Dynamics of gas in barred spiral galaxies – I. NGC 6221

The southern galaxy NGC 6221, which exhibits very large non-circular gas motions, has been observed as part of a continuing study into the gas dynamics in barred spirals. A deprojected image of NGC 6221, simulating the face-on view of the galaxy, clearly shows the bar which is foreshortened on ordinary photographs. A faint third outer arm, or linear extension, indicates a possible tidal interaction with the nearby spiral companion NGC 6215, as does the presence of an asymmetric high velocity tail on the integrated H I profile. The gas velocity field has been mapped with a grid of long-slit spectra taken with the AAT. The 465 velocity measurements are listed in the Appendix. We derive a systemic velocity of 1475 km s−1, an inclination of 45° and a position angle of the line of nodes of 118° which is exactly perpendicular to the long axis of the bar. The approximate mass within a radius of 5.4 kpc is 7 × 1010M⊙. The isovelocity contour map shows a conspicuous S-shaped pattern of closely spaced contours indicating a very sudden change in velocity of about 150 km s−1 at the position of the dust lanes in the bar. The gas is moving radially outward as it approaches the dust lane, and then inward after passing through the shock, in good agreement with theoretical predictions. The transition from outward to inward motion occurs in a narrow region less than 200 pc wide. The small, very bright nucleus shows signs of moderate activity. The emission lines have an intense narrow peak which is blueshifted 60 km s−1 relative to the systemic velocity, and a weak asymmetric broad component. The blueshifted emission, which is commonly seen in other galaxies, seems to be related to the gas streaming around the bar.