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Large metallicity variations in the Galactic interstellar medium

An Author Correction to this article was published on 03 November 2021

This article has been updated


The interstellar medium (ISM) comprises gases at different temperatures and densities, including ionized, atomic and molecular species, and dust particles1. The neutral ISM is dominated by neutral hydrogen2 and has ionization fractions of up to eight per cent3. The concentration of chemical elements heavier than helium (metallicity) spans orders of magnitudes in Galactic stars4, because they formed at different times. However, the gas in the vicinity of the Sun is assumed to be well mixed and to have a solar metallicity in traditional chemical evolution models5. The ISM chemical abundances can be accurately measured with ultraviolet absorption-line spectroscopy. However, the effects of dust depletion6,7,8,9—which removes part of the metals from the observable gaseous phase and incorporates it into solid grains—have prevented, until recently, a deeper investigation of the ISM metallicity. Here we report the dust-corrected metallicity of the neutral ISM measured towards 25 stars in our Galaxy. We find large variations in metallicity over a factor of ten (with an average of 55 ± 7 per cent solar metallicity and a standard deviation of 0.28 dex), including many regions of low metallicity, down to about 17 per cent solar metallicity and possibly below. Pristine gas falling onto the Galactic disk in the form of high-velocity clouds can cause the observed chemical inhomogeneities on scales of tens of parsecs. Our results suggest that this low-metallicity accreting gas does not efficiently mix into the ISM, which may help us understand metallicity deviations in nearby coeval stars.

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Fig. 1: Location of our targets on the Galactic plane.
Fig. 2: Dust-corrected abundances in the neutral ISM.
Fig. 3: Metallicities in the neutral ISM.

Data availability

The observational data used in this work are publicly available in the Mikulski Archive for Space Telescope (HST/STIS data, programme ID 15335, principal investigator A.D.C., and the ESO Science Archive Facility (VLT/UVES data The data used in the figures and tables are available as electronically readable source data files, except for the publicly available observational data (Extended Data Fig. 1). Source data are provided with this paper.

Code availability

The VoigtFit software is publicly available on GitHub at

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A.D.C. thanks C. Chiosi and the ‘Galaxies and the Universe’ group at the University of Geneva for discussions, and B. Holl for help with navigating the Gaia archive. A.D.C., T.R.-H., C.K. and J.-K.K. acknowledge support by the Swiss National Science Foundation under grant 185692. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. E.B.J. was supported by grant number HST-GO-15335.002-A from STScI to Princeton University. Based on data obtained from the ESO Science Archive Facility. This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The background image in Fig. 1 is courtesy of NASA/JPL-Caltech/R. Hurt (SSC/Caltech).

Author information




A.D.C. initiated, designed and directed the project, is the principal investigator of the HST data, analysed and interpreted the data, developed and applied the main methodology, and wrote the bulk of the manuscript. E.B.J. reduced the HST data and retrieved the UVES data, analysed and interpreted the data, measured the column densities, developed and applied one of the two methods to measure the metallicity, contributed to the writing and produced Fig. 3. A.J.F. contributed to the writing and scientific design of the paper. C.L. checked the consistency of the analysis, helped interpret the data and contributed to the writing. E.B.J., C.L., A.J.F. and P.P. are co-investigators of the HST data. T.R.-H. measured the position of our targets within the Galaxy and produced Fig. 1 and Extended Data Fig. 4. C.K. reviewed the depletion methods and assumptions, and collected data from Galactic extinction maps. P.P. contributed to the prioritization of the scientific goals and the writing. J.-K.K. assessed the ionization effects and contributed to the writing. J.-K.K., T.R.-H., C.K., C.L. and A.D.C. measured the column densities towards eight targets with an independent method for a cross-check of the results. All authors participated in the scientific interpretation, edited the manuscript and contributed to its revision.

Corresponding author

Correspondence to Annalisa De Cia.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Line profiles of Zn ii λ2026 (black), Cr ii λ2056 (blue) and Fe ii λ2260 (green) in our sample.

The Mg i λ2026 line is separated by ~50 km s−1 from Zn ii. Vertical lines mark the zero-velocity central wavelength of the Zn ii line. The yellow curve shows the 1σ uncertainties.

Extended Data Fig. 2 Determination of the metallicity and strength of depletion with the relative method.

The variables and coefficients of the linear relation are defined in equations (4) to (7), where the y intercept gives the [M/H]tot and the slope of the relation the strength of depletion [Zn/Fe]fit. The error bars show the 1σ uncertainties.

Source data

Extended Data Fig. 3 Determination of [M/H]tot and F* with the F* method.

The variables are described in equation (8). The most volatile elements (red) are taken from the literature (Extended Data Table 6) and shown for reference: their discrepancy with respect to the more refractory elements suggests a mix between high-metallicity and pristine gas, see Methods. The error bars show the 1σ uncertainties.

Source data

Extended Data Fig. 4 Metallicity towards our targets and their Galactic location.

a, Metallicities towards our targets and their Galactic radii. The green dotted line shows the metallcity gradient measured in H ii regions by ref. 10, although without dust corrections. The solar Galactic radius (red cross) is assumed at 8.29 kpc 59. The error bars show the 1σ uncertainties. b, Metallicities towards our targets and their height above the Galactic Disk. The error bars show the 1σ uncertainties.

Source data

Extended Data Table 1 Target sample characteristics
Extended Data Table 2 Column densities
Extended Data Table 3 Metallicities of the neutral ISM
Extended Data Table 4 Absorption lines that we use in this work and their oscillator strengths
Extended Data Table 5 Coefficients used in equations (6) and (7)
Extended Data Table 6 Column densities of the volatile elements

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De Cia, A., Jenkins, E.B., Fox, A.J. et al. Large metallicity variations in the Galactic interstellar medium. Nature 597, 206–208 (2021).

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