90 percent of distant galaxies have gone undiscovered, say astronomers

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Washington, March 26 (ANI): Astronomers, using two of the four giant 8.2-metre telescopes that make up ESO's Very Large Telescope (VLT), have determined that 90 percent of galaxies whose light took 10 billion years to reach us have gone undiscovered.

Astronomers frequently use the strong, characteristic "fingerprint" of light emitted by hydrogen known as the Lyman-alpha line, to probe the amount of stars formed in the very distant Universe.

Yet there have long been suspicions that many distant galaxies go unnoticed in these surveys.

The new VLT survey demonstrates for the first time that this is exactly what is happening.

Most of the Lyman-alpha light is trapped within the galaxy that emits it, and 90 percent of galaxies do not show up in Lyman-alpha surveys.

"Astronomers always knew they were missing some fraction of the galaxies in Lyman-alpha surveys, but for the first time we now have a measurement. The number of missed galaxies is substantial," explained Matthew Hayes, the lead author of the research paper.

To figure out how much of the total luminosity was missed, Hayes and his team used the FORS camera at the VLT and a custom-built narrowband filter to measure this Lyman-alpha light, following the methodology of standard Lyman-alpha surveys.

Then, using the new HAWK-I camera, attached to another VLT Unit Telescope, they surveyed the same area of space for light emitted at a different wavelength, also by glowing hydrogen, and known as the H-alpha line.

They specifically looked at galaxies whose light has been travelling for 10 billion years, in a well-studied area of the sky, known as the GOODS-South field.

"This is the first time we have observed a patch of the sky so deeply in light coming from hydrogen at these two very specific wavelengths, and this proved crucial," said team member Goran Ostlin.

The survey was extremely deep, and uncovered some of the faintest galaxies known at this early epoch in the life of the Universe.

The astronomers could thereby conclude that traditional surveys done using Lyman-alpha only see a tiny part of the total light that is produced, since most of the Lyman-alpha photons are destroyed by interaction with the interstellar clouds of gas and dust.

This effect is dramatically more significant for Lyman-alpha than for H-alpha light. As a result, many galaxies, a proportion as high as 90 percent, go unseen by these surveys.

"If there are ten galaxies seen, there could be a hundred there," Hayes said. (ANI)

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