A census of 37 nearby gas-rich dwarf galaxies using NASA/ESA Hubble Space Telescope observations suggests that these systems retain only about 11% of the oxygen produced by their stars. The findings support the idea that supernova-driven outflows expel much of the element into intergalactic space, while also indicating that a significant amount may remain in the galaxies’ surrounding gas halos.
This Hubble image shows NGC 4449, one of the dwarf galaxies in the Wind Oxygen Loss Galaxies (GLOW) study. Image credit: NASA / ESA / Hubble Heritage-ESA / Hubble Collaboration / Alessandra Aloisi, STScI & ESA.
Tracking oxygen in dwarf galaxies
“Oxygen is produced by stars in galaxies,” said Dr. Grace Telford, an astronomer at the University of Utah.
“We know that oxygen must have been present inside the galaxy at some point in the past, so it must have been released.”
“And that requires the energy from massive stars and supernova explosions to expel material from the host galaxy and into intergalactic space.”
As part of the Wind Oxygen Loss Galaxies (GLOW) project, Telford and her colleagues studied dwarf galaxies located within about 19.5 million light-years of Earth. The galaxies have stellar masses ranging from about 3 million to 3 billion times the mass of the Sun.
To estimate how much oxygen each galaxy should have produced, the researchers used Hubble observations of individual stars to reconstruct the galaxies’ star-formation histories. They then calculated how much oxygen was present in each galaxy’s gas, stars and dust.
The galaxies retained between 4% and 25% of the oxygen produced by their stars, with an average of about 11%. Almost all of the retained oxygen was found in gas rather than in stars or dust.
The remaining oxygen was probably expelled by supernova-driven outflows, a long-standing idea for which the study provides a quantitative estimate.
The role of galactic environments
Galaxies located more than 3.3 million light-years from massive neighbors retained less than 15% of their oxygen at any given time. Galaxies in denser environments, or within 3.3 million light-years of large galaxies, showed greater variation and included most of the highest oxygen-retention values.
This pattern could mean that oxygen is being recycled into these galaxies. It could also indicate that they are accreting gas already enriched by neighboring galaxies.
The researchers also examined whether oxygen expelled into the gas halo surrounding each galaxy, sometimes called the perigalactic medium, escaped entirely.
Early ultraviolet observations of highly ionized oxygen accounted for approximately 1% to 7% of the oxygen produced by the galaxies. After accounting for other ionization states, the authors estimated that the halo may retain between 5% and 35% of the oxygen. A large portion may therefore still remain outside the galaxy’s gravitational range.
Comparisons with simulations
The astronomers compared their results with three cosmological simulations. All three reproduced the mass-metallicity relationship, but they predicted that dwarf galaxies would retain more metals than the observations indicate.
Using a simple galaxy-evolution model, the researchers found that the data fit best when outflows carried away proportionately more mass from smaller galaxies, the outflowing gas was about twice as metal-rich as the galaxies’ interstellar medium, and gas accretion onto the galaxies was suppressed by about 40%.
“So we chose oxygen for practical reasons,” said GLOW principal investigator Kristen Wingfield-McQuinn, Ph.D., mission director at NASA’s Nancy Grace Roman Space Telescope Science Operations Center at the Space Telescope Science Institute and an astronomer at Rutgers University.
“Oxygen is one of the main elements formed by stars. Being able to track how much oxygen is produced and how much is distributed inside and outside galaxies will help us understand the evolution of galaxies.”
“It also helps us understand where and how many galaxies enrich the space between galaxies, known as the intergalactic medium.”
The results were published in the Astrophysical Journal.
Kristen B.W. Mackin et al. 2026. “GLOW. II. Investigation of Oxygen in Low-Mass Galaxies.” Astrophysical Journal 1011, 158. doi: 10.3847/1538-4357/ae93a6
Source: www.sci.news