Prevalence of Compact Nuclear Starbursts in Nearby Seyfert Galaxies

Abstract

We present an imaging survey of CO(1‐0), HCN(1‐0), and HCO+(1‐0) lines in nearby Seyfert galaxies using the Nobeyama Millimeter Array and RAINBOW Interferometer. Some of the observed Seyfert galaxies, including NGC 1068, NGC 1097, NGC 5033, and NGC 5194, show strong HCN(1‐0) emission at a few 100 pc scales; the observed HCN(1‐0)/CO(1‐0) and HCN(1‐0)/HCO+(1‐0) line ratios in these Seyfert nuclei (> 0.2 and > 1.8, respectively) are never observed in the central regions of nuclear starburst galaxies. On the other hand, the molecular line ratios in the nuclei of NGC 3079, NGC 3227, NGC 4051, NGC 6764, NGC 7479, and NGC 7469 are comparable with those in nuclear starburst galaxies. We suggest that the elevated HCN emission is originated from X‐ray irradiated dense molecular tori or X‐ray dominated regions (XDRs) close to the active nuclei, and our HCN/CO and HCN/HCO+ diagram will provide us with a new powerful diagnostic of the nuclear power source in active galaxies. Based on our diagnostic, we find that 3 of 5 type‐1 Seyferts (6 of 10 in total) host compact nuclear starbursts. Our results are also supported by observations at other wavelengths such as infrared L‐band spectroscopy. The proposed method based on the HCN and HCO+ spectroscopy will be very important toward heavily dusty nuclei such as Ultra‐Luminous Infrared Galaxies and high redshift submillimeter galaxies, because these molecular lines are free from dust extinction. As an example of such application, we present HCN and HCO+ observations of the Luminous Infrared Galaxy NGC 4418. We find an high HCN/HCO+ ratio of 1.8, suggesting a presence of a buried active nucleus.