The Distribution of Metallicities in the Local Galactic Interstellar Medium*

Abstract

In this investigation, we present an analysis of the metallicity distribution that pertains to neutral gas in the local Galactic interstellar medium (ISM). We derive relative ISM metallicities for a sample of 84 sight lines probing diffuse atomic and molecular gas within 4 kpc of the Sun. Our analysis is based in large part on column density measurements reported in the literature for 22 different elements that are commonly studied in interstellar clouds. We supplement the literature data with new column density determinations for certain key elements and several individual sight lines important to our analysis. Our methodology involves comparing the relative gas-phase abundances of many different elements for a given sight line to simultaneously determine the strength of dust depletion in that direction and the overall metallicity offset. We find that many sight lines probe multiple distinct gas regions with different depletion properties, which complicates the metallicity analysis. Nevertheless, our results provide clear evidence that the dispersion in the metallicities of neutral interstellar clouds in the solar neighborhood is small (∼0.10 dex) and only slightly larger than the typical measurement uncertainties. We find no evidence for the existence of very low metallicity gas (as has recently been reported by De Cia et al.) along any of the 84 sight lines in our sample. Our results are consistent with a local Galactic ISM that is well mixed and chemically homogeneous.

α-element enhancements in the ISM of the LMC and SMC: Evidence of recent star formation

“…The low-metallicity gas may be distributed in pockets along the line of sight and their mass may be small (De Cia et al 2022). On the other hand, Ritchey et al (2023) find no evidence for low-metallicity gas in the Milky Way. This discrepancy arises from the different strategy in the analysis of the abundance patterns.…”

“…performs a linear fit to the abundance patterns of either both volatiles and refractory elements, or excluding the refractory ones). Overall, both De Cia et al ( 2021) and Ritchey et al (2023) observe deviations from a linear behaviour of the abundance patterns that are due to the complexity of the ISM medium, such as differences in depletion and/or metallicity. We discuss this further in Sect.…”

“…Studies of abundance patterns that systematically include the volatile metals in a single fit to the abundance patterns, even when non-linear deviations are evident (e.g. Ritchey et al 2023) are not sensitive to the presence of low-metallicity gas, but are instead dominated by metal-rich gas.…”

“…The results of De Cia et al ( 2021) are based mostly on the study of refractory and mildly refractory elements, which are more sensitive to less dusty and metal-poor gas. The results of Ritchey et al (2023) are based mostly on the study of volatile and mildly volatile elements. The inclusion of the volatile elements in the analysis of the abundance patterns makes it very hard to detect the signatures of any low-metallicity gas, if present along the line of sight.…”

“…This is because the determination of the volatile element abundances is dominated by the gas of higher metallicity, hiding the possible presence of low-metallicity gas. The methodology of Ritchey et al (2023) is not sensitive to the presence of low-metallicity gas because it always includes the volatile elements in the analysis of the abundance patterns and assumes that all the gas along the line of sight has the same metallicity (i.e. performs a linear fit to the abundance patterns of either both volatiles and refractory elements, or excluding the refractory ones).…”