Dense Molecular Gas and Star Formation in Nearby Seyfert Galaxies

TL;DR

This study uses molecular line imaging to identify unique gas signatures in Seyfert galaxies, proposing a new diagnostic method to distinguish pure AGNs from composite nuclei based on HCN and HCO$^+$ ratios.

Contribution

It introduces the use of HCN/CO and HCN/HCO$^+$ ratios as diagnostics for identifying pure AGNs in nearby Seyfert galaxies, based on high-resolution molecular line observations.

Findings

3 out of 7 Seyferts show abnormally high HCN/CO and HCN/HCO$^+$ ratios.

Enhanced HCN emission likely from X-ray irradiated dense tori.

NGC 1068, NGC 1097, NGC 5194 host pure AGNs; others may be composite.

Abstract

An imaging survey of CO(1-0), HCN(1-0), and HCO$^+$(1-0) lines in the centers of nearby Seyfert galaxies has been conducted using the Nobeyama Millimeter Array and the RAINBOW interferometer. Preliminary results reveal that 3 Seyferts out of 7 show abnormally high HCN/CO and HCN/HCO$^+$ ratios, which cannot occur even in nuclear starburst galaxies. We suggest that the enhanced HCN emission originated from X-ray irradiated dense obscuring tori, and that these molecular line ratios can be a new diagnostic tool to search for ``pure'' AGNs. According to our HCN diagram, we suggest that NGC 1068, NGC 1097, and NGC 5194 host ``pure'' AGNs, whereas Seyfert nuclei of NGC 3079, NGC 6764, and NGC 7469 may be ``composite'' in nature.