The mystery behind the power source of the giant "Lyman-alpha blob," one of the largest single objects known in the universe, has finally been revealed by astronomers at the European Southern Observatory. Observations from ESO's Very Large Telescope have led to the conclusion that rare vast cloud of glowing gas from the earliest days of the universe must be powered by galaxies embedded within it. The results appear in the Aug. 18 issue of the journal Nature.
Lyman-alpha blobs are huge gaseous structures emitting Lyman-alpha frequency light. Some of these are more than 400,000 light years across in size. The ESO team found that the light coming from one of these blobs is polarized. This is the first time that polarization has ever been found in a Lyman-alpha blob, and this observation will help unlock the mystery of how the blobs shine.
"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." explains Matthew Hayes of the University of Toulouse in France, lead author of the paper.
The team studied one of the first and brightest of these blobs LAB-1, discovered in 2000, which is so far away that its light takes about 11.5 billion years to reach earth. 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.
Some have argued that Lyman-alpha blobs shine when cool gas is pulled in by the blob's powerful gravity, and heats up. Another idea was 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, which power LAB-1.
The team tested the two theories by measuring whether the 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 on Earth. If it is reflected or scattered it becomes polarized and this subtle effect can be detected by a very sensitive instrument. However, measuring polarization of the light from a Lyman-alpha blob is very challenging, because of their great distance.
"These observations couldn't have been done without the VLT and its FORS instrument. We clearly needed two things: a telescope with at least an eight-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," says Claudia Scarlata of the University of Minnesota, a co-author of the paper.
By observing their target for about 15 hours with the Very Large Telescope, the team found that 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 centre. 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.
This video zoom sequence starts with a wide-field view of the dim constellation of Aquarius and slowly closes in on one of the largest known single objects in the Universe, the Lyman-alpha blob LAB 1. Observations with the ESO VLT show, for the first time, that the giant "blob" must be powered by galaxies embedded within the cloud.
Credit:ESO/A. Fujii/Digitized Sky Survey 2/M. Hayes
Music: John Dyson (from the album Moonwind)
This animation of a Lyman-alpha blob begins with a close-up view of a supermassive black hole in the center of a galaxy. Material falls into the black hole via a hot, rotating disk, and powers an outflow that pushes out into the galaxy.