ESO: Giant Space Blob Glows from Within

bystander · Post by **bystander** » Wed Aug 17, 2011 10:21 pm

Giant Space Blob Glows from Within
European Southern Observatory | VLT | 2011 Aug 17

VLT finds primordial cloud of hydrogen to be centrally powered

Observations from ESO’s Very Large Telescope have shed light on the power source of a rare vast cloud of glowing gas in the early Universe. The observations show for the first time that this giant “Lyman-alpha blob” — one of the largest single objects known — must be powered by galaxies embedded within it. The results appear in the 18 August issue of the journal Nature.

A team of astronomers has used ESO’s Very Large Telescope (VLT) to study an unusual object called a Lyman-alpha blob. These huge and very 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 polarised. In everyday life, for example, polarised light is used to create 3D effects in cinemas. This is the first time that polarisation has ever been found in a Lyman-alpha blob, and this observation helps to 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 (University of Toulouse, France), lead author of the paper.

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 which 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 it is so far away that its light has taken about 11.5 billion years to reach us (redshift 3.1). 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 the light from the blob was polarised. By studying how light is polarised 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 polarised and this subtle effect can be detected by a very sensitive instrument. To measure polarisation of the light from a Lyman-alpha blob is, however, a very challenging observation, 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-metre mirror to collect enough light, and a camera capable of measuring the polarisation of light. Not many observatories in the world offer this combination.” adds Claudia Scarlata (University of Minnesota, USA), co-author of the paper.

By observing their target for about 15 hours with the Very Large Telescope, the team found that the light from the Lyman-alpha blob LAB-1 was polarised in a ring around the central region, and that there was no polarisation 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.

Credit: ESO/M. Hayes

Zoomable Image

Central powering of the largest Lyman-α nebula is revealed by polarized radiation - M Hayes, C Scarlata, B Siana

Nature (online 17 Aug 2011) DOI: 10.1038/nature10320

Giant Space 'Blob' Glows Green From Galaxies Within
Space.com | 2011 Aug 17

Click to play embedded YouTube video.
A giant, glowing blob of gas – a cosmic relic of the early universe – is lit by galaxies within it, according to a new study.

"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," lead author Matthew Hayes, of the University of Toulouse in France, said about the Lyman-alpha blob, a rare and brightly lit gas cloud structure that is among the largest known objects.

A team of astronomers used the European Southern Observatory's Very Large Telescope (VLT), located at the Paranal Observatory in Chile, to study the Lyman-alpha blob. These huge structures of hydrogen gas are usually seen in regions of the early universe where matter is concentrated.

The astronomers found that the light coming from the blob was strangely polarized, and by studying this effect they were able to unlock the mystery of how the blob shines. [See photo of the mysterious green space "blob"]

The findings of the new study are published in the Aug. 18, 2011 issue of the journal Nature.

Studying how light is polarized allows astronomers to discern how the light is produced. If light is reflected or scattered, it becomes polarized, and this subtle effect can be detected using very sensitive ground-based instruments. However, measuring the polarization of light from a Lyman-alpha blob is tricky, the researchers said, because the target is so far away.

The astronomers used the VLT to study the blob for 15 hours, and they found that the light was polarized in a ring around the central region but that there was no polarization at the center.

This effect is almost impossible if light simply comes from the gas falling into the blob due to gravity, the researchers said. Instead, it would be expected if the light originated from galaxies embedded in the blob's central region before becoming scattered by the gas. [6 Weird Facts About Gravity]

Lyman-alpha blobs can measure a few times larger than the Milky Way, and are as powerful as the brightest galaxies.

The blobs are typically seen from vast distances, so they appear as they were when the universe was only a few billion years old. As a result, they are important for understanding how galaxies formed and evolved when the universe was much younger.

Hayes and his team studied one of the first, largest and brightest known Lyman-alpha blobs – LAB-1, which was discovered in 2000 and is so far away that its light takes about 11.5 billion years to reach Earth.

LAB-1 measures about 300,000 light-years across. Embedded within it are several primordial galaxies, including one active galaxy.

The researchers said more of these blobs need to be examined to see whether the results obtained for LAB-1 are also true for them.

Galaxies Make Giant Gas Clouds Glow
Sky & Telescope | Shweta Krishnan | 2011 Aug 17

About a decade ago, Caltech astronomer Charles Steidel and others embarked on a mission to count faint galaxies in the early universe. That’s when the team stumbled on a surprising discovery: two giant clouds of glowing gas, larger and more massive than anything seen previously.

Subsequent hunts using the Subaru and the Keck telescopes on Mauna Kea helped locate 30 more such behemoths, each holding up to 10 times the Milky Way’s mass. Spectroscopic studies showed Lyman-alpha lines — emission lines seen when excited hydrogen atoms lose energy — and these clouds were dubbed Lyman-alpha blobs. These especially large, bright objects possibly formed shortly after the universe had begun to generate stars.

But what illuminates these enormous gas clouds? Unfortunately, the speculations that sprouted over the last decade spawned a debate but no real answer. While some theorists think the glow could be due to embedded galaxies, others have argued that gas filaments gravitating toward central masses of dark matter get heated and emit light.

Now, a study to be published in tomorrow’s issue of the journal Nature points in favor of the embedded-galaxy idea. Using ESO’s Very Large Telescope in Chile, Matthew Hayes (University of Toulouse), Claudia Scarlata (University of Minnesota), and Brian Siana (University of Geneva) studied the largest Lyman-alpha blob, LAB1, and postulated that an active galaxy at its heart generates the glow.

Located 11.5 billion light-years away, at a redshift of 3.1, LAB1 spans 300,000 light-years across and was one of the first blobs seen. But the three researchers noticed something new during their recent observation: photons from the lighted cloud were all polarized.

Polarization is common in nature. In fact, some sunglasses eliminate glare using this principle. Photons oscillate along three dimensions as they travel in space. When they encounter a hurdle — here, it’s gas molecules — the obstacle can orient the photons so they oscillate only in one direction.

In this study, the photons coming from LAB1 appear to be polarized in a radial pattern, which Hayes attributes to the unique geometry within the blob. The distinct polarization pattern would occur if a central source of light were bouncing off all the gas molecules. This favors a galaxy at the core. If the light were from random gas filaments, the polarization would not be so uniform.

“Observations of this gas are typically very difficult to do in astronomy,” says Hayes, “and this polarization signal provides a completely new way of measuring the properties of this gas.”

The structure of the central galaxies is still a mystery. From the large amount of observed ultraviolet light, they must house a large number of very young stars. But nothing more is known. “We are more concerned with the gas that surrounds the galaxies rather than the galaxies themselves,” Hayes explains.

This study was entirely based on visual data, and the team now plans to follow up with spectroscopic studies. While the results so far do favor a central galaxy, the scheme involving interacting gas filaments has found a lot of support in collaborative evidence. And a future study with a larger sample size might help make a stronger case one way or the other.