in the distant Universe. We are eager to unmask these new enigmatic sources with future ALMA observations.”

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