CO EMISSION FROM THE NUCLEUS OF INFRARED GALAXY NGC-4418 - AN EARLY AGN PHASE

The 12CO (J = 1-0) emission has been detected in an extremely extinguished galaxy NGC 4418. The inferred mass of molecular gas is 4 × 108 M⊙ in the central 15″ region of the galaxy. Combining the narrow CO line width (85 km s-1) with the extinction value derived from the 10 μm silicate absorption, it is suggested that the molecular gas clouds are highly concentrated toward the center or dynamically decoupled from the underlying stellar system, rotating more slowly than the stars and falling to the center. We hypothesize that NGC 4418 is at the very early phase of its AGN activity, and that the radiation from the nucleus is efficiently absorbed by dense ambient gas and reemitted in the far-infrared, taking the following peculiar properties into account: (1) the 1.5 GHz map recently reported by Condon et al. that measured 38.5 mJy in a 0″.5 × 0″.3 core (brightness temperature of 150,000 K), (2) no emission lines in the optical and near-infrared, (3) the ratio of the 1.5 GHz flux relative to the IRAS flux which is 10 times smaller than those of other IRAS galaxies, and (4) the ratio (= 170) of the IR luminosity to the molecular gas mass which is 10 times greater than those of other galaxies with the comparable IR luminosity (L = 7 × 1010 L⊙) and comparable to that of an ultraluminous IRAS galaxy Mrk 231 (Seyfert 1/QSO with L = 2 × 1012 L⊙).