Home » Lyman-alpha Blob: the great challenge to understand the formation of the Universe

Lyman-alpha Blob: the great challenge to understand the formation of the Universe

Understanding the dynamics behind the formation and evolution of galaxies has always been a major challenge. This is why astronomers have focused part of their studies on the Lyman-alpha blobs, which are considered, perhaps rightly, to be the places where the most massive primordial galaxies in the Universe form.

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The Lyman-alpha Blob (LAB, from Lyman-alpha Blob) are giant clouds of hydrogen gas, covering areas thousands of light-years across, among the largest in the Universe. The LABs are visible from planet Earth as a diffuse glow of extreme brightness. The name of these gas clouds refers to the characteristic wavelength of ultraviolet light they emit, known as Lyman-alpha radiation.

Snapshot taken from a cosmological simulation of a Lyman-alpha  Lyman-alpha Blob universe
Snapshot taken from a cosmological simulation of a LAB-1-like Lyman-alpha “blob”. Credits: J.Geach/D.Narayanan/R.Crain

Astronomers initially did not understand what made these huge clouds of gas glow so brightly. Subsequent observations have shown that it is not the gas, which is heated by the blob’s gravitational pull, that makes the LABs glow. Rather, it is the galaxies within the blob that generate this particular radiation.

Lyman-alpha blobs, rare objects in the Universe

Lyman-alpha Blob, also known as LAB (Lyman-alpha Blob), are among the largest objects in the Universe. These huge, very bright clusters are quite rare and are generally found in the regions of the early Universe where matter is most concentrated.

They are gigantic, brilliant clouds of hydrogen gas, reaching a diameter of several hundred thousand light years and far exceeding the size of the Milky Way. The glow that results from observing them can be likened to that of the Milky Way, as suggested by one of the scholars, Jim Geach (of the Centre for Astrophysics Research at the University of Hertfordshire). “It’s similar to a streetlight on a foggy night. You see a diffuse glow because the light from the streetlamp is scattered by the tiny water droplets in the fog.”

The LAB are located at enormous distances from the Earth, so we can observe them as they were when the Universe was only a few billion years old. The study of them is therefore fundamental to our understanding of how galaxies formed and evolved at the time when the Universe was younger.

ESO's Lyman-alpha blob observation reconstruction. Credits: ESO / J. Geach. Lyman-alpha Blob universe
Reconstruction of ESO’s observation of the Lyman-alpha Blob. Credits: ESO / J. Geach.

The name LABs comes from the characteristic wavelength of the ultraviolet light they emit, known as Lyman-alpha radiation, which is produced when the electrons of the hydrogen atom drop from the penultimate to the lowest energy level. Recall that when light waves are polarised, their electric and magnetic fields assume a specific orientation, whereas in unpolarised light the orientation of the fields is random and does not follow any preferential direction.

Theories behind Lyman-alpha Blob

Since their discovery, the processes behind LABs have been an astronomical enigma.Astronomers could not understand what made these enormous clouds of gas glow so brightly.The energy source of their high luminosity and the true nature of the “blobs” were unclear.

There were several theories that attempted to explain the brightness of the Lyman-alpha blob. One theory was that the luminosity was produced by cold gas being heated as a result of its attraction by the blob’s gravity. Another theory suggested that LABs glow because of the bright objects inside them. Specifically, galaxies that are either highly star-forming or host voracious black holes that engulf their matter.

The discovery of Lyman-alpha Blob and their role in the Universe

Studies conducted by an international team through the use of the ALMA (Atacama Large Millimeter/Submillimeter Array) telescope, along with the VLT (Very Large Telescope) of the ESO and the use of other telescopes, have helped determine the true nature of Lyman-alpha Blob.

Remember that the ESO (European Southern Observatory) is the leading intergovernmental astronomy organization in Europe and the most productive astronomical observatory in the world. It is supported by 15 countries: Austria, Belgium, Brazil, Denmark, Finland, France, Germany, Great Britain, Italy, the Netherlands, Portugal, the Czech Republic, Spain, Sweden, and Switzerland. The ESO carries out a program that focuses on the design, construction, and operation of powerful ground-based astronomical instruments that enable astronomers to make important scientific discoveries.

First observations with the ALMA telescope revealed the reason for the intense glow at the heart of one of the LAB observed. Specifically, it was two galaxies that were frantically forming stars. It was this process that was illuminating the surroundings. The two large galaxies were at the center of a multitude of other similar smaller ones, which appeared to be in the process of forming a massive cluster of galaxies. From this it was deduced that the strong glow of the LAB was therefore due to the furious formation of new stars by the two galaxies, immersed in the mist of a huge cloud of intergalactic gas.

Demonstration of the nature of Lyman-alpha band phenomena

The giant LAB-1 blob illuminated by galaxies hidden within the cloud. Credits: ESO / M. Hayes. Lyman-alpha Blob universe
The giant LAB-1 blob illuminated by galaxies hidden within the cloud. Credits: ESO / M. Hayes.

The astronomers had thus discovered for the first time that the light emanating from one of the observed blobs was polarised. Data from subsequent observations of the polarisation of LABs helped to unravel the mystery of the source of illumination of these huge concentrations of gas. Specifically, the diffuse luminosity in Lyman-alpha has helped to provide insight into what is happening in the clouds of primordial gas that envelop young galaxies, a region that is complex to study but crucial to our understanding of such dynamics.

In particular, an international team of astronomers chose one of the largest Lyman-alpha blobs for their study. It is SSA22-Lyman-alpha blob 1, known as LAB-1, one of the first to be discovered in 2000 and one of the brightest. LAB-1 is one of the largest known blobs, with a diameter of about 300 000 light years.

It is so far away that its light takes 11.5 billion years to reach us. Inside LAB-1 are several primordial galaxies, including an active galaxy, whose bright core is powered by an enormous black hole. The gravitational force of the black hole attracts the surrounding matter and causes it to heat up, leading to the generation of these luminous phenomena known as LAB.

Which theory is confirmed by telescope observations?

By checking whether the light from the blobs was polarised, the team was able to compare and discard one of the two theories mentioned above. The first theory assumed that the brightness was produced by the cold gas, while the second theory assumed that the brightness was produced by the luminous objects inside. By studying how light is polarised, astronomers have shown that it is the galaxies that are responsible for the glow. And not, instead, the cold gas that is being heated.

Through an observation of about 15 hours using the VLT, it was found that the light from LAB-1 was polarized around the central region only in a ring-shaped area, while there was no polarization in the center. This effect would be impossible to reproduce if the light were simply produced by the gas falling under the effect of gravity and heating up. In contrast, this effect is exactly what would be expected if light were originally produced by galaxies.

Astronomers continue to study and observe other similar objects. The goal is to prove and verify that the results obtained for LAB-1 can be applied to other known blobs. The challenge to understand the laws that govern the Universe continues. The latter is also played out through the study of what happens to these young, growing galaxies.

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