Green glow from the old galaxies surprises scientists

This image shows the results of a deep field survey of the Subaru Telescope. A new analysis used a Subaru deep field image to identify the location of nearly 1,300 distant galaxies. The analysis is then combined with the UV-radiation of these galaxies to learn about their composition. A few bright supernova can also be seen in the picture.

(Subaru Telescope, NAOJ)

GRAPEVINE, Texas — The galaxies are greener on the other side of the universe, it seems: A new analysis shows that some of the most distant galaxies ever observed are bright green light, a finding that could deeply impact on the development of models of the first galaxies.

“It is a very characteristic, intense green light that these galaxies only to pour out,” Matt Malkin, a professor of physics at the University of California in Los Angeles, who led the new research, said on Dec. 8 during a press conference at the 229th American Astronomical Society meeting.

Researchers have seen that this is the same kind of green light from a few galaxies in the nearby universe, but according to Malkin, the “shocking” part of the new analysis is in the number of galaxies involved: The green light that researchers have observed in the new study turns out to be from the majority, or all, of the galaxies formed in the first 1 billion or 2 billion years of the universe. The finding is further evidence that the scientists don’t fully understand many of the characteristics of the stars and galaxies in this early period of the universe life. Fortunately, that is an era that has a couple of upcoming telescopes will illuminate. [The History And Structure of the Universe (Infographic)]


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  • The History And Structure of the Universe (Infographic)

Green is not a typical color for a galaxy. Most galaxies scientists observe tend to be whitish in colour, with a few subtle deviations — some may have a red, orange or yellow tint as they contain a lot of cool, red stars; others bluish-white if they contain many of the hot, short-lived blue stars. Regions with a lot of hot gas can have strong, pronounced colors. For example, regions of the recent heavy star formation, such as the orion nebula , may radiate with a pink glow created by the presence of hydrogen. (With thanks to a technique called spectroscopy, scientists can look to the light of an object and find out what chemicals are present.)

But there are a few exceptions in the relatively nearby universe: a sprinkling of dwarf galaxies with a high star formation rate, known as the “green pea galaxies” that were identified by the citizen scientist project Galaxy Zoo. The greenish tint comes in particularly of oxygen atoms that had two of their electrons ripped away (the symbol for this type of oxygen atom is O++).

For a star to produce enough energy to doubly ionize oxygen atoms, it would have a temperature of around 50,000 Kelvin, according to Malkin; the hottest known stars in the universe reach temperatures of 50,000 or 60,000 Kelvin. It may be that these green peas have an unusually high number of very hot stars that have their energy through the oxygen-rich gas clouds, and deliver a solid slap on the oxygen atoms therein, the removal of the two electrons of the atoms. But a full explanation for how green-pea galaxies as a large amount of these specific forms of oxygen atoms is still a matter of debate, according to a statement from UCLA about the new work.

Green peas make up less than one hundredth of 1 percent of the known galaxies. The galaxies that Malkin and the colleagues studied are all very, very far away, and because light takes time to travel through space, that means that astronomers see galaxies on the way in which they have a long, long time ago . The light of the galaxies that Malkin and his team study was emitted about 11 billion years ago, when the universe was only about 2 billion or 3 billion years old.

That early galaxies were a little less chemically polluted than modern. The oven in the heart of a star fuses atoms together, creating things such as carbon, iron and oxygen. When massive stars die, they explode and spread of these elements in the space, and that explosions can also compress and heat material for the creation of more heavy elements . After more than 10 billion years of star formation star death, modern galaxies are a rich stew of heavy elements (astronomers characterize these stars have “high metallicity”) — a condition that was not present in these young galaxies.

When that heavy elements in the inner core of a star, they have the tendency to absorb the energy of the stars of the internal oven, so that the stars in the modern universe (where there are more heavy elements) are usually cooler than the stars in the early universe. In other words, the lower metallicity stars tend to be warmer and more high-energy radiation, Daniel Stark, an assistant professor in astronomy at the University of Arizona, who was not involved in the new analysis, told Modern galaxies can certainly be a bit of the green light is an indication of O++ atoms, according to Malkin, but that light does not dominate the colours of the galaxy.

As early galaxies don’t contain as many heavy elements, then there must be more of those hot stars. That energy can pass, by means of oxygen-rich interstellar gas clouds, and the possible making of a stronger O++ green glow. Models of these early, massive stars indicate that this is probably the case, but the new finding shows that the green light is a factor of two more intense than what the models predict. Malkin said the models is dependent upon a lot of unknown parameters”, and are currently quite simplistic, but he estimates that the light of O++ atoms is “four or more times stronger than the pink light of gaseous hydrogen; most of the models predict that the signature of hydrogen should be equal to or stronger than the O++. (A study earlier this year confirmed the presence of oxygen in a distant galaxy).

Malkin new results highlight how underdeveloped these models are, Stark said.

“We can study the massive stellar populations in the Milky way in great detail. But stars that are present in the early universe are likely to be much lower level,” said Stark. “The massive stars in such low metallicity systems remain very poorly understood.”

It is possible that the stars are actually “one way or another, much hotter, more energy than even the hottest energetic star we know today,” Malkin said during his presentation at AAS. “So they are not only emitting UV photons, they emit extremely high energy UV photons. They are almost out of X-rays, the coming out of these stars.”

For comparison, the temperature of the surface of the sun is about 5000 or 6000 Kelvin and so the central body of our nearest star is not hot enough to produce X-rays or knock two electrons off an oxygen atom. (The surrounding corona reaches a temperature of a million Kelvin, and can produce X-rays.)

Greenlighting new telescopes

One of the reasons models of old galaxies are not very robust, because those galaxies are incredibly far away, and so they are incredibly difficult to study; there simply is not a lot of observational data to go. Malkin and his colleagues could not see the green glow of the individual galaxies they studied; by itself, the galaxies are too weak. But by combining the observations of the Subaru Deep Field of 1,294 galaxies (a technique called “stacking”), Malkin and his colleagues were able to the general characteristics of these galaxies. They were shocked when they saw that the green light that they saw was so much stronger than what their models expect; in fact, it is the strongest emission line emitted by these galaxies, which means that even though hydrogen must be the most common element in these galaxies, the light they emit is dominated by doubly ionized oxygen.

According to Malkin, the signal is so strong that it can not be easily explained by a few outliers; it turns out that most of the young galaxies that existed at that particular point in the universe’s evolution was emitting this green light.

A new generation of telescopes will be needed to answer important questions about the stars and the galaxies, both Malkin and Stark said. Now, scientists are not able to see the very high-energy UV-light that these galaxies are emitting. But the James Webb Space Telescope are sensitive to green light, that Malkin and his colleagues detected; the instrument will also deliver very fast a resolution that is high enough to study that distant galaxies individually.

If that strong O++ green glow is really a characteristic of most of the galaxies in that time, when scientists could use that double-ionized oxygen signal, to help date the age of distant galaxies, and confirm the detection of new galaxies from that period in the history of the universe.

The results of the new analysis of Malkin and his colleagues are currently being revised for publication.

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