Measuring the composition of galaxies is a difficult task that requires studying the light that they emit at various wavelengths. This information can help us understand the different types and amounts of matter that make up a galaxy, such as stars, gas, dust, and dark matter.
A study using data from Earth and sky telescopes solves a problem that has plagued infrared astronomers and could aid in better observations of the composition of the universe with the James Webb Space Telescope and other instruments. Nature Astronomy published the findings.
“We’re trying to measure the composition of gases inside galaxies,” Yuguang Chen, a postdoctoral researcher at the University of California, Davis’ Department of Physics and Astronomy, explained.
Because most elements other than hydrogen, helium, and lithium are produced inside stars, astronomers can use the composition and distribution of heavier elements, particularly the oxygen-to-hydrogen ratio, to determine how many and what types of stars are being formed in a distant object.
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.
Yuguang Chen
Astronomers use two methods to measure oxygen in a galaxy, but the results are inconsistent. One common method, collisionally excited lines, produces a strong signal, but the results are thought to be temperature sensitive, according to Chen. A second method employs a different set of lines known as recombination lines, which are fainter but not thought to be temperature sensitive.
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 that was mounted on a Boeing 747. The aircraft could reach above 99% of the water vapor in Earth’s atmosphere by flying at 38,000 to 45,000 feet, effectively blocking infrared light from deep space from reaching ground level. SOFIA, a joint NASA-German space agency project, completed its final operational flight in September 2022 and is now on display in Tucson.
The Herschel Space Observatory, named after astronomers William and Caroline Herschel, was a European Space Agency infrared space telescope. It was operational 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. According to him, there is no agreement on an explanation for the disparity. Chen stated that the team intends to look at additional objects to determine what properties of galaxies correlate with this variation.
The James Webb Space Telescope, which will be launched in 2022, will make infrared observations of the composition of distant galaxies in the first billion years of the universe. The new findings lay the groundwork for future measurements using the JWST and Chile’s Atacama Large Millimeter Array.
