Direct Constraints on the Extremely Metal-Poor Massive Stars Underlying Nebular C IV Emission from Ultra-Deep HST/COS Ultraviolet Spectroscopy

Abstract

Metal-poor nearby galaxies hosting massive stars have a fundamental role to play in our understanding of both high-redshift galaxies and low metallicity stellar populations. But while much attention has been focused on their bright nebular gas emission, the massive stars that power it remain challenging to constrain. Here we present exceptionally deep Hubble Space Telescope ultraviolet spectra targeting six galaxies that power strong nebular C IV emission approaching that encountered at 𝑧 > 6. We find that the strength and spectral profile of the nebular C IV in these new spectra follow a sequence evocative of resonant scattering models, indicating that the hot circumgalactic medium likely plays a key role in regulating C IV escape locally. We constrain the metallicity of the massive stars in each galaxy by fitting the forest of photospheric absorption lines, reporting measurements driven by iron that lie uniformly below 10% solar. Comparison with the gas-phase oxygen abundances reveals evidence for enhancement in O/Fe above solar across the sample, robust to assumptions about the absolute gas-phase metallicity scale. This supports the idea that these local systems are more chemically-similar to their primordial high-redshift counterparts than to the bulk of nearby galaxies. Finally, we find significant tension between the strong stellar wind profiles observed and our population synthesis models constrained by the photospheric forest in our highest-quality spectra. This reinforces the need for caution in interpreting wind lines in isolation at high-redshift, but also suggests a unique path towards validating fundamental massive star physics at extremely low metallicity with integrated ultraviolet spectra.

Haro 11 -- Untying the knots of the nuclear starburst

“…fig. 7 in Senchyna et al 2021). The residuals of the UV continuum are comparable with the statistical errors of the data.…”

Strong Lyman continuum emitting galaxies show intense CIV 1550 emission

“…They are low-mass low-metallicity galaxies at z ∼ 0.01 that both show very strong C iv emission with EW(C iv) ∼ 6 − 10 Å (Berg et al 2019b,a). Based on high-resolution COS spectra, which show strong nebular C iv emission in the two doublet lines and indications for minor radiation transfer effects in these lines, Berg et al (2019a) have suggested that J104457 is optically thin to ionizing radiation, at least at energies above 47.9 eV, the ionization potential of C 3+ (see also Senchyna et al 2021). It also shares other properties of strong LyC leakers, such as a very high O32= 16.2.…”