Gas dynamics in the barred Seyfert galaxy NGC 4151 - I. H I streaming shocks and inflow along the bar

We present sensitive, high-resolution observations of neutral hydrogen (H I) in the unusually gas-rich, oval distortion of the Seyfert galaxy NGC 4151. The gas dynamics of the oval are found to be consistent with those of a kinematically weak bar, fully confirming previous suggestions for the presence of a fat bar, and, for the first time, individual gaseous features in the bar are spatially resolved. In particular, the two bright regions close to the leading edges of the bar in NGC 4151 exhibit kinematics strikingly similar to the signature of bar shocks seen in gas-dynamical simulations, and demonstrate how strong the gaseous response may be even in such a weak bar potential. The residual velocity field, showing deviations from circular motion, is largely consistent with streaming in a bar potential, and, in addition, clearly shows that inflow is concentrated in narrow regions originating in the shocks. This inflow may represent an early stage in the fuelling process of the AGN. The presence and properties of the shocks in NGC 4151 indicate that, in addition to the x1 orbits, the family of x2 orbits exists and is of significant extent in the bar of NGC 4151, with gas streaming from the shocks making the transition between the two families. We therefore suggest that the circumnuclear ellipse, identified optically by previous authors and associated with gas flowing in x2 orbits, has formed as a natural consequence of the gas flows in the bar without the requirement for a second, inner bar. Observations of H I were previously thought to be ill-suited to the study of bar shocks because of limitations in angular resolution and sensitivity. However, our observations show that, following recent instrumental enhancements, such measurements are now feasible, albeit at the limits of instrumental capability.