Stars make oxygen for small galaxies, but they lose most of it

Scientists found that dozens of small galaxies in the GLOW study contain far less oxygen than their stars are estimated to have produced. The study included 37 dwarf galaxies, which are much smaller than the Milky Way and located within 22 million light-years of Earth. The galaxies retained between 4% and 25% of the oxygen, with an average of about 11%. This suggests that galactic winds, driven by massive stars and supernovae, remove oxygen more effectively than current computer models predict. The researchers used Hubble images to track individual stars and their brightness and colors to determine when and how the galaxies formed stars and changed their chemical makeup. The findings challenge previous predictions about how much oxygen is lost by galaxies.

• "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 co-author 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.

• The researchers combined these images with data from NASA's retired Spitzer 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.

The findings suggest that galactic winds, driven by massive stars and supernovae, remove oxygen more effectively than current computer models predict. This challenges previous predictions about how much oxygen is lost by galaxies. The study extends previous research on galaxies similar to the Milky Way that are still forming stars, where much of their oxygen had left the galaxies' main bodies, with some detected in the vast clouds of gas surrounding them. The GLOW study includes smaller galaxies with a range of masses and surroundings, and 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 launched Nancy Grace Roman Space Telescope at the Space Telescope Science Institute in Baltimore. In the future, McQuinn 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's James 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."

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