Hubble Finds the Oxygen Dwarf Galaxies Could Not Keep

The Vanishing Inventory

A small galaxy makes stars. Those stars manufacture oxygen. So why does the galaxy seem to keep so little of it?

A new Hubble-led study reported by NASA examined 37 nearby dwarf galaxies and compared the oxygen associated with their stars, gas and dust with the amount expected from their star-formation histories. The result: much of the expected oxygen is not retained in the galaxies. Oxygen here is a tracer of stellar chemical enrichment, not a measure of whether these places have breathable air.

A Cosmic Escape Route

The team used Hubble's archival optical images to resolve individual stars and reconstruct when the galaxies formed them. Spitzer infrared data and radio observations from the Very Large Array and Australian Telescope Compact Array helped with the wider census. These galaxies were close enough to make that stellar accounting possible.

• The likely engine: energetic massive stars and supernova explosions drive galactic winds.
• The mismatch: lower-mass galaxies appear to lose more of their produced oxygen than current simulations predict.
• The unresolved destination: some oxygen may be in surrounding gas, but the study does not establish whether most sits in those halos or escaped farther into intergalactic space.

The oxygen was not watched leaving in a single explosion. This is an inference from the chemical inventory and models, with an open question about where the missing share ultimately resides. The research paper in The Astrophysical Journal is the underlying scientific report.

Why the Gap Matters

Oxygen is comparatively easy for astronomers to measure and is produced by stars in understood ways. If small galaxies are poorer at retaining it than models allow, simulations of how galaxies enrich the space between them need another look. The team plans to extend the sample toward fainter dwarfs using Roman to find targets and Hubble and Webb to examine their stars; those future observations are not yet results.

The stars made oxygen. Their galaxies could not keep it all.

The accompanying image is a real Hubble observation of NGC 4449, one member of the study, not an image of oxygen escaping. Image credit: NASA, ESA, Hubble Heritage–ESA/Hubble Collaboration, Alessandra Aloisi (STScI, ESA). NASA's media guidelines permit attributed factual editorial use without implying endorsement.