Hubble Finds 37 Dwarf Galaxies Kept Only About 11% of the Oxygen Their Stars Made

Astronomers using NASA’s Hubble Space Telescope report that 37 nearby dwarf galaxies have kept only about 11% of the oxygen their stars produced. The study, published Monday, Nov. 2, in The Astrophysical Journal, offers a new census of dwarf galaxies’ oxygen and shows supernova-driven winds clear out far more than models predict.

The work comes from the Galaxies Losing Oxygen via Winds project, known as GLOW, which is funded by NASA and the National Science Foundation. It is led by Kristen McQuinn of the Space Telescope Science Institute in Baltimore and Rutgers University, NASA said in its announcement.

How Hubble tracked dwarf galaxies’ oxygen

Oxygen is forged inside stars and scattered by supernova explosions. Because its production is well understood and it is easy to measure in glowing gas, it serves as a tracer for heavier elements in general. “So, it’s really for practical reasons that we’ve chosen oxygen,” McQuinn said in NASA’s release.

The team used archival Hubble images to resolve individual stars in each galaxy and reconstruct its star-formation history. They combined that with data from NASA’s retired Spitzer Space Telescope and radio observations from the Very Large Array and the Australia Telescope Compact Array, which map the hydrogen gas. From those pieces, they estimated how much oxygen each galaxy has made over its lifetime and how much still sits in its stars, gas and dust.

The 37 galaxies lie within about 6 megaparsecs, or roughly 20 million light-years, and span stellar masses from a few million to a few billion times the mass of the sun, according to the GLOW II paper. They are close enough for Hubble to see individual stars, which is not true of most known dwarf galaxies.

By the numbers: most of the oxygen is gone

On average the galaxies retain 10.7% of the oxygen their stars produced, with a standard deviation of 5.1 percentage points. The lowest value, about 3.8%, belongs to NGC 4163, and the highest, 24.8%, to UGC 5139, a spread of about 6.5 times. In all, 29 of the 37 galaxies, or about 78%, kept 15% or less, the paper reports.

Almost all of the oxygen that remains is in gas rather than stars. In NGC 4449, the galaxy pictured above, about 8.9% of the oxygen produced is in gas, 6.0% in stars and 1.8% in dust, for a total of 16.7%. By The Boston Report’s calculation, that means roughly 83% of the oxygen made in NGC 4449 is unaccounted for within the galaxy itself.

Size matters. The paper places its results alongside other studies: the tiny galaxy Leo P retains about 5% of its oxygen, galaxies of about a billion solar masses retain roughly 20% in a study based on scaling relations, and the Andromeda galaxy retains about 38% of its metals. That trend fits the idea that smaller galaxies, with weaker gravity, lose more of what their stars make.

Why the simulations got it wrong

Computer simulations that correctly reproduce how metal content scales with galaxy mass still predict much higher retention than GLOW measured. One set of simulations overshoots by a factor of about two to four across the GLOW mass range, the authors write. Their simple modeling suggests that the amount of gas carried by winds and the metal content of those winds control how much oxygen a galaxy keeps.

“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,” co-investigator Grace Telford of the University of Utah said in NASA’s release. “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.”

Where it went remains open. Ionized oxygen has been detected in the halos of gas around dwarf galaxies, but the paper estimates those halos may hold only about 5% to 35% of the oxygen produced, leaving most of it unaccounted for. Galaxies in denser neighborhoods, near a massive galaxy, tended to retain more oxygen, possibly because they recapture or receive metals from their surroundings.

What comes next

McQuinn has an approved program to use NASA’s Nancy Grace Roman Space Telescope to find even fainter dwarf galaxies, then follow up with Hubble and the James Webb Space Telescope. Telford plans to use Hubble to study the massive stars that make and expel oxygen. The study adds to a busy stretch for space telescopes, after Hubble and Webb recently imaged the Cat’s Head Nebula in Carina and Webb spotted three black holes in one early galaxy.

Sources

• NASA: Hubble uncovers case of missing oxygen in nearby dwarf galaxies (Nov. 2, 2026)
• The Astrophysical Journal: McQuinn et al., GLOW. II. A Census of Oxygen in Low-mass Galaxies
• arXiv: GLOW II, full text and Table 2 oxygen budget