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Astronomy team discovers evidence of a galactic metal cocooned in dust

By: April Carson

A complete grasp of galaxy evolution is reliant in part on an accurate assessment of the metal content in the intergalactic medium—the space between stars—but dust may confound optical observations. By examining infrared data gathered during a multiyear monitoring effort, an international team of researchers at the University of California, Irvine, Oxford University in England, and other institutions discovered evidence that local galaxies have greater amounts of heavier components—which previous research indicated were lacking. This "metal cocoon" of dust could explain some discrepancies in our understanding of how metals are dispersed throughout the universe.

"The idea that there might be this missing population of metals was first proposed about 10 years ago, but it's been very hard to find direct evidence for it," said lead author Andrew Bunker, UCI associate professor of physics & astronomy. "Our new study provides the first clear evidence that this metal-enriched dust exists in galaxies like our own."

The researchers studied five galaxies that are dim in visible light but trillions of times more brilliant than the sun in the infrared for a paper published recently in Nature Astronomy. Gas is pushed and collapsed as a result of interactions between these galaxies and neighboring star systems, generating conditions conducive to spectacular star birth.

"Astronomers studying the gas content of these galaxies with optical instruments were certain that they were significantly metal-poor when compared with other galaxies of comparable mass," said Nima Chartab, a UCI postdoctoral fellow in physics and astronomy. "However, when we observed emission lines from these dusty galaxies in infrared wavelengths, we got a good look at them, and there was no sign of a metal deficiency."

The astronomers sought to collect data on the ratios of proxies, oxygen and nitrogen, because infrared emissions from these elements are less obscured by galactic dust.

"We're seeking for signs of baryon recycling in which stars reduce hydrogen and helium to create carbon, nitrogen, and oxygen," said co-author Asantha Cooray, a UCI physics & astronomy professor. "The stars eventually explode and go supernovae, at which point all of that gas in the outskirts of the stars is transformed into clouds that are dispersed. Material in them is loose and diffuse, but it will gradually clump and collapse under gravitational pressure caused by other stars orbiting about. The formation of new stars is imminent."

Observations in the infrared region are hampered by water vapor in Earth's atmosphere, which blocks radiation on this portion of the electromagnetic spectrum. Even the highest-altitude ground telescopes—such as those at the Keck Observatory in Hawaii—cannot provide enough data to measure this process.

The team used data from the now-retired Herschel Space Telescope, but because it lacked a spectrometer capable of reading a specific emission line, the UCI-led study did not have access to it. The researchers' solution was to take to the skies—reaching more than 45,000 feet above sea level—in NASA's Boeing 747 aircraft equipped with a 2.5-meter telescope.

The team's observations revealed that the emission line was redshifted, or stretched to longer wavelengths, indicating that the gas was moving away from us. The amount of redshift corresponded to a velocity of about 250 kilometers per second—faster than the escape velocity of our galaxy.

"It took us nearly three years to gather all of the data using NASA's SOFIA observatory because these flights don't last all night; they're more in the 45-minute observing time range, so the study necessitated a lot of flight planning and coordination," Cooray added. "But in the end, we had enough data to piece together a detailed image of this hidden galactic structure."

By analyzing infrared radiation, the researchers were able to compare the metallicity of their target ultraluminous infrared galaxies to less dusty counterparts with comparable mass and star formation rates. According to Chartab, these new findings support the fundamental metallicity relation, which was determined by stellar mass, metal abundance, and star formation rate.

The new findings revealed that the lack of metals produced from optical emission lines is most probably caused by "heavy dust obscuration associated with starburst."

"This work provides evidence that the fundamental metallicity relation is still valid in the most obscured starburst galaxies," said Chartab. "It also helps to explain why we haven't seen as many metals in these galaxies as we would expect."

"We use a lot of infrared light for this study because it was important for us to get a complete picture of what's going on in some of these galaxies," said Cooray. "When the optical findings first came out suggesting that these galaxies had little metals, theorists went and published papers; there were a lot of calculations trying to understand what was happening. People thought, 'Maybe they are really low-metal galaxies,' but we discovered they were not. Having a comprehensive view of the universe across the whole electromagnetic spectrum is critical, I believe."

The finding could help to explain why these galaxies appear to be deficient in metals, even though they are actively forming new stars.

The team's findings are published in the Nature Astronomy Journal.

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About the Blogger:

April Carson is the daughter of Billy Carson. She received her bachelor's degree in Social Sciences from Jacksonville University, where she was also on the Women's Basketball team. She now has a successful clothing company that specializes in organic baby clothes and other items. Take a look at their most popular fall fashions on

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