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

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.

💡 Research Summary

This paper presents the final set of observations from a systematic interferometric survey of luminous infrared galaxies (LIRGs) using the Nobeyama Millimeter Array (NMA). The authors targeted four LIRGs—NGC 2623, Mrk 266, Arp 193, and NGC 1377—to obtain simultaneous, spatially resolved measurements of the HCN (1‑0) and HCO⁺ (1‑0) molecular lines at 3 mm. By measuring the brightness‑temperature ratio (HCN/HCO⁺) in the nuclear regions, they aim to test whether this ratio can serve as a robust diagnostic for distinguishing between nuclei dominated by a buried active galactic nucleus (AGN) and those powered primarily by starburst activity.

Observations were carried out between 2005 and 2007 in the compact C and D configurations of the NMA, delivering an angular resolution of roughly 2–3 arcseconds (≈0.5–1 kpc at the distances of the targets). The data were calibrated and imaged using the standard MIRIAD pipeline, with CLEAN deconvolution applied to recover the complex nuclear structures. For each galaxy the authors extracted peak intensities and integrated fluxes for both lines, assumed a common source size for the two transitions, and derived the corresponding brightness temperatures, from which the HCN/HCO⁺ ratios were calculated.

The results show a clear dichotomy that mirrors the infrared spectroscopic classifications of the nuclei. NGC 2623, which exhibits deep silicate absorption and other buried‑AGN signatures in its mid‑infrared spectrum, displays a high HCN/HCO⁺ ratio of ≈1.8. Mrk 266 is a double‑nucleus system; the northern component, identified as AGN‑like in the infrared, has a ratio near 2.0, whereas the southern, starburst‑dominated component shows a low ratio of ≈0.7. Arp 193, a classic starburst LIRG, has a ratio of ≈0.6, consistent with strong HCO⁺ emission and relatively weak HCN. NGC 1377, an extremely dust‑enshrouded galaxy with ambiguous energy sources, yields an intermediate ratio of ≈0.9, suggesting a mixed or transitional state.

In the discussion the authors interpret these trends in terms of the underlying chemistry and excitation conditions. In AGN environments, intense X‑ray irradiation and high‑energy particles enhance the formation pathways of HCN while simultaneously destroying HCO⁺ through electron collisions, leading to elevated HCN/HCO⁺ ratios. Conversely, starburst regions are dominated by far‑ultraviolet photons and shock heating, which boost HCO⁺ production and suppress HCN, resulting in low ratios. The paper also acknowledges that gas density, kinetic temperature, optical depth, and chemical timescales can modulate the ratio, implying that it is not a simple binary indicator but a nuanced probe of the nuclear interstellar medium.

The authors conclude that the HCN (1‑0)/HCO⁺ (1‑0) brightness‑temperature ratio is a reliable, observationally inexpensive tool for separating AGN‑important from starburst‑dominant nuclei in LIRGs, corroborating earlier findings with a larger, uniformly observed sample. They advocate for follow‑up studies with higher‑resolution facilities such as ALMA, extending the analysis to higher‑J transitions (e.g., HCN (3‑2), HCO⁺ (3‑2)) and additional molecular tracers, to refine the physical interpretation of the ratio and to quantify the contribution of buried AGN throughout the evolutionary sequence of infrared‑luminous galaxies.