A comprehensive study on the relation between the metal enrichment of ionised and atomic gas in star-forming galaxies

TL;DR

This study uses EAGLE simulations to analyze the metallicity relationship between ionised and atomic gas in star-forming galaxies across redshifts 0-3, revealing central regions have more mixed gas phases and that metallicity differences depend on galaxy position.

Contribution

It provides a detailed simulation-based analysis of the metallicity relation between ionised and atomic gas in galaxies, highlighting the role of galaxy position and gas mixing processes.

Findings

Z_{SFR} is generally higher than Z_{HI} in sightlines.

Metallicity difference decreases towards galaxy centers.

Good agreement with observational data, except in nuclear-starburst regions.

Abstract

We study the relation between the metallicities of ionised and atomic gas in star-forming galaxies at z=0-3 using the EAGLE cosmological, hydrodynamical simulations. This is done by constructing a dense grid of sightlines through the simulated galaxies and obtaining the star formation rate- and HI column density-weighted metallicities, Z_{SFR} and Z_{HI}, for each sightline as proxies for the metallicities of ionised and atomic gas, respectively. We find Z_{SFR} > Z_{HI} for almost all sightlines, with their difference generally increasing with decreasing metallicity. The stellar masses of galaxies do not have a significant effect on this trend, but the positions of the sightlines with respect to the galaxy centres play an important role: the difference between the two metallicities decreases when moving towards the galaxy centres, and saturates to a minimum value in the central regions of galaxies, irrespective of redshift and stellar mass. This implies that the mixing of the two gas phases is most efficient in the central regions of galaxies where sightlines generally have high column densities of HI. However, a high HI column density alone doesn't guarantee a small difference between the two metallicities. In galaxy outskirts, the inefficiency of the mixing of star-forming gas with HI seems to dominate over the dilution of heavy elements in HI through mixing with the pristine gas. We find good agreement between the available observational data and the Z_{SFR}-Z_{HI} relation predicted by the EAGLE simulations. Though, observed regions with a nuclear-starburst mode of star formation appear not to follow the same relation.