A temperature or FUV tracer? The HNC/HCN ratio in M83 on the GMC scale

TL;DR

This study investigates whether the HNC/HCN ratio in M83 acts as a temperature indicator or is influenced by UV radiation, finding it correlates more with UV effects than temperature on GMC scales.

Contribution

It provides high-resolution observations of the HNC/HCN ratio in M83, clarifying its dependence on UV radiation rather than temperature, challenging previous assumptions.

Findings

HNC/HCN ratio does not vary with star formation rate in the bar-end region.

In the central region, higher star formation rates correlate with higher HNC/HCN ratios.

The ratio decreases with increasing dust temperature, indicating UV influence.

Abstract

The HNC/HCN ratio is observationally known as a thermometer in Galactic interstellar molecular clouds. A recent study has alternatively suggested that the HNC/HCN ratio is affected by the ultraviolet (UV) field, not by the temperature. We aim to study this ratio on the scale of giant molecular clouds in the barred spiral galaxy M83 towards the southwestern bar end and the central region from ALMA observations, and if possible, distinguish the above scenarios. We compare the high (40-50 pc) resolution HNC/HCN ratios with the star formation rate from the 3-mm continuum intensity and the molecular mass inferred from the HCN intensities. Our results show that the HNC/HCN ratios do not vary with the star formation rates, star formation efficiencies, or column densities in the bar-end region. In the central region, the HNC/HCN ratios become higher with higher star formation rates, which tend to cause higher temperatures. This result is not consistent with the previously proposed scenario in which the HNC/HCN ratio decreases with increasing temperature. Spectral shapes suggest that this trend may be due to optically thick HCN and optically thin HNC. In addition, we compare the large-scale ($\sim 200$ pc) correlation between the dust temperature from the FIR ratio and the HNC/HCN ratio for the southwestern bar-end region. The HNC/HCN ratio is lower when the dust temperatures are higher. We suggest from the above results that the HNC/HCN ratio depends on the UV radiation field that affects the interstellar medium on the $\sim100\,$pc scale where the column densities are low.