ABSTRACT. We present new interferometric observations of the ¹²CO (1-0), ¹²CO (2-1), and HCN (1-0) molecular line emission in NGC 3227 obtained with the IRAM Plateau de Bure interferometer (PdBI). We achieved an unprecedented angular resolution in the ¹²CO (2-1) line of about 06 corresponding to only about 50 pc at a distance of 17.3 Mpc. The mapped ¹²CO emission is concentrated in the inner 8'' and accounts for 20% of the total 30 m ¹²CO line flux. The ¹²CO emission is resolved into an asymmetric nuclear ring with a diameter of about 3''. The HCN line emission is mostly unresolved at our resolution of ~24 and contains all of the single-dish flux. We have decomposed the observed molecular gas motions into a circular and noncircular component revealing that about 80% of the gas in the circumnuclear region exhibits pure circular rotation. We find evidence for bar streaming onto the nuclear ring and a redshifted emission knot on the ring perimeter.

In the central arcsecond the gas shows apparent counterrotation. This behavior can be best explained by a warping of the inner molecular gas disk rather than gas motion in a nuclear bar potential. We detected molecular gas at a distance from the nucleus of only ~13 pc with a velocity of about 75 km s^-1 with respect to the systemic velocity and find that within the central arcsecond the rotation curve is rising again. This is the first time that millimetric CO line emission has been detected interferometrically at such small distances to the nucleus of a Seyfert galaxy. These measurements indicate a lower limit on the enclosed mass of about 2 × 10⁷ M in the inner 25 pc.

Subject headings: galaxies: individual (NGC 3227); galaxies: ISM; galaxies: nuclei; radio lines: ISM

Print publication: Issue 2 (2000 April 20)
Received 1999 August 23, accepted for publication 1999 November 18