Rotating Molecular Gas Associated with a Silhouette Disk at the Center of the Radio Galaxy 3C 31

Abstract

Aperture synthesis observations of ¹²CO(J=1-0) emissions of the radio galaxy 3C 31 (NGC 383) have been made using the Nobeyama Millimeter Array (NMA) and the RAINBOW interferometer (NMA plus the Nobeyama 45 m radio telescope). Our high-resolution (1.9"×1.4", or 640pc×470pc, at D=70 Mpc) ¹²CO image shows a circularly rotating molecular gas disk that closely coincides with a silhouette disk observed in the Hubble Space Telescope (HST) optical images. The molecular gas mass (Mgas) of the disk is estimated to be 9.7×10⁸M_solar within a radius of 1 kpc, and the peak gas surface density Σgas is 4.0×10²M_solar pc^-2 at 440 pc from the center, if a Galactic I(CO)-to-N(H₂) conversion factor (1.8×10²⁰ cm^-2 [K km s^-1]^-1) is applied. The rotation velocity of the disk is 460 km s^-1 at a radius of 1 kpc, giving an enclosed mass (dynamical mass) of M_dyn=5.0×10¹⁰M_solar within this radius. The ratio of gas mass to dynamical mass, Mgas/Mdyn, is less than 0.02, indicating that the gas disk at the center of 3C 31 is stable against gravitational instabilities, although the total gas mass of the nuclear disk in 3C 31 is fairly large compared with the nuclear gas concentration observed in late-type spirals.