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# The ramp-up of interstellar medium enrichment at z > 4

## Abstract

Fluorine is one of the most interesting elements for nuclear and stellar astrophysics1,2. Fluorine abundance was first measured for stars other than the Sun in 19921, then for a handful of metal-poor stars3, which are likely to have formed in the early Universe. The main production sites of fluorine are under debate and include asymptotic giant branch stars, the ν-process in core-collapse supernovae, and Wolf–Rayet stars4,5,6,7,8,9,10. Due to the difference in the mass and lifetime of progenitor stars, high-redshift observations of fluorine can help constrain the mechanism of fluorine production in massive galaxies. Here, we report the detection of HF (signal-to-noise ratio of 8) in absorption in a gravitationally lensed dusty star-forming galaxy at redshift z = 4.4 with NHF/$${N}_{{{{{\rm{H}}}}}_{2}}$$ as high as ~2 × 10−9, indicating a very quick ramp-up of the chemical enrichment in this high-z galaxy. At z = 4.4, asymptotic giant branch stars of a few solar masses are very unlikely to dominate the enrichment. Instead, we show that Wolf–Rayet stars are required to produce the observed fluorine abundance at this time, with other production mechanisms becoming important at later times. These observations therefore provide an insight into the underlying processes driving the ramp-up phase of chemical enrichment alongside rapid stellar mass assembly in a young massive galaxy.

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## Data availability

This paper makes use of the following ALMA data: ADS/JAO.ALMA#2017.1.00510.S, archived at https://almascience.nrao.edu/alma-data/archive. Tabulated spectral data used in this study are provided at https://github.com/maximilienfranco/f21_fluorine_spectrum.

## Code availability

The ALMA data are processed using the CASA ALMA pipeline (v.5.6.1-8) available at https://almascience.nrao.edu/processing/science-pipeline. The lens model was produced using the VISILENS package publicly available at https://github.com/jspilker/visilens/.

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## Acknowledgements

M.F. is grateful to Y. Hezaveh for his advice on the lens model. M.F. and K.E.K.C. acknowledge support from the UK Science and Technology Facilities Council (STFC) (grant ST/R000905/1). K.E.K.C. acknowledges support from a Royal Society Leverhulme Trust Senior Research Fellowship (grant RSLT SRF/R1/191013). J.E.G. acknowledges support from a Royal Society University Research Fellowship. C.K. acknowledges funding from the UK STFC through grants ST/M000958/1 and ST/ R000905/1. S.C.C. acknowledges the Natural Sciences and Engineering Research Council of Canada (NSERC). C.Y. acknowledges support from an ESO Fellowship. J.S.S. is a NHFP Hubble Fellow supported by NASA Hubble Fellowship grant HF2-51446 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, for NASA, under contract NAS5-26555. E.G.-A. thanks the Spanish Ministerio de Economía y Competitividad for support under projects ESP2017-86582-C4-1-R and PID2019-105552RB-C41. M.J.M. acknowledges the support of the National Science Centre, Poland, through SONATA BIS grant 2018/30/E/ST9/00208. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2017.1.00510.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

## Author information

Authors

### Contributions

M.F. reduced and analysed the data; M.F., K.E.K.C., J.E.G. and C.K. interpreted the results and wrote the paper. C.K. created the chemical evolution models. C.Y. interpreted the results. J.S.S. helped create the lens model. E.G.-A. computed the HF column density and contributed to various aspects of the analysis. S.C.C. provided, and helped analyse the data and contributed to the manuscript. All other authors contributed to the ALMA proposals and to the scientific discussion, and provided comments on the manuscript.

### Corresponding author

Correspondence to M. Franco.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Astronomy thanks Andreas Faisst and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Franco, M., Coppin, K.E.K., Geach, J.E. et al. The ramp-up of interstellar medium enrichment at z > 4. Nat Astron 5, 1240–1246 (2021). https://doi.org/10.1038/s41550-021-01515-9

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• DOI: https://doi.org/10.1038/s41550-021-01515-9

• ### Observing early stellar nucleosynthesis

• Nils Ryde
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Nature Astronomy (2021)