Dwarf Galaxies Have Lost Most of Their Oxygen, Hubble Study of 37 Finds

Small galaxies near the Milky Way appear to be leaking most of the oxygen their stars create. According to NASA, as reported by Universe Today, a study in The Astrophysical Journal that used the Hubble Space Telescope found that much of the oxygen astronomers expected to see in 37 nearby dwarf galaxies is missing.

The result suggests these galaxies can hold on to only a small share of what their stars produce. It also indicates that current computer models of galaxy formation underestimate how much oxygen is lost.

The key numbers behind the oxygen shortfall

• 37 galaxies. This is the number of nearby dwarf galaxies in the study. NASA describes dwarf galaxies as small systems with anywhere from a few thousand to hundreds of millions of stars.
• 15 percent. Grace Telford, the paper's second author, said that typically about this share of the oxygen produced by stars in these low-mass galaxies is still inside them today. Most of it is in their gas, with smaller amounts in stars and dust.
• One oxygen atom for every 2,000 hydrogen atoms. Telford said this is the approximate ratio in the Milky Way.
• One oxygen atom for every 10,000-20,000 hydrogen atoms. This is the ratio she gave for low-mass galaxies like those in the study, meaning they have far less oxygen relative to hydrogen than the Milky Way does.

How astronomers tracked oxygen that is no longer there

Oxygen is the third most abundant element in the cosmos, according to NASA. It is relatively easy to detect and measure, and astronomers understand well how it is made, which makes it a dependable marker of how chemical elements are spread through a galaxy.

Telford, an assistant professor in the Department of Physics and Astronomy at the University of Utah, told Universe Today by email that this is the first time researchers have measured both how much stellar oxygen remains in low-mass galaxies and how much has been lost over their lifetimes.

The team combined Hubble data with observations from NASA's retired Spitzer Space Telescope and from two ground-based facilities, the Very Large Array and the Australian Telescope Compact Array, NASA said. The work is part of the Galaxies Losing Oxygen via Winds, or GLOW, project, which is jointly funded by NASA and the National Science Foundation. The project aims to understand how gas and heavy elements move between low-mass galaxies, the halos of gas around them and the space between galaxies. Telford said it requires combining a large set of new and archival observations from ground and space telescopes.

Where the missing oxygen likely went

Telford said most oxygen comes from core-collapse supernovae, the explosive deaths of massive stars. A very small fraction comes from other types of supernovae, which involve the remnants of low-mass stars becoming unstable or merging.

According to Telford, energetic radiation from massive stars, along with their deaths as supernovae, drives oxygen-rich gas out of the galaxies over time. The missing oxygen is therefore likely in the gaseous halos around the galaxies, known as the circumgalactic medium. Some of it has probably traveled even farther, into the intergalactic medium.

Similar measurements of lost oxygen and other heavy elements have been made for massive galaxies such as the Milky Way, Telford said. Low-mass galaxies are expected to lose oxygen more easily because their gravity is weaker, and she said the new paper shows that is the case.

Why it matters for understanding galaxies

For general readers, this is a result about how the raw materials of planets and life are distributed across the universe. Oxygen serves as a tracer for heavy elements such as iron, carbon, nickel, magnesium and sulfur, which Universe Today describes as the building blocks of habitable planets. Knowing how much of them stays in a galaxy and how much is pushed out helps astronomers trace the chemical history of the cosmos.

The finding also points to a gap in scientific models. Telford said the GLOW project found that low-mass galaxies lose more oxygen than current galaxy formation simulations predict. In her words, that implies researchers do not yet fully understand how massive stars interact with their host galaxies.

The conclusions rest on a single paper and on observations of 37 galaxies. The sources do not describe any follow-up studies, so the picture may be refined as the GLOW project continues.

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Around Prague, based on reporting by Universe Today