A Temperature or Far-ultraviolet Tracer? The HNC/HCN Ratio in M83 on the Scale of Giant Molecular Clouds

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 Atacama Large Millimeter/submillimeter Array observations, and if possible, distinguish the above scenarios. We compare the high-resolution (40–50 pc) 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 (∼200 pc) correlation between the dust temperature from the far-IR 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 ∼100 pc scale where the column densities are low.