A thorough understanding of galaxy evolution depends in part on accurately measuring the abundance of metals in the intergalactic medium – the space between stars – but dust can hamper observations at optical wavelengths. An international team of astronomers from the University of California at Irvine, the University of Oxford in England and other institutions have found evidence of heavier elements in local galaxies – found to be deficient in previous studies – by analyzing the infrared data collected during a multi-year campaign.
For an article recently published in natural astronomy, the researchers examined five galaxies that are faint in the visible wavelengths but billions of times brighter than the sun in the infrared. Interactions between these galaxies and nearby star systems cause the gas to shift and collapse, creating the conditions for prodigious star formation.
“By studying the gas content of these galaxies with optical instruments, astronomers were confident that they were significantly metal-poor compared to other galaxies of similar mass,” said lead author Nima Chartab, postdoctoral researcher UCI in physics and astronomy. “But when we observed the emission lines from these dusty galaxies in the infrared wavelengths, we got a clear view of them and found no significant metal deficiencies.”
To determine the abundance of gas-phase metals in the intergalactic medium, astronomers have sought to acquire data on the ratios of proxies, oxygen, and nitrogen, since the infrared emissions of these elements are less obscured by galactic dust.
“We are looking for evidence of the baryon cycle in which stars process elements such as hydrogen and helium to produce carbon, nitrogen and oxygen,” said co-author Asantha Cooray, professor of physics and astronomy at the UCI. “Stars eventually become supernovae and explode, and then all that gas on the outskirts of stars turns into clouds that are thrown up. The material in them is loose and diffuse, but eventually by gravitational disturbances caused by other stars moving in, the gas will begin to clump together and collapse, leading to the formation of new stars.”
Observing this process in infrared wavelengths is a challenge for astronomers, because water vapor in the Earth’s atmosphere blocks radiation on this part of the electromagnetic spectrum, rendering measurements from ground-based telescopes insufficient. even at the highest altitude, such as those at the Keck Observatory in Hawaii.
Part of the dataset used by the team came from the now-retired Herschel Space Telescope, but Herschel was not equipped with a spectrometer capable of reading a specific emission line which the team led by the UCI needed for its study. The researchers’ solution was to soar – more than 45,000 feet above sea level – in the Stratospheric Infrared Astronomy Observatory, NASA’s Boeing 747 telescope-equipped 2. 5 meters.
“It took us almost three years to collect all the data using NASA’s SOFIA Observatory, because these flights are not all night; they are more like 45 minutes of observing time, so the study took a lot of flight planning and coordination,” Cooray said.
By analyzing the infrared emissions, the researchers were able to compare the metallicity of their target ultrabright infrared galaxies with less dusty galaxies with similar masses and star formation rates. Chartab explained that these new data show that ultrabright infrared galaxies conform to the fundamental metallicity relationship determined by stellar mass, metal abundance and star formation rate.
The new data further show that the underabundance of metals derived from optical emission lines is likely due to “heavy dust obscuration associated with starbursts,” according to the paper.
“This study is an example where it was critical for us to use this infrared wavelength to fully understand what’s going on in some of these galaxies,” Cooray said. “When the optical observations first came out suggesting that these galaxies had little metal in them, theorists went and wrote papers, there were a lot of simulations trying to explain what was going on. People thought, ‘Maybe “Maybe they really are low-metal galaxies.” But we found that they weren’t. Having a full view of the universe across the entire electromagnetic spectrum is really crucial, I think.
The far-infrared emission from galaxies with active supermassive black holes
Nima Chartab et al, Low gas-phase metallicities of ultraluminous infrared galaxies are the result of dust obscuration, natural astronomy (2022). DOI: 10.1038/s41550-022-01679-y
Provided by University of California, Irvine
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