Large metallicity variations in the Galactic interstellar medium

Abstract

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 depletion^6-9-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.