Early Galaxies Moved Metals Outward—But Their Stars May Have Had Weaker Winds

Two observations put different parts of early-galaxy models under scrutiny. JWST’s SPURS observations indicate that metal-bearing gas was moving outward from three distant galaxies. Hubble’s TEMPOS survey, meanwhile, suggests that massive stars in nearby, metal-poor galaxies can have unexpectedly weak winds. The nearby stars are useful analogues of early stellar populations, not direct observations of them.45

What shows that the early galaxies were dispersing metals?

In the three SPURS galaxies, at redshifts 7.2–9.3, ultraviolet absorption from heavy elements appears in several ionization states. Those absorption features are blueshifted relative to the galaxies, consistent with metal-bearing gas moving outward along our line of sight. This is evidence for early outflows, but the absorption measurements alone do not establish how far the material travelled or whether it escaped into intergalactic space.

That distinction matters for the search for Population III stars, which would form from essentially metal-free gas. If early outflows reached nearby star-forming regions, they could have reduced the supply of pristine gas there. The SPURS detections do not show that enrichment was universal, however: metal-free pockets could still have existed elsewhere.13

What did TEMPOS find about stellar winds?

TEMPOS used Hubble ultraviolet observations to examine 29 massive stars in six nearby metal-poor dwarf galaxies. The survey reports unexpectedly weak winds, with evidence that stars below roughly one-tenth of the Sun’s heavy-element abundance may depart from a smooth extrapolation of wind behaviour at higher metallicities. That departure is tentative, not a settled new law.4510

The result has a physical basis worth testing: heavy-element ions help transfer radiation’s momentum to a massive star’s atmosphere, driving gas away. Less effective winds would mean less mass lost during the star’s lifetime.3 But the evidence is not uniform across studies. An earlier analysis of 11 O-type stars at low metallicity found very weak winds consistent with extrapolated wind-momentum trends, underscoring the need to compare samples and modelling methods carefully.15

What could change in models of the first galaxies?

If the TEMPOS wind measurements hold across a broader stellar population, massive stars could retain more mass before dying. Models would then need to revisit their predicted explosions and compact remnants, as well as how stars return energy and material to gas available for later star formation.34 TEMPOS did not observe those eventual deaths, and SPURS did not identify which processes launched its galaxies’ outflows. A galaxy-scale outflow and a wind from an individual star are related questions, not the same measurement.

Follow-up spectroscopy and modelling can test whether the weak-wind trend persists and how stellar metal abundances affect it. Deeper JWST observations of very low-metallicity candidates can test Population III interpretations, while further measurements of early-galaxy gas can clarify where the observed metals ultimately went.51013