Stars Make Oxygen for Small Galaxies. They Lose Most of It.

A Rutgers astrophysicist and colleagues investigate how powerful winds driven by stars shape the galaxies around them

Kristen Wingfield McQuinn, an associate professor in the Department of Physics and Astronomy, leads the Galaxies Losing Oxygen via Winds (GLOW) project, funded by NASA and the National Science Foundation. She also is mission head of the Science Operations Center for NASA's newly launched Nancy Grace Roman Space Telescope at the Space Telescope Science Institute in Baltimore.

Scientists examining data collected by NASA's Hubble Space Telescope have found that dozens of small galaxies contain far less oxygen than their stars are estimated to have produced.

Researchers, including a Rutgers astrophysicist, studied 37 dwarf galaxies which are much smaller than the Milky Way and considered to be nearby, within 22 million light years from Earth. Theresults were published in The Astrophysical Journal.

Members of the team compared estimates of how much oxygen the galaxies' stars had made over their lifetimes with the amount remaining in the galaxies' stars, gas and dust. The galaxies retained between 4% and 25% of the oxygen, with an average of about 11%.

Tracking oxygen helps scientists understand how galaxies change over time and how elements made by stars spread through the universe.

Oxygen, the most abundant element in the universe after hydrogen and helium, is relatively easy to detect and measure, according to scientists. That makes it a useful clue to the movement of other elements produced by stars.

"Oxygen is one of the primary elements formed by stars," said Kristen Wingfield McQuinn, an associate professor in the Department of Physics and Astronomy at the Rutgers School of Arts and Sciences and a coauthor of the study. "So, it's really for practical reasons that we've chosen oxygen."

Oxygen gives off light at specific colors that instruments on telescopes on Earth can easily detect. By separating that light into its different colors and measuring its brightness, scientists can identify oxygen and estimate how much is present.

McQuinn leads the Galaxies Losing Oxygen via Winds (GLOW) project, funded by NASA and the National Science Foundation. The project examines how much oxygen stars produce, how much their galaxies keep and how much escapes into surrounding space.

So where did the oxygen go?

The findings suggest galactic winds, streams of gas driven outward by energy from massive stars and stellar explosions, remove oxygen more effectively than current computer models predict. These explosions, called supernovae, occur when certain stars reach the end of their lives.

"We know that the oxygen had to be inside of the galaxies at some point in the past, so the oxygen must have been ejected," said Grace Telford, an assistant professor in the Department of Physics and Astronomy at the University of Utah and a coinvestigator in the study. "And to do that, you need energy from massive stars and supernova explosions to drive material out of the host galaxies and into the space between galaxies."

The researchers used previously collected images fromHubble to distinguish individual stars within the galaxies. The stars' brightness and colors helped researchers piece together when the galaxies formed stars and how their chemical makeup changed.

The researchers combined these images with data from NASA's retiredSpitzer Space Telescope as well as the ground-based Very Large Array and Australia Telescope Compact Array to help estimate the oxygen produced and the amount remaining.

The team studied these small galaxies to test predictions about how much oxygen they lose. Computer simulations suggested much of the oxygen would escape, but the observations showed that even less remained than predicted. All 37 are close enough for Hubble to pick out individual stars instead of seeing their light blended together. Most known dwarf galaxies are too far away for that level of detail.

Although astronomers have suspected that galaxies expel a large fraction of oxygen and other heavy elements, they haven't fully accounted for where those elements go. Aprevious study from 2014 examined galaxies similar to the Milky Way that are still forming stars. It found that much of their oxygen had left the galaxies' main bodies, with some detected in the vast clouds of gas surrounding them. Astronomers call this surrounding region the circumgalactic medium.

In the GLOW study, McQuinn and her team extended this research to smaller galaxies with a range of masses and surroundings. Other researchers have detected oxygen in the gas around dwarf galaxies, but scientists still do not know whether most of the missing oxygen remains there or has traveled farther into the space between galaxies.

McQuinn is mission head of the Science Operations Center for NASA's newly launchedNancy Grace Roman Space Telescope at the Space Telescope Science Institute in Baltimore.

In the future, McQuinn said she hopes to study even smaller galaxies in the nearby universe. She has an approved program to use the Roman Space Telescope to find these faint and tiny galaxies across a large area of the sky. Once she has identified these galaxies, McQuinn hopes to use Hubble and NASA'sJames Webb Space Telescope to study their stars in greater detail. Those observations would help her piece together when the galaxies formed their stars and how their star formation changed over billions of years.

Telford said she plans to use Hubble to study individual massive stars in some of the GLOW galaxies. Her observations will examine the properties of the stars that produce oxygen and help push it out of their galaxies.

"Being able to trace how much oxygen gets made, and then how much is distributed inside and outside of galaxies, helps us understand galaxy evolution," McQuinn said. "It also helps us learn where and how much galaxies are enriching the space between galaxies, called the intergalactic medium."

This story includes information from a news release from NASA and the Space Telescope Science Institute.

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