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How the cosmic fog cleared

M. Kornmesser / ESO

This artist's impression shows galaxies at a time less than a billion years after the big bang, when the Universe was still partially filled with hydrogen fog that absorbed ultraviolet light. New observations with the European Southern Observatory's Very Large Telescope are probing this phase of the early universe by studying the light from some of the most distant galaxies ever detected.

Two studies shed additional light on a murky question: How did the cosmic fog that enveloped the universe in its early days dissipate?

In one study, researchers suggest that whatever happened, happened quickly ... and they say it probably had to do with the hot blast of the first generation of stars. Another suggests how the fog-blasting mechanism worked ... and why it might be tricky to see the effect.

First, about that cosmic fog: Cosmologists have worked out a model for the development of the early cosmos that's a good match for their observations, and the model indicates that for the first few hundred thousand years of its existence, the universe consisted of a hot, murky stew of subatomic particles.


About 400,000 years after the big bang, things had cooled down enough for electrons and protons to come together and form a fog of neutral hydrogen gas. This marked a period that astronomers call the "Dark Ages." Eventually, gravity did its magic, and clouds of hydrogen coalesced to create the first stars and galaxies. The remaining hydrogen became electrically charged — "reionized," in geek-speak — and was cleared away.

Today, astronomers can see only as far back as the period of reionization, even if they're using the most powerful telescopes in the world. The best they can do is observe what was happening to galaxies while the reionization was taking place. And that's exactly what astronomers did during a three-year survey that's described in a research paper to be published in the Astrophysical Journal.

Timeline of the early universe
"Archaeologists can reconstruct a timeline of the past from the artifacts they find in different layers of soil. Astronomers can go one better: We can look directly into the remote past and observe the faint light from different galaxies in cosmic evolution," the project's leader, Adriano Fontana of INAF Rome Astronomical Observatory, said today in a news release from the European Southern Observatory. "The differences between the galaxies tell us about the changing conditions in the universe over this important period, and how quickly these changes were occurring."

The team conducted their survey using the ESO's Very Large Telescope in Chile. Astronomers identified five extremely faraway galaxies, based on their redshift, and placed them in a timeline that started at 780 million years after the big bang (which is thought to have occurred 13.7 billion years ago) and ended about a billion years after the big bang. They also measured how much of the galaxies' ultraviolet light was absorbed by the hydrogen fog surrounding the galaxies.

The paper's lead author, Laura Pentericci of INAF Rome Astronomical Observatory, said there was a "dramatic difference" in the amount of light blocked by the oldest vs. the youngest galaxies in the sample.

"When the universe was only 780 million years old, this neutral hydrogen was quite abundant, filling from 10 to 50 percent of the universe's volume," she said in the news release. "But only 200 million years later, the amount of neutral hydrogen had dropped to a very low level, similar to what we see today. It seems that reionization must have happened quicker than astronomers previously thought."

The findings also favor a particular hypothesis for the mechanism behind the reionization. Some theorists say the fog was cleared by radiation blazing forth from the first generation of stars, while others point to the intense radiation given off as matter falls toward black holes.

"The detailed analysis of the faint light from two of the most distant galaxies we found suggsts that the very first generation of stars may have contributed to the energy output observed," said another member of the research team, Eros Vanzella of the INAF Trieste Observatory. "These would have been very young and massive stars, about 5,000 times younger and 100 times more massive than the sun, and they may have been able to dissolve the primordial fog and make it transparent."

Confirming or disproving that hypothesis would require further observations, either from space telescopes or from better ground-based instruments such as the ESO's planned European Extremely Large Telescope.

Jordan Zastrow / Univ. of Mich.

In this three-color image of the dwarf starburst galaxy NGC 5253, green corresponds to starlight. The yellow shows the gas that is being lit up by the starburst at the galaxy's core. The red shows where ultraviolet light from massive stars is evaporating gas, exposing the central starburst along a narrow cone.

Building the case for blazing stars
Another study, published in Astrophysical Journal Letters, provides further support for the blazing-star hypothesis. Astronomers observed a dwarf starburst galaxy known as NGC 5253, about 11 million light-years away in the constellation Centaurus, using the Magellan Telescopes at Las Campanas Observatory in Chile.

NGC 5253 is nowhere near as old as the galaxies that the Italian researchers surveyed, but it does provide a clearer, closer-up view of the phenomenon that might have been at work during the reionization period. "This galaxy is nearby, but we're trying to use it to better understand what was going on in the early universe," the study's lead author, Jordan Zastrow of the University of Michigan, told me.

When Zastrow and her colleagues used special filters to analyze the light from the galaxy, they determined that extreme ultraviolet radiation was blasting out of the galaxy's center and causing hydrogen gas in the interstellar medium to dissipate. "We are not directly seeing the ultraviolet light," Zastrow emphasized in a news release. "We are seeing its signature in the gas around the galaxy."

The signature of the blast shows up in the team's color-coded picture of NGC 5253 as a reddish-yellow tail snaking out toward the lower left corner of the frame. "It appears to be happening over a very narrow region, a very narrow cone," Zastrow said.

The gas within such a galaxy would normally absorb the ultraviolet radiation, but the researchers suggest that superwinds from the galaxy's massive stars helped clear a passageway through the galactic gas, letting more of the light break through.

Starburst galaxies are rarely found in the nearby universe, but they're thought to have been very common in the early universe. NGC 5253 just might be showing astronomers a rerun of the gas-clearing process that marked the age of ionization. But the galaxy is also showing astronomers why it's been hard to see similar processes at work in other galaxies.

"The opening that is letting the UV light out is very small, which makes this light challenging to detect," Zastrow said. "We can think of it as a lighthouse. If the lamp is pointed toward you, you can see the light. If it's pointed away from you, you can't see it. We believe the orientation of the galaxy is important as to whether we can detect escaping UV radiation."

Astronomers might want to take this narrow-beam effect into account as they build their scenarios for how the cosmic fog cleared, Zastrow told me. "Particularly because this issue is so interesting, and so important for our cosmic history, the important thing is to better understand what is actually possible in terms of learning how it could have happened," she said.

More about cosmic frontiers:


In addition to Pentericci, Fontana and Vanzella, authors of "Spectroscopic Confirmation of Z~7 LBGs: Probing the Earliest Galaxies and the Epoch of Reionization" include M. Castellaon, A. Grazian, M. Dijkstra, K. Boutsia, S. Cristiani, M. Dickinson, E. Giallongo, M. Giavalisco, R. Maiolino, A. Moorwood and P. Santini.

In addition to Zastrow, authors of "An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253" include M.S. Oey, Sylvain Veilleux, Michael McDonald and Crystal L. Martin.

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