Hubble finds dwarf galaxies kept only 11% of the oxygen their stars made

The small galaxies around the Milky Way have lost almost all of the oxygen their own stars made. A census built on Hubble Space Telescope observations finds that a typical dwarf galaxy still holds only about one oxygen atom in every nine its stars have forged; the rest is no longer inside the galaxy, where astronomers can count it.

Oxygen is the most abundant element after hydrogen and helium, and astronomers use it as a stand-in for every heavier element, the raw material of rocky planets, water and living things. Dwarf galaxies are the most common kind in the universe. If they routinely throw away most of what their stars produce, a large share of the cosmos’s heavy elements has ended up in the near-empty space between galaxies. And the computer simulations that get the rest of a galaxy’s chemistry right say far more should have stayed behind.

The project, called Galaxies Losing Oxygen via Winds (GLOW), studied 37 gas-rich dwarf galaxies that are still forming stars. They retain between 4% and 25% of the oxygen their stars have made, 10.7% on average. One widely used set of galaxy simulations predicts two to four times more.

The analysis was led by Kristen McQuinn, an astronomer at the Space Telescope Science Institute and Rutgers University who heads the science operations center for NASA’s Nancy Grace Roman Space Telescope, and it appears in The Astrophysical Journal. “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 co-investigator Grace Telford, an assistant professor at the University of Utah. “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.”

How to take an inventory of oxygen in a galaxy

The study works like an audit. First the team estimated how much oxygen each galaxy has produced over its life. Most oxygen comes from massive stars that end as supernovae, so the amount made tracks the total mass of stars a galaxy has formed. The researchers measured that mass from infrared light recorded by NASA’s retired Spitzer Space Telescope and converted it into oxygen with a standard yield, the mass of new oxygen released for every unit of gas turned into stars.

Then they counted what is still there. Hubble’s archival images resolve the individual stars of each galaxy, and the colors and brightnesses of those stars reveal when they formed and how much oxygen they locked up. Radio telescopes, the Very Large Array in New Mexico and the Australia Telescope Compact Array, weighed the hydrogen gas through the faint radio signal it gives off. Optical spectra of the glowing clouds around young stars gave the oxygen content of that gas, and a correction for the oxygen trapped in dust grains completed the tally.

The galaxies were chosen on purpose. They lie roughly 3 to 20 million light-years away, close enough for Hubble to pick out single stars, which is impossible for most known dwarf galaxies. Their masses range from a few million to a few billion times the mass of the Sun, the range where theory predicts supernova winds should do the most damage. None of them orbits a big galaxy as a satellite. Of the oxygen they kept, nearly all sits in their gas, with much smaller amounts in stars and dust.

Bigger galaxies keep three to four times more

The losses look extreme next to larger galaxies. The study compiles earlier results showing that retention climbs by a factor of three to four for galaxies of around ten billion solar masses and up, where most of the kept oxygen is locked in stars rather than gas. Andromeda, the large spiral nearest the Milky Way, has held on to 38% of its heavy elements, most of it in stars, according to an earlier analysis by Telford and colleagues using a similar method.

The usual explanation is gravity. A small galaxy has a shallow gravitational well, so the blast waves of exploding stars can drive gas out at speeds the galaxy cannot hold. The GLOW data fit a simple model in which the escaping winds carry about twice the oxygen concentration of the gas left behind, and in which these galaxies also draw in about 40% less fresh gas from intergalactic space than their share of the universe’s ordinary matter would suggest.

At the small end, Leo P, a faint dwarf the same group studied earlier, has kept 5% of its oxygen. Some of the Milky Way’s own satellites hold only 1% to 3%, but the bigger galaxy has also stripped them of their gas, so their losses have a different cause.

Environment seems to matter too. Dwarf galaxies more than about 3 million light-years from any large neighbor consistently keep a low share of their oxygen. Those closer to a big galaxy, in denser surroundings, tend to retain more, possibly because outside pressure pushes expelled gas back in or because they sweep up oxygen thrown out by others. And, contrary to what the team expected, how much oxygen a galaxy kept did not line up with its position on the standard relation between a galaxy’s mass and the oxygen content of its gas.

Simulations get the chemistry right and the oxygen budget wrong

That last result is why the study matters beyond dwarf galaxies. Cosmological simulations are tuned to reproduce the mass-metallicity relation, and the three the team compared against all do so well at these masses. Yet the same simulations keep too much oxygen inside their galaxies; one set predicts retention two to four times higher than measured. They also predict that retention should rise steeply with mass, which the combined observations do not show.

The simulations land on the right average chemistry while moving the oxygen around the wrong way. The authors argue that this kind of mismatch can separate competing recipes for stellar feedback, the energy injected by supernovae and massive stars that every simulation has to approximate because it cannot resolve individual explosions.

What the census does not settle

The biggest open question is where the missing oxygen went. Ultraviolet observations of the halo of diffuse gas around some of these galaxies have detected highly ionized oxygen, but it accounts for only 1% to 7% of the oxygen produced. Correcting for oxygen in other states could raise the halo’s share to somewhere between 5% and 35%, a range that leaves open whether most of the oxygen lingers near its galaxy or has escaped into intergalactic space for good. The team notes that even that correction may not hold if the halo gas is out of equilibrium.

The audit also rests on assumptions. The stellar masses carry an uncertainty of about 33%, and the oxygen produced depends on the adopted supernova yields and on the mix of star masses assumed at birth. The simulations also use different yields and different definitions of where a galaxy ends, so the authors warn against strict number-to-number comparisons, while calling the overall trends robust. The environmental pattern comes from a small sample and is described as tentative, and the 37 galaxies are all gas-rich, star-forming field dwarfs in our cosmic neighborhood, not every kind of small galaxy.

Common questions about dwarf galaxies and missing oxygen

What is a dwarf galaxy?

A dwarf galaxy is a small galaxy with a few thousand to hundreds of millions of stars, against hundreds of billions in the Milky Way. Dwarfs are the most numerous galaxies in the universe. The ones in this study are rich in gas and still forming stars.

Where does the oxygen in a galaxy come from?

Almost all of it is made inside massive stars and released when they explode as supernovae. Each generation of stars enriches the surrounding gas, and later stars and planets form from that enriched material. The oxygen in Earth’s air and water was forged this way long before the Sun existed.

Where did the missing oxygen go?

Supernova-driven winds pushed it out of the galaxies. Part of it sits in a diffuse halo of gas around each galaxy, but the measured halo oxygen covers only a small fraction of the deficit. The rest may have escaped into intergalactic space entirely, and current data cannot yet tell which.

Why do small galaxies lose more oxygen than big ones?

Their gravity is weaker, so the energy from exploding stars can more easily lift gas out of them. Large spirals such as Andromeda keep a much bigger share, mostly locked inside their stars. The GLOW galaxies kept about a tenth on average.

Roman will look for even fainter galaxies

The GLOW census was published on November 2, 2026, in The Astrophysical Journal, alongside a companion atlas of the galaxy data in the Astrophysical Journal Supplement Series; preprints had been posted in August. McQuinn has an approved program on the newly launched Roman Space Telescope to find even smaller and fainter galaxies across a wide area of sky, which she then hopes to study in depth with Hubble and the James Webb Space Telescope. Telford is using Hubble to examine the individual massive stars that make the oxygen in several GLOW galaxies and drive it out of them.

Reference: McQuinn et al., “GLOW. II. A Census of Oxygen in Low-mass Galaxies,” The Astrophysical Journal, 2026. DOI: 10.3847/1538-4357/ae93a6

Tags: astronomy, hubble, dwarf galaxies, NGC 4449, oxygen, supernovae