Giant space blob glows from within
Researchers discover that Lyman-alpha blobs, gigantic primordial clouds of hydrogen, shine due to embedded galaxies within the structures.
August 17, 2011
A team of astronomers has used the European Southern Observatory’s (ESO) Very Large Telescope (VLT) to study an unusual object called a Lyman-alpha blob. These huge and luminous rare structures are normally seen in regions of the early universe where matter is concentrated. The team found that the light coming from one of these blobs is polarized. In everyday life, for example, polarized light is used to create 3-D effects in cinemas. This is the first time that polarization has ever been found in a Lyman-alpha blob, and this observation helps unlock the mystery of how the blobs shine.
This image shows one of the largest known single objects in the universe, the Lyman-alpha blob LAB-1. This picture is a composite of two different images taken with the FORS instrument on the Very Large Telescope — a wider image showing the surrounding galaxies and a much deeper observation of the blob itself at the center made to detect its polarization. The intense Lyman-alpha ultraviolet radiation from the blob appears green after it has been stretched by the expansion of the universe during its long journey to Earth. These new observations show for the first time that the light from this object is polarized. This means that the giant "blob" must be powered by galaxies embedded within the cloud. Image credit: ESO/M. Hayes
“We have shown for the first time that the glow of this enigmatic object is scattered light from brilliant galaxies hidden within, rather than the gas throughout the cloud itself shining,” said Matthew Hayes from the University of Toulouse, France.
Lyman-alpha blobs are some of the biggest objects in the universe:
gigantic clouds of hydrogen gas that can reach diameters of a few hundred thousand light-years — a few times larger than the size of the Milky Way — and are as powerful as the brightest galaxies. They are typically found at large distances, so we see them as they were when the universe was only a few billion years old. They are, therefore, important in our understanding of how galaxies formed and evolved when the universe was younger. But the power source for their extreme luminosity, and the precise nature of the blobs, has remained unclear.
The team studied one of the first and brightest of these blobs to be found. Known as LAB-1, it was discovered in 2000 and is so far away that its light has taken about 11.5 billion years to reach us. With a diameter of about 300,000 light-years, it is also one of the largest known and has several primordial galaxies inside it, including an active galaxy.
There are several competing theories to explain Lyman-alpha blobs. One idea is that they shine when cool gas is pulled in by the blob’s powerful gravity and heats up. Another is that they are shining because of brilliant objects inside them: galaxies undergoing vigorous star formation or containing voracious black holes engulfing matter. The new observations show that it is embedded galaxies, and not gas being pulled in, that power LAB-1.
The team tested the two theories by measuring whether light from the blob was polarized. By studying how light is polarized, astronomers can find out about the physical processes that produced the light, or what has happened to it between its origin and its arrival at Earth. If it is reflected or scattered, it becomes polarized, and sensitive instruments can detect this subtle effect. To measure polarization of the light from a Lyman-alpha blob is, however, a very challenging observation because of its great distance.
“These observations couldn’t have been done without the VLT and its FORS instrument,” said Claudia Scarlata from the University of Minnesota, Minneapolis. “We clearly needed two things: a telescope with at least an 8-meter mirror to collect enough light and a camera capable of measuring the polarization of light. Not many observatories in the world offer this combination.”
By observing their target for about 15 hours with the VLT, the team found that the light from the Lyman-alpha blob LAB-1 was polarized in a ring around the central region, and that there was no polarization in the center. This effect is almost impossible to produce if light simply comes from the gas falling into the blob under gravity, but it is just what is expected if the light originally comes from galaxies embedded in the central region before being scattered by the gas.
The astronomers now plan to look at more of these objects to see if the results obtained for LAB-1 are true of other blobs.