ISM chemistry in metal rich environments: molecular tracers of metallicity

TL;DR

This study investigates how molecular tracers like CS and methanol can serve as indicators of metallicity in metal-rich galaxies, revealing systematic variations and potential for future metallicity calibration.

Contribution

It presents the first survey of CS and methanol emission in early-type galaxies and links molecular ratios to galaxy metallicity, advancing understanding of ISM chemistry in different environments.

Findings

CS is a better tracer of dense star-forming gas than HCN.

Methanol emission may be driven by dust mantle destruction or shocks.

Molecular ratios show systematic variation with metallicity, matching chemical model predictions.

Abstract

In this paper we use observations of molecular tracers in metal rich and alpha-enhanced galaxies to study the effect of abundance changes on molecular chemistry. We selected a sample of metal rich spiral and star bursting objects from the literature, and present here new data for a sample of early-type galaxies (ETGs). We conducted the first survey of CS and methanol emission in ETGs, detecting 7 objects in CS, and 5 in methanol emission. We find evidence to support the hypothesis that CS is a better tracer of dense star-forming gas than HCN. We suggest that the methanol emission in these sources is driven by dust mantle destruction due to ionisation from high mass star formation, but cannot rule out shocks dominating in some sources. The derived source averaged CS/methanol column densities and rotation temperatures are similar to those found in normal spiral and starburst galaxies, suggesting dense clouds are little affected by the differences between galaxy types. Finally we used the total column density ratios for our galaxy samples to show for the first time that some molecular tracers do seem to show systematic variations that appear to correlate with metallicity, and that these variations roughly match those predicted by chemical models. Using this fact, the chemical models of Bayet et al. (2012b), and assumptions about the optical depth we are able to roughly predict the metallicity of our spiral and ETG sample, with a scatter of ~0.3 dex. We provide the community with linear approximations to the relationship between the HCN and CS column density ratio and metallicity. Further study will clearly be required to determine if this, or any, molecular tracer can be used to robustly determine gas-phase metallically, but that a relationship exists at all suggests that in the future it may be possible to calibrate a metallicity indicator for the molecular interstellar medium (abridged).