Kinematics of the dusty circumnuclear ring in the barred Seyfert galaxy NGC 4151

We present optical and neutral hydrogen observations of the central ∼2 kpc in NGC 4151, and examine the distribution and kinematics of neutral hydrogen in the vicinity of the optical circumnuclear dust ‘ellipse’. The V-I colour map, derived from broad-band images obtained using TAURUS II in imaging mode, shows clearly the red arcs which delineate the 11′′ × 18′′ dust ellipse detected previously. The new, high resolution (∼ 6′′ × 5′′) HI image reveals two arcs of HI emission, which lie on the inner edges of the main bar and coincide closely with the optical arcs. The peak column densities in the HI arcs, of ∼ 1.8 × 1021 cm−2 and ∼ 0.7× 1021 cm−2, imply optical extinctions of Av ∼1.16 and 0.45 in the northern and southern arcs respectively, which are comparable with those derived from broad-band imaging. The kinematics of the HI, measured along the two arcs, are presented. The first order circular fit to the velocity curve yields a value of 22±5◦ for the PA of the line of nodes, which agrees well with that of the galaxy. If the HI is associated with the optical ellipse, the PA of the spatial major axis of the ellipse (∼48◦) is offset from its kinematic major axis (∼22◦), providing evidence of non-circular orbits and suggesting that ring is not an inclined circular disk, but is intrinsically elliptical and lies in the plane of the galaxy.