Chemical tracers of high-metallicity environments

TL;DR

This study investigates molecular gas properties in high-metallicity environments like early-type galaxies, analyzing chemical reactions and line ratios to identify indicators of metallicity and element enhancement.

Contribution

It provides the first detailed modeling of molecular chemistry in metal-rich environments, including the effects of $ ext{alpha}$-element enhancement and observable line ratios.

Findings

Certain molecules like CS, H$_{2}$S, and H$_{2}$CN are invariant to metallicity changes.

Species such as C$^{+}$, CO, and HCN are highly sensitive to metallicity variations.

Line brightness ratios are identified as potential tools for estimating super-solar metallicities.

Abstract

We present for the first time a detailed study of the properties of molecular gas in metal-rich environments such as early-type galaxies (ETGs). We have explored Photon-Dominated Region (PDR) chemistry for a wide range of physical conditions likely to be appropriate for these sources. We derive fractional abundances of the 20 most chemically reactive species as a function of the metallicity, as a function of the optical depth and for various volume number gas densities, Far-Ultra Violet (FUV) radiation fields and cosmic ray ionisation rates. We also investigate the response of the chemistry to the changes in $\alpha-$element enhancement as seen in ETGs. We find that the fractional abundances of CS, H$_{2}$S, H$_{2}$CS, H$_{2}$O, H$_{3}$O$^{+}$, HCO$^{+}$ and H$_{2}$CN seem invariant to an increase of metallicity whereas C$^{+}$, CO, C$_{2}$H, CN, HCN, HNC and OCS appear to be the species most sensitive to this change. The most sensitive species to the change in the fractional abundance of $\alpha-$elements are C$^{+}$, C, CN, HCN, HNC, SO, SO$_{2}$, H$_{2}$O and CS. Finally, we provide line brightness ratios for the most abundant species, especially in the range observable with ALMA. Discussion of favorable line ratios to use for the estimation of super-solar metallicities and $\alpha$-elements are also provided.