Prevalence of compact nuclear starbursts in nearby Seyfert galaxies

We present an imaging survey of the CO(1–0), HCN(1–0), and HCO(1–0) lines in the 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 exhibit strong HCN(1–0) emission on a few 100 pc scales. The observed HCN(1–0)/CO(1–0) and HCN(1–0)/HCO(1–0) line ratios in the Seyfert nuclei (>0.2 and >1.8, respectively) have never been 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 the nuclear starburst galaxies. We propose that the elevated HCN emission originates from the X-ray irradiated dense molecular tori or X-ray dominated regions (XDRs) in close proximity to the active nuclei. Further, our HCN/CO and HCN/HCO diagrams will provide a new powerful diagnostic of the nuclear power source in active galaxies. Based on our diagnostic, we observe three of five type-1 Seyferts (six of ten in total) host compact nuclear starbursts. Our results are also supported by observations at other wavelengths such as those by infrared L-band PAH spectroscopy. The proposed method based on the HCN and HCO spectroscopies will be crucial for investigating extremely dusty nuclei, such as the ultraluminous infrared galaxies and high-redshift submillimeter galaxies, because these molecular lines are devoid of dust extinction. As an example, we present the HCN and HCO observations of the luminous infrared galaxy NGC 4418. We have obtained a high HCN/HCO ratio of 1.8, which suggests the presence of a buried active nucleus. SURVEY OF DENSE MOLECULAR GAS IN SEYFERT GALAXIES USING NOBEYAMA MILLIMETER ARRAY A dense molecular medium plays various roles in the vicinity of active galactic nuclei (AGNs). The presence of dense and dusty interstellar matter (ISM), which obscures the broad line regions in the AGNs, is inevitable at < 1pc — a few 10 pc scales according to the proposed unified model of Seyfert galaxies. This circumnuclear dense ISM could be a reservoir of fuel for the active nuclei and a site for massive star formation. In fact, strong HCN (1–0) emission, which requires dense (nH2 > 10 4 cm−3) environments for its collisional excitation, has been detected in the prototypical type-2 Seyfert NGC 1068 [1, 2, 3]. Similar enhancements in the low-luminosity Seyfert galaxies such as NGC 1097 [4] and NGC 5194 [5] have been reported. In these Seyfert nuclei,the HCN (1–0) to CO (1–0) integrated intensity ratios in the brightness temperature scale, RHCN/CO, are enhanced up to approximately 0.4-0.6, and the kinematics of the HCN line indicates that this dense molecular medium could be the outer envelope of the abovementioned obscuring material [1, 2, 5]. Howevwer, not every Seyfert galaxy exhibits such HCN enhancement. For example, TABLE 1. Galaxy sample for the survey using the Nobeyama Millimeter Array Name Class Morphology D∗ CO† HCN† HCO+† Seyfert galaxies listed in Palomar Northern sample NGC 1068 S1.9 (R)SA(rs)b 14.4 © © NGC 1667 S2 SAB(r)c 61.2 © © © NGC 3079 S2 SB(s)c 20.4 © © © NGC 3227 S1.5 SAB(s)a pec 20.6 © © © NGC 3982 S1.9 SAB(r)b: 17.0 © © © NGC 4051 S1.2 SAB(rs)bc 17.0 © © © NGC 4151 S1.5 (R’)SAB(rs)ab: 20.3 © NGC 4258 S1.9 SAB(s)bc 6.8 © △ △ NGC 4388 S2 SA(s)b: sp 16.8 © © © NGC 4501 S2 SA(rs)b 16.8 © © © NGC 4579 S1.9/L1.9 SAB(rs)b 16.8 © NGC 5033 S1.5 SA(s)c 18.7 © © © NGC 5194 S2 SA(s)bc pec 7.7 © © © NGC 6951 S2 SAB(rs)bc 24.1 © © △ NGC 7479 S1.9 SB(s)c 32.4 © © © Southern Seyfert galaxies NGC 1097 S1 (R’)SB(r’l)b 14.0 © © © NGC 5135 S2 SB(l)ab 54.8 © NGC 6764 S2 SB(s)bc 32.2 © © © NGC 6814 S1.5 SAB(rs)bc 20.8 © NGC 7465 S2 (R’)SB(s)0 26.2 © © © NGC 7469 S1.2 (R’)SAB(rs)a 65.2 © © © ∗ Distance in Mpc. † The survey status. © = observed; △ = observations in progress; blank = not observed. there is no significant enhancement in RHCN/CO within the central region extending over a few 100 pc of the Seyfert galaxy NGC 6951 [6], although a slight increase in the ratio can be observed in the circumnuclear starburst/star-forming rings of NGC 6951. Questions regarding the difference between these Seyfert galaxies with and without HCN enhancement and the nature of the strong HCN emission toward some of Seyfert galaxies arise. In order to address these issues, we have conducted an extensive “3D” imaging survey of the CO(1–0), HCN(1–0), and HCO(1–0) lines in the local Seyfert galaxies using the Nobeyama Millimeter Array (NMA) and the RAINBOW interferometer with typical resolutions of ∼ 2 to 6 and sensitivities of a few mJy beam−1 for a ∼ 50 km s−1 velocity channel. The sample of galaxies for the survey is listed in Table 1. The majority of the galaxies are from the Palomar Northern Seyfert sample [7]. Some southern Seyfert galaxies are also included in the sample. It should be noted that the HCN(1–0) and HCO(1–0) lines are observed simultaneously by using the Ultra Wide Band Correlator [8]. This is essential to obtain accurate HCN/HCO integrated intensity ratios (RHCN/HCO+ hereafter).