Adolescent Galaxies are Incandescent and Contain Unexpected Elements

If the Universe has adolescent galaxies, they’re the ones that formed about 2 to 3 billion years after the Big Bang. New research based on the James Webb Space Telescope shows that these teenage galaxies are unusually hot. Not only that, but they contain some unexpected chemical elements. The most surprising element found in these galaxies is nickel.

The new observations are part of CECILIA, which stands for Chemical Evolution Constrained Using Ionized Lines in Interstellar Aurorae. It uses the JWST’s NIRSpec instrument to study the spectra of 33 galaxies at z ~ 1-3. That corresponds roughly to 2 to 3 billion years post-Big Bang. But in an interesting twist, the spectra weren’t studied individually; instead, the researchers combined 23 of them into one composite spectrum.

“This washes out the details of individual galaxies but gives us a better sense of an average galaxy. It also allows us to see fainter features,” said Allison Strom from Northwestern University. Strom is the lead author of a new paper presenting CECILIA’s results. “It’s significantly deeper and more detailed than any spectrum we could collect with ground-based telescopes of galaxies from this time period in the universe’s history.”

The new paper is titled “CECILIA: The Faint Emission Line Spectrum of z ~ 2-3 Star-Forming Galaxies.” It’s published in The Astrophysical Journal Letters.

“Never in my wildest dreams did I imagine we would see nickel.”

Dr. Alison Strom, Northwestern University

“We’re trying to understand how galaxies grew and changed over the 14 billion years of cosmic history,” said Strom. “Using the JWST, our program targets teenage galaxies when they were going through a messy time of growth spurts and change. Teenagers often have experiences that determine their trajectories into adulthood. For galaxies, it’s the same.”

This work is based on star-forming regions in these adolescent galaxies. Active star formation produces lots of light. That light creates nebular emissions. “The nebular emission lines originating in galaxies’ star-forming regions are among the most powerful tools available for investigating the physical conditions in galaxies at all redshifts,” the authors write.

The nebular emissions have spectral lines that are like galactic DNA. Examining this “chemical fingerprint” in adolescent galaxies gives researchers insight into how the galaxies formed and what their future evolution will look like. Galaxy formation and evolution is a hot topic in space science, and “Galaxies Over Time” is one of the JWST’s main science objectives.

The study of galaxies is one of the JWST’s primary activities. This image from the NASA/ESA/CSA James Webb Space Telescope shows the heart of M74, otherwise known as the Phantom Galaxy. Webb’s sharp vision has revealed delicate filaments of gas and dust in the grandiose spiral arms which wind outwards from the centre of this image. While this image is not part of CECILIA, it does show how powerful the JWST is and what it’s capable of when it studies galaxies. Image Credit: NASA / ESA / CSA / Judy Schmidt (CC BY 2.0)

By combining the spectra of multiple adolescent galaxies, the researchers produced an ultra-deep composite spectrum. The spectrum contained the chemical signatures of eight distinct elements: Hydrogen, helium, nitrogen, oxygen, silicon, sulphur, argon and nickel. In astronomy, all elements heavier than hydrogen and helium are called metals. Finding metals in galaxies is not unusual. But the presence of nickel comes as a surprise.

“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. 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.”

This figure from the study is the composite spectrum for the CECILIA sample of 23 galaxies. Hydrogen and helium are expected, but the other metals are objects of interest. Most interesting and surprising is the presence of nickel, something not seen before. The inset panel shows the lines for H-alpha and SII, the only lines routinely observed in ground-based observations and shallower JWST spectra of individual high-z galaxies. Image Credit: Strom et al. 2023.

The CECILIA galaxies are also surprisingly hot. What exactly the temperature and the presence of nickel tell us about these galaxies is yet to be determined. But temperature and chemistry are linked, and both findings drive home an important point, one that the JWST has repeatedly made since it began observations.

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

As hoped, the JWST is driving us toward a new understanding of how galaxies form and evolve and what types of chemistry they contain. This is just one of the space telescope’s results that force us to rethink some of our theories. It’s already found the oldest and most distant spectroscopically-confirmed galaxy known. It’s also found early galaxies that are more fully formed than we thought they should be. It also found grand spiral galaxies that formed 11 billion years ago, far sooner than we thought.

But behind all these surprises are the complexity and the newness of JWST’s data. As this study shows, the data can reveal even more than scientists thought when analyzed in novel ways.

“Deep observations, such as those obtained as part of CECILIA and outlined here, will be critical for developing and testing the new tools necessary to accurately interpret this wealth of data,” the researchers write in their article.

“JWST is still a very new observatory,” said Ryan Trainor, paper co-author and associate professor of physics at Franklin & Marshall College. “Astronomers around the world are still trying to figure out the best ways to analyze the data we receive from the telescope.”

If CECILIA is any indication of what new JWST data analysis techniques can reveal, then the telescope’s study of galaxies is only going to get more interesting.

Evan Gough

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