Letter to the Editor

Circumnuclear dense gas disk fuelling the active galactic nucleus in the nearby radio galaxy NGC 4261

¹ The Research Institute for Time Studies, Yamaguchi University, 1677-1 Yoshida, Yamaguchi, Yamaguchi 753-8511, Japan
e-mail: swdsth@gmail.com
² Graduate School of Science, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
³ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago 763-0355, Chile
⁴ NAOJ Chile Observatory, Alonso de Córdova 3788, Oficina 61B, Vitacura, Santiago, Chile
⁵ Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

Received: 18 May 2022
Accepted: 2 August 2022

Abstract

The cold molecular gas in the circumnuclear disk (CND) of radio galaxies provides critical information for understanding the mass accretion onto active galactic nuclei. We present the first detection and maps of HCN J = 1–0 and HCO⁺J = 1–0 emission lines from the circumnuclear region of a nearby radio galaxy, NGC 4261, using the Northern Extended Millimeter Array. Both molecular lines are detected at a radial velocity of ±700 km s⁻¹ relative to the systemic velocity of the galaxy, and they arise from a CND with an outer radius of 100 pc. The velocity fields of HCN and HCO⁺ are fitted with a Keplerian disk rotation. The enclosed mass is (1.6 ± 0.1) × 10⁹ M_⊙, assuming a disk inclination angle of 64°. The continuum image at 80 GHz reveals a weak two-sided jet structure extending over 5 kpc along the east–west direction and a bright core at the centre. The continuum spectrum between 80 and 230 GHz shows a spectral index of −0.34 ± 0.02, which suggests optically thin synchrotron radiation. The dense gas mass associated with the CND is calculated to be 6.03 × 10⁷ M_⊙. It supports a positive correlation between the dense gas mass in the CND and the accretion rate onto the supermassive black hole, though there are uncertainties in the parameters of the correlation.