New insights on the dense molecular gas in NGC253 as traced by HCN and HCO+

TL;DR

This study uses high-resolution observations of NGC253's starburst disk to analyze dense molecular gas via HCN and HCO+ lines, revealing uniform ratios and subthermal excitation, which inform galaxy chemistry and star formation understanding.

Contribution

It provides the first detailed, high-resolution analysis of dense gas tracers in NGC253, showing constant HCN/HCO+ ratios and subthermal excitation across the starburst disk.

Findings

HCN and HCO+ emissions trace similar dense gas regions.

The HCN/HCO+ ratio remains constant at about 1.1 across the disk.

Lines are subthermally excited and optically thick.

Abstract

We have imaged the central ~1kpc of the circumnuclear starburst disk in the galaxy NGC253 in the HCN(1-0), HCO+(1-0), and CO(1-0) transitions at 60pc resolution using the Owens Valley Radio Observatory Millimeter-Wavelength Array (OVRO). We have also obtained Atacama Pathfinder Experiment (APEX) observations of the HCN(4-3) and the HCO+(4-3) lines of the starburst disk. We find that the emission from the HCN(1-0) and HCO+(1-0) transitions, both indicators of dense molecular gas, trace regions which are non-distinguishable within the uncertainties of our observations. Even though the continuum flux varies by more than a factor 10 across the starburst disk, the HCN/HCO+ ratio is constant throughout the disk, and we derive an average ratio of 1.1+/-0.2. From an excitation analysis we find that all lines from both molecules are subthermally excited and that they are optically thick. This subthermal excitation implies that the observed HCN/HCO+ line ratio is sensitive to the underlying chemistry. The constant line ratio thus implies that there are no strong abundance gradients across the starburst disk of NGC253. This finding may also explain the variations in L'(HCN)/L'(HCO+) between different star forming galaxies both nearby and at high redshifts.