Ancient starburst galaxies that formed 2-3 billion years after the Big Bang are unusually hot and contain unexpected elements, like nickel, according to a new analysis of data collected by the CECILIA (Chemical Evolution Constrained using Ionized Lines in Interstellar Aurorae) survey.
This Webb image shows member galaxies of the galaxy cluster A2744-z7p9OD. Image credit: NASA / ESA / CSA / T. Morishita, IPAC / A. Pagan, STScI.
In the Universe’s youth, many galaxies experienced a period of intense star formation.
Today, some galaxies, such as our own Milky Way, still form new stars, albeit not as rapidly. Other galaxies have stopped forming stars altogether.
This new work can help astronomers understand the reasons behind these different trajectories.
“We’re trying to understand how galaxies grew and changed over the 14 billion years of cosmic history,” said Dr. Allison Strom, an astronomer at Northwestern University.
“Using the NASA/ESA/CSA James Webb Space Telescope, our program targets ‘teenage’ galaxies when they were going through a messy time of growth spurts and change.”
Dr. Strom and colleagues studied the spectra from 33 distant galaxies, separating their light into its component wavelengths.
“We averaged together the spectra from all 33 galaxies to create the deepest spectrum of a distant galaxy ever seen — which it would take 600 hours of telescope time to replicate,” said Dr. Gwen Rudie, an astronomer at Carnegie Observatories.
“This enabled us to create an atlas, of sorts, that will inform future Webb observations of very distant objects.”
Using the spectra, the astronomers were able to identify eight distinct elements: hydrogen, helium, nitrogen, oxygen, silicon, sulfur, argon and nickel.
“These elements existing in these galaxies are not a surprise, but our ability to measure their light is unprecedented and shows the power of Webb,” Dr. Rudie said.
All elements that are heavier than hydrogen and helium form inside stars.
When stars explode in violent events like supernovae, they spew these elements out into the cosmic surroundings, where they are incorporated into the next stellar generation.
So, by revealing the presence of certain elements in these early galaxies, astronomers can learn about how star formation changes over the course of their evolution.
The authors were surprised by the presence of nickel, which is particularly difficult to observe.
“Never in my wildest dreams did I imagine we would see nickel. Even in nearby galaxies, people don’t observe this,” Dr. Strom said.
“There has to be enough of an element present in a galaxy and the right conditions to observe it.”
“No one ever talks about observing nickel. Elements have to be glowing in gas in order for us to see them.”
“So, in order for us to see nickel, there may be something unique about the stars within the galaxies.”
The teenage galaxies were also extremely hot: while the hottest pockets with galaxies can reach over 9,700 degrees Celsius, the early galaxies clock in at higher than 13,350 degrees Celsius.
“We expected these early galaxies to have very, very different chemistry from our own Milky Way and the galaxies that surround us today,” Dr. Rudie said.
“But we were still surprised by what Webb revealed.”
The discovery is described in a paper in the Astrophysical Journal Letters.
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Allison L. Strom et al. 2023. CECILIA: The Faint Emission Line Spectrum of z ∼ 2–3 Star-forming Galaxies. ApJL 958, L11; doi: 10.3847/2041-8213/ad07dc
