A dense, solar metallicity ISM in the z=4.2 dusty star-forming galaxy SPT0418-47

TL;DR

This study investigates the physical conditions of the interstellar medium in a high-redshift dusty star-forming galaxy, revealing dense, solar-metallicity gas dominated by photo-dissociation regions through far-infrared line observations.

Contribution

First detailed analysis of multiple far-infrared lines in a z=4.2 galaxy showing dense, solar-metallicity ISM with high gas densities and photo-dissociation region dominance.

Findings

ISM is dominated by photo-dissociation regions with high gas densities

Line ratios indicate near-solar metallicity in the galaxy's ISM

Detection of lines was enabled by gravitational lensing

Abstract

We present a study of six far-infrared fine structure lines in the z=4.225 lensed dusty star-forming galaxy SPT0418-47 to probe the physical conditions of its InterStellar Medium (ISM). In particular, we report Atacama Pathfinder EXperiment (APEX) detections of the [OI]145um and [OIII]88um lines and Atacama Compact Array (ACA) detections of the [NII]122 and 205um lines. The [OI]145um / [CII]158um line ratio is ~5x higher compared to the average of local galaxies. We interpret this as evidence that the ISM is dominated by photo-dissociation regions with high gas densities. The line ratios, and in particular those of [OIII]88um and [NII]122um imply that the ISM in SPT0418-47 is already chemically enriched close to solar metallicity. While the strong gravitational amplification was required to detect these lines with APEX, larger samples can be observed with the Atacama Large Millimeter/submillimeter Array (ALMA), and should allow to determine if the observed dense, solar metallicity ISM is common among these highly star-forming galaxies.