‘Teenage Galaxies’ Are Unusually Hot, Sparkling With Unexpected Elements

Similar to human teenagers, teenage galaxies are weird, experience growth spurts, and like heavy metal – that is, nickel.

A team of astrophysicists led by Northwestern University analyzed early results from the CECILIA (Chemical Evolution Constrained using Ionized Lines in Interstellar Aurorae) survey, a program that uses NASA’s James Webb Space Telescope (JWST) to study the chemistry of distant galaxies.

According to early results, so-called “teenage galaxies” – which formed two to three billion years after the Big Bang – are unusually hot and contain unexpected elements, such as nickel, that are notoriously difficult to observe.

The research, “CECILIA: Weak emission line spectrum of z~2-3 star-forming galaxies,” was published Nov. 20 in The letters from the astrophysical journal. It is the first in a series of future studies of CECILIA Research.

“We’re trying to understand how galaxies have grown and changed over 14 billion years of cosmic history,” said Northwestern’s Allison Strom, who led the study. “Using JWST, our program targets adolescent galaxies when they were going through a complicated period of growth spurts and changes. Teenagers often have experiences that determine their trajectories into adulthood. For galaxies, it’s the same thing .”

One of the principal investigators of the CECILIA Survey, Strom is an assistant professor of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences and a member of Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Strom co-leads CECILIA Research with Gwen Rudie, staff scientist at Carnegie Observatories.

‘Chemical DNA’ gives insight into galaxy formation

Named in honor of Cecilia Payne-Gaposchkin, one of the first women to earn a doctorate. in astronomy, the CECILIA Survey observes spectra (or the amount of light at different wavelengths) of distant galaxies. Strom compares a galaxy’s spectrum to its “chemical DNA.” By examining this DNA during a galaxy’s “teenage” years, researchers can better understand how it grew and how it will evolve into a more mature galaxy.

For example, astrophysicists still don’t understand why some galaxies appear “red and dead” while others, like our Milky Way, are still forming stars. A galaxy’s spectrum can reveal its key elements, such as oxygen and sulfur, which provide a window into what a galaxy was doing previously and what it might do in the future.

“This adolescence is really important because that’s when the most growth occurs,” Strom said. “By studying this, we can begin to explore the physics that made the Milky Way look like the Milky Way – and why it might look different from its neighboring galaxies.”

In the new study, Strom and his collaborators used JWST to observe 33 distant teenage galaxies for 30 continuous hours last summer. They then combined spectra from 23 of these galaxies to construct a composite image.

“This eliminates the details of individual galaxies, but gives us a better sense of an average galaxy. It also allows us to see fainter features,” said Strom. “It is significantly deeper and more detailed than any spectrum we could collect with ground-based telescopes of galaxies from this period in the history of the universe.”

Spectral surprises

The ultra-deep spectrum revealed eight distinct elements: Hydrogen, helium, nitrogen, oxygen, silicon, sulfur, argon and nickel. All elements heavier than hydrogen and helium form inside stars. Thus, the presence of certain elements provides information about star formation throughout the evolution of a galaxy.

Although Strom expected to see lighter elements, she was particularly surprised by the presence of nickel. Heavier than iron, nickel is rare and incredibly difficult to observe.

“Never in my wildest dreams did I imagine we would see nickel,” Strom said. “Even in nearby galaxies, people don’t observe this. There has to be enough of an element present in a galaxy and the right conditions to observe it. Nobody ever talks about observing nickel. The elements have to be glowing in gas to be able to for us to see them. So for us to see nickel, there may be something unique about stars within galaxies.”

Another surprise: teenage galaxies were extremely hot. By examining spectra, physicists can calculate the temperature of a galaxy. While the hottest pockets of galaxies can reach over 9,700° Celsius (17,492° Fahrenheit), teenage galaxies reach temperatures in excess of 13,350° Celsius (24,062° Fahrenheit).

“This is just additional evidence of how different galaxies probably were when they were younger,” Strom said. “Ultimately, the fact that we observe a higher characteristic temperature is just another manifestation of their different chemical DNA, because the temperature and gas chemistry in galaxies are intrinsically linked.”

Data for the study were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute and the WM Keck Observatory.