Large Metallicity Variations in the Galactic Interstellar Medium

Abstract

The Interstellar Medium (ISM) comprises gases at different temperatures and densities, including ionized, atomic, molecular species, and dust particles [1]. The neutral ISM is dominated by neutral hydrogen [2] and has ionization fractions up to 8% [3]. The concentration of chemical elements heavier than helium (metallicity) spans orders of magnitudes in Galactic stars [4], because they formed at different times. Instead, the gas in the Solar vicinity is assumed to be well mixed and have Solar metallicity in traditional chemical evolution models [5]. The ISM chemical abundances can be accurately measured with UV absorption-line spectroscopy. However, the effects of dust depletion [6,7,8,9], 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 10 (with an average 55 ± 7% Solar and standard deviation 0.28 dex) and including many regions of low metallicity, down to ∼ 17% Solar and possibly below. Pristine gas falling onto the disk in the form of high-velocity clouds can cause the observed chemical inhomogeneities on scales of tens of pc. 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.

Dust depletion of metals from local to distant galaxies

“…We only considered systems observed with high spectral resolution (R = 35 000-58 000) when measuring Ti and Ni column densities. For the Milky Way, we used ISM abundances from Jenkins (2009), De Cia et al (2021), Welty & Crowther (2010), and Phillips et al (1982). For the LMC, we used Roman-Duval et al (2021), and for the SMC, we used Welty & Crowther (2010), Tchernyshyov et al (2015), Jenkins & Wallerstein (2017), the abundances of QSO-DLAs were taken from Berg et al (2015), De Cia et al (2018), and those for DLAs toward GRBs from Bolmer et al (2019).…”

“…We included a sample of ISM metal column densities from the Milky Way, the LMC, and the SMC. The Milky Way sample includes a compilation of elemental abundances from 243 lines of sight in the Milky Way (Jenkins 2009), 25 bright O and B stars observed with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) with a resolving power of R ∼ 30 000 (De Cia et al 2021) and Al measurements from Phillips et al (1982). The SMC sample is composed of elemental abundances toward 19 lines of sight from Welty & Crowther (2010), Tchernyshyov et al (2015), Jenkins & Wallerstein (2017).…”