The Origin of Light that Bathes Our Universe: Quasars or Galaxies?

First Posted: Aug 27, 2014 09:10 AM EDT

Scientists may have just taken one step closer to uncovering the origin of the ultraviolet light that bathes our universe. The findings could tell researchers a bit more about how galaxies were first formed.

 "Which produces more light? A country's biggest cities, or its many tiny towns?" asked Andrew Pontzen, lead author of the new study, in a news release. "Cities are brighter, but towns are far more numerous. Understanding the balance would tell you something about the organization of the country. We're posing a similar question about the universe: does ultraviolet light come from numerous but faint galaxies, or from a smaller number of quasars?"

Quasars are some of the brightest objects of the universe. Their light is created by gas as it falls toward a black hole. In fact, quasars make galaxies with billions of stars look practically dim by comparisons. By understanding whether numerous small galaxies can outshine bright quasars, scientists can find out the way the universe built up today's populations of stars and planets.

In order to find out a bit more about light, the researchers examined the light from quasars and how it interacted with hydrogen gas on its journey to Earth. Two types of hydrogen gas are found in the universe: a plain, neutral form and one charged form created by the bombardment of UV light. These two forms of light can be distinguished by studying a particular wavelength, called "Lyman-alpha." In theory, scientists can study where in the universe this Lyman-alpha light has been absorbed to map the neutral hydrogen.

By examining quasars that are located billions of light-years away, scientists can see what the universe looked like in the distant past. In fact, the resulting map will reveal where neutral hydrogen was located billions of years ago. An even distribution of neutral hydrogen case would suggest numerous galaxies as the source of most light, while an uneven pattern would suggest that quasars were the primary source.

"It's amazing how little is known about the objects that bathed the universe in ultraviolet radiation while galaxies assembled into their present form," said Hiranya Peiris, co-author of the new study. "This technique gives us a novel handle on the intergalactic environment during this critical time in the universe's history."

The findings are published in the journal The Astrophysical Journal Letters.

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