Resolving the riddle of the infrared background
Have you ever seen the Milky Way? It’s a faint band of light in the sky. It’s only visible from a dark location, on a clear, moonless night. No one knew what this faint glow was, until the invention of the telescope. Then, it became clear that the Milky Way consists of many thousands of faint stars. Without a telescope, you just can’t resolve the individual stars, only their combined glow.
At infrared and submillimeter wavelengths, a similar glow fills the entire sky. It’s called the Cosmic Infrared Background, or CIB for short. Our eyes can’t see this faint ‘heat glow’ at all, but infrared telescopes and satellites have detected it wherever they looked. And no one knew what it was.
Now, ALMA has solved the mystery. A team of Japanese astronomers combed through the existing archives of the Atacama Large Millimeter/submillimeter Array, in search of very faint sources of millimeter and submillimeter radiation. They found 133 of those faint sources. When they compared the ALMA observations with photographs from the Hubble Space Telescope and the Japanese Subaru-telescope in Hawaii, about eighty of those (some 60 percent) turned out to be very remote galaxies.
Most likely, the astronomers have only detected the brightest of the faint millimeter- and submillimeter sources – there must be many more that are too faint to be seen even by ALMA. Therefore, the team suggests that the Cosmic Infrared Background is not a uniform glow at all. Instead, it consists of countless of those faint, discrete sources. Earlier telescopes just couldn’t see them individually – just like our eyes can’t make out individual stars in the faint glowing band of the Milky Way.
If 60 percent of the faint sources are remote galaxies, what about the remaining 40 percent? No one knows, but they might be small galaxies that contain huge amounts of dust. So much dust, in fact, that visible light and infrared radiation is not able to escape.
In the sky, we can see the Sun, the Moon, the planets and the stars. Using telescopes, astronomers have also discovered nebulae, star clusters and galaxies. But at various wavelengths, the space between the stars appears to be filled with a faint, uniform glow. The glow at visible wavelengths is produced by countless faint stars at large distances. The glow at millimeter wavelengths is the remnant glow of the Big Bang. But the true nature of the Cosmic Infrared Background has never been established. Until now.
Seiji Fujimoto, a graduate student at the University of Tokyo, led the research team that made this discovery. Seiji worked together with Masami Ouchi, Yoshiaki Ono, Takatoshi Shibuya, Masafumi Ishigaki, Hiroshi Nagai and Rieko Momose. They searched through observational data from ALMA that have been collected over a period of two and a half years. Seiji and his colleagues published the results in The Astrophysical Journal Supplement on 28 December 2015.