ALMA has helped to solve a fifteen-year-old astronomical riddle. Observations with the 66-dish observatory have revealed the true nature of a huge mysterious object in the distant universe.
The object is known as LAB-1. LAB stands for Lyman Alpha Blob. It’s a huge smudge of light in the night sky, about three times as large as our own Milky Way galaxy. But because it’s so far away – some 11.5 billion light-years – the blob can only be seen by large telescopes.
The visible light from LAB-1 has traveled through the expanding universe. As a result, its light waves have been stretched. Astronomers have calculated that the glow must have been emitted at ultraviolet (UV) wavelengths. UV is the same type of radiation that gives your skin a tan when exposed to sunlight. But no one knew the origin of the ultraviolet glow of LAB-1.
ALMA has now found the answer. At millimeter wavelengths, ALMA has detected the radiation of cool dust in two large galaxies. The two galaxies are in the center of the blob. Other telescopes (including the Hubble Space Telescope and the Keck Telescope on Hawaii) discovered that the two galaxies are part of a large swarm. This swarm also contains a lot of smaller galaxies.
Astronomers think that the galaxy swarm is a so-called proto-cluster: a cluster of galaxies that is still forming. Small dwarf galaxies may be crashing into the two larger ones. The two large galaxies are influencing each other through their mutual gravity. Because of all these interactions, many new stars are being born in the two galaxies – about 100 times as much as in our own Milky Way.
Young, hot stars emit a lot of ultraviolet radiation. But this UV light cannot easily reach us. That’s because the proto-cluster is embedded in a huge cloud of colder gas. The cold gas reflects and scatters the ultraviolet light from within. As a result, it looks like a glowing blob – just like a mist cloud surrounding a lamp post.
Astronomers have discovered more Lyman Alpha Blobs like LAB-1. They may all contain star-forming galaxies in proto-clusters. The new discovery will help astronomers to better understand the very first stages of the formation of galaxy clusters.
SSA22-Lyman-alpha blob 1, or LAB-1 for short, was discovered in 2000. It looks like a small and faint smudge of light, but in reality, it is some 300,000 light-years across. It looks so small because it is located at a distance of some 11.5 billion light-years, in the constellation Aquarius, the Water Bearer. So the light from LAB-1 that we receive today was emitted 11.5 billion years ago. Back then, the universe was only 2.3 billion years old – one sixth of its present age.
LAB-1 was studied by many large telescopes: the European Very Large Telescope at Cerro Paranal in northern Chile, the Keck Telescope at Mauna Kea (Hawaii), the Hubble Space Telescope, and ALMA. the observing campaign was led by Jim Geach of the University of Hertfordshire in Hatfield, United Kingdom. Jim worked together with dozens of other researchers from all over the world. The results of their observations were published in The Astrophysical Journal.