Upper limit on HF(1-0) absorption in a dusty star-forming galaxy at $z = 6$: Constraints on early fluorine enrichment

TL;DR

This study sets an upper limit on HF absorption in a high-redshift galaxy, providing insights into early fluorine production and the role of Wolf-Rayet stars in chemical enrichment shortly after the Big Bang.

Contribution

First observational constraint on fluorine abundance at z=6, testing the contribution of massive stars and Wolf-Rayet yields to early chemical enrichment.

Findings

HF absorption not significantly detected, with an upper limit much lower than local values.

Fluorine enrichment was inefficient about 0.9 Gyr after the Big Bang.

Results align with models excluding Wolf-Rayet yields at this epoch.

Abstract

Wolf-Rayet (WR) stars have recently attracted attention as possible drivers of early chemical enrichment, including the production of fluorine, whose nucleosynthetic origin remains debated. To test the contribution of massive stars to fluorine production in the early Universe, we conducted Atacama Large Millimeter/submillimeter Array Band 5 spectroscopy of the HF(1-0) absorption line toward a dusty star-forming galaxy at $z=6.024$. This galaxy has a known gas-phase metallicity and is too young for low-mass AGB stars to have contributed significantly, providing a clean environment to isolate massive-star yields. We do not detect significant HF absorption ($\sim2\sigma$) and derive a conservative 5$\sigma$ upper limit of $N_\mathrm{HF}/N_\mathrm{H_2} < 2.2\times10^{-9}$. This limit is about an order of magnitude below typical local measurements, indicating inefficient fluorine enrichment $\sim0.9$ Gyr after the Big Bang. Comparison with chemical evolution models shows that our constraint is consistent with scenarios without WR yields at this epoch. Expanding the sample of HF absorption measurements in high-redshift galaxies with well-characterized metallicities will be crucial for tracing the onset of WR enrichment and fluorine production across cosmic time.