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EI2GYB > ASTRO 20.04.23 20:22l 96 Lines 5286 Bytes #999 (0) @ WW
BID : 13309_EI2GYB
Read: GUEST
Subj: Making better measurements of the composition of galaxies
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Sent: 230420/1908Z @:EI2GYB.DGL.IRL.EURO #:13309 LinBPQ6.0.23
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Making better measurements of the composition of galaxies
A study using data from telescopes on Earth and in the sky resolves a problem
plaguing astronomers working in the infrared, and could help make better
observations of the composition of the universe with the James Webb Space
Telescope and other instruments. The work is published April 20 in Nature
Astronomy.
"We're trying to measure the composition of gases inside galaxies," said
Yuguang Chen, a postdoctoral researcher working with Professor Tucker Jones in
the Department of Physics and Astronomy at the University of California, Davis.
Most elements other than hydrogen, helium and lithium are produced inside
stars, so the composition and distribution of heavier elements-especially the
ratio of oxygen to hydrogen-can help astronomers understand how many and what
kinds of stars are being formed in a distant object.
Astronomers use two methods to measure oxygen in a galaxy, but unfortunately,
they give different results. One common method, collisionally excited lines,
gives a strong signal, but the results are thought to be sensitive to
temperature changes, Chen said. A second method uses a different set of lines,
called recombination lines, which are fainter but not thought to be affected by
temperature.
The recombination line method consistently produces measurements about double
those from collisionally excited lines. Scientists attribute the discrepancy to
temperature fluctuations in gas clouds, but this has not been directly proven,
Chen said.
Chen, Jones and colleagues used optical and infrared astronomy to measure
oxygen abundance in dwarf galaxy Markarian 71, about 11 million light years
from Earth. They used archived data from the recently retired SOFIA flying
telescope and the retired Herschel Space Observatory, as well as making
observations with telescopes at the W.M. Keck Observatory in Mauna Kea, Hawaii.
SOFIA (Stratospheric Observatory For Infrared Astronomy) was a telescope
mounted in a Boeing 747 aircraft. By flying at 38,000 to 45,000 feet, the
aircraft could get above 99% of the water vapor in Earth's atmosphere, which
effectively blocks infrared light from deep space from reaching ground level. A
joint project of NASA and the German space agency, SOFIA made its last
operational flight in September 2022 and is now headed for a museum display in
Tucson.
The Herschel Space Observatory, named after astronomers William and Caroline
Herschel, was an infrared space telescope operated by the European Space
Agency. It was active from 2009 to 2013.
A surprising result
With data from these instruments, Chen and Jones examined oxygen abundance in
Markarian 71 while correcting for temperature fluctuations. They found that the
result from collisionally excited infrared lines was still 50% less than that
from the recombination line method, even after eliminating the effect of
temperature.
"This result is very surprising to us," Chen said. There is no consensus on an
explanation for the discrepancy, he said. The team plans to look at additional
objects to figure out what properties of galaxies correlate with this
variation, Chen said.
One of the goals of the James Webb Space Telescope, launched in 2022, is to
make infrared observations of the composition of distant galaxies in the first
billion years of the universe. The new results provide a framework for making
these measurements with the JWST and the Atacama Large Millimeter Array in
Chile.
Additional co-authors on the paper are: Ryan Sanders and Erin Huntzinger, UC
Davis; Dario Fadder, Jessica Sutter and Robert Minchin, SOFIA Science Center,
NASA Ames Research Center; Peter Senchyna, Observatories of the Carnegie
Institute for Science, Pasadena; Daniel Stark and Benjamin Weiner, Steward
Observatory, University of Arizona; Justin Spilker, Texas A&M University; and
Guido Roberts-Borsani, UCLA.
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