Millimeter Interferometric HCN(1-0) and HCO+(1-0) Observations of Luminous Infrared Galaxies

TL;DR

This study uses millimeter interferometry to analyze molecular lines in luminous infrared galaxies, revealing a correlation between high HCN/HCO+ ratios and buried AGN activity, supporting the idea of enhanced HCN abundance near AGNs.

Contribution

First comprehensive interferometric analysis of HCN and HCO+ lines in multiple LIRGs, linking molecular line ratios to buried AGN signatures.

Findings

High HCN/HCO+ ratios correlate with buried AGN presence.

Wolf-Rayet galaxy He 2-10 shows low HCN/HCO+ ratio.

Enhanced HCN abundance likely due to X-ray irradiation from AGNs.

Abstract

We present the results on millimeter interferometric observations of four luminous infrared galaxies (LIRGs), Arp 220, Mrk 231, IRAS 08572+3915, and VV 114, and one Wolf-Rayet galaxy, He 2-10, using the Nobeyama Millimeter Array (NMA). Both the HCN(1-0) and HCO+(1-0) molecular lines were observed simultaneously and their brightness-temperature ratios were derived. High-quality infrared L-band (2.8-4.1 micron) spectra were also obtained for the four LIRGs to better constrain their energy sources deeply buried in dust and molecular gas. When combined with other LIRGs we have previously observed with NMA, the final sample comprised nine LIRGs (12 LIRGs' nuclei) with available interferometric HCN(1-0) and HCO+(1-0) data-sufficient to investigate the overall trend in comparison with known AGNs and starburst galaxies. We found that LIRGs with luminous buried AGN signatures at other wavelengths tend to show high HCN(1-0)/HCO+(1-0) brightness-temperature ratios as seen in AGN-dominated galaxies, while the Wolf-Rayet galaxy He 2-10 displays a small ratio. An enhanced HCN abundance in the interstellar gas surrounding a strongly X-ray-emitting AGN, as predicted by some chemical calculations, is a natural explanation of our results.