Dwarf galaxies keep about 11% of the oxygen their stars made, on average. In the worst case it is 4%, and in the best case 25%. That is the finding of a project called GLOW, led by Kristen Wingfield McQuinn of Rutgers University. The team studied 37 nearby galaxies using data from the Hubble telescope. The results were published on 2 November 2026 in The Astrophysical Journal.
No one yet knows where the rest of the oxygen went. Computer models predict that dwarf galaxies should hold on to more of it.
Why these 37 galaxies were chosen
GLOW stands for "Galaxies that lose oxygen through winds". NASA and the US National Science Foundation fund the project. The team picked these galaxies because, in theory, supernovae push oxygen out of them most easily. They are also close enough for Hubble to pick out individual stars, which it cannot do for most known dwarf galaxies. Rutgers says they lie up to about 22 million light years away, while Sci.News gives 19.5 million. A galaxy like this has anywhere from a few thousand to hundreds of millions of stars.
The team chose oxygen for practical reasons. After hydrogen and helium it is the most common element, it is easy to detect, and scientists know well how stars make it.
How to work out how much oxygen is missing
The sum has two parts. First, astronomers use the brightness and colour of individual stars to work out when stars formed in a galaxy and how many. From that they calculate how much oxygen the stars made over their whole lifetime. Then they measure how much oxygen is there now, in stars, gas and dust. The data come from the now retired Spitzer space telescope and from the Very Large Array and Australia Telescope Compact Array radio telescopes.
It is like a kitchen: you know how much flour you bought over the years, but the pantry holds only a tenth of it. According to Sci.News, almost all the oxygen that is left sits in gas, not in stars or dust.
Galaxies more than 3.3 million light years from a large neighbour keep less than 15% of their oxygen. Those in denser surroundings, or close to a large galaxy, vary more, and they include most of the highest values.
Supernova winds and where the oxygen goes
The main suspects are galactic winds, which are streams of gas that leave a galaxy. Grace Telford of the University of Utah, a co-investigator on the project, says: "you need energy from massive stars and supernova explosions to drive material out of the host galaxies". In other words, gas needs the energy of massive stars and supernova blasts to leave a galaxy.
But the models do not match the observations. Three cosmological simulations cited by Sci.News reproduce the link between a galaxy's mass and the amount of heavy elements in it. But they predict that dwarf galaxies hold more metals than observers see. NASA says the winds appear to throw out oxygen better than current models suggest. Rutgers adds that the simulations expected a large share of the oxygen to escape, while the observations show even less is left.
The second open question is where the lost oxygen ends up. It may be in the circumgalactic medium, the halo of gas around a galaxy, or it may have left the surrounding area and reached the space between galaxies. Other studies have already found oxygen in the gas around dwarf galaxies, but they do not say how much of the missing oxygen is there. According to Sci.News, ultraviolet observations of highly ionised oxygen cover only 1–7% of what the stars made. After a correction for other ionisation states, the authors estimate that the halo could hold 5–35%.
Another open question is whether the wider spread among galaxies in dense surroundings means oxygen falls back in, or that these galaxies take in gas already enriched by their neighbours. A simple model fits the data best when the wind removes proportionally more mass from smaller galaxies, the gas it throws out is about twice as rich in metals as the gas between stars, and the inflow of new gas is cut by about 40%.
McQuinn plans to use an approved programme on the Roman space telescope to find fainter and smaller galaxies. Hubble and James Webb will then study them. Telford will use Hubble to watch individual massive stars in some of the GLOW galaxies and study the properties of the stars that make and throw out oxygen.