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

TL;DR

This study detects and maps dense molecular gas in the circumnuclear disk of NGC 4261, revealing a Keplerian rotation and providing insights into gas fueling the active galactic nucleus.

Contribution

First detection of HCN and HCO+ lines in NGC 4261's CND, with detailed kinematic analysis and mass estimates, advancing understanding of AGN fueling mechanisms.

Findings

Detection of HCN and HCO+ emission lines at 700 km/s

CND with a radius of 100 pc and Keplerian rotation

Dense gas mass of approximately 6.03x10^7 solar masses

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 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)x10^9 M_solar, assuming a disk inclination angle of 64 degree. 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 6.03x10^7 M_solar. 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.