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

TL;DR

This study presents systematic interferometric observations of HCN(1-0) and HCO+(1-0) in four luminous infrared galaxies, confirming that the brightness-temperature ratio helps distinguish between AGN-dominated and starburst-dominated nuclei.

Contribution

It provides the most comprehensive spatially-resolved HCN and HCO+ data for LIRGs, reinforcing the ratio's use as an AGN/starburst diagnostic.

Findings

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

Starburst-dominated nuclei show low HCN/HCO+ ratios.

The ratio effectively separates AGN and starburst activity in LIRGs.

Abstract

We report the results of interferometric HCN(1-0) and HCO+(1-0) observations of four luminous infrared galaxies (LIRGs), NGC 2623, Mrk 266, Arp 193, and NGC 1377, as a final sample of our systematic survey using the Nobeyama Millimeter Array. Our survey contains the most systematic interferometric, spatially-resolved, simultaneous HCN(1-0) and HCO+(1-0) observations of LIRGs. Ground-based infrared spectra of these LIRGs are also presented to elucidate the nature of the energy sources at the nuclei. We derive the HCN(1-0)/HCO+(1-0) brightness-temperature ratios of these LIRGs and confirm the previously discovered trend that LIRG nuclei with luminous buried AGN signatures in infrared spectra tend to show high HCN(1-0)/HCO+(1-0) brightness-temperature ratios, as seen in AGNs, while starburst-classified LIRG nuclei in infrared spectra display small ratios, as observed in starburst-dominated galaxies. Our new results further support the argument that the HCN(1-0)/HCO+(1-0) brightness-temperature ratio can be used to observationally separate AGN-important and starburst-dominant galaxy nuclei.