Neutral hydrogen studies of the Seyfert galaxy NGC 3227

Abstract

We have used the VLA to image the neutral hydrogen emission from the Seyfert galaxy NGC 3227, with angular resolutions ranging from 12 to 60 arcsec. We detect plumes of Hi extending to ~ 70 kpc north and ~ 31 kpc south of the galaxy which may be a consequence of interaction. Complex motions in the galactic disc have been resolved into emission from the disc, which is in approximate solid body rotation, and an anomalous velocity cloud, situated northwest of the disc at the base of the northern plume. The cloud has a mean velocity of ~ 150 km s-1 above the systemic velocity of NGC 3227 and shows evidence of rotation. We suggest that this cloud is a gas-rich dwarf galaxy that either is one of the bodies responsible for the interaction, or has formed as a consequence of the interaction. No neutral hydrogen emission is associated with the companion galaxy NGC 3226, and the presence of low velocity (500 km s-1) gas reported in single dish measurements is not detected in the present observations. The continuum image of NGC 3227 shows an extended component, in addition to the compact core, that may be enhanced disc emission due to the interaction. In the central 15 arcsec of the disc we detect evidence of Hi absorption against the nuclear continuum source. Higher resolution observations are required to confirm this. Although the velocities in the disc are remarkably close to solid body velocities (considering the presence of a disturbing companion), the integrated neutral hydrogen emission shows a bar of enhanced emission crossing the nucleus, in a northwest to southeast direction, that has a 'Z' shaped morphology and trails in the same sense as the optical spiral arms. This H i bar seems to be a continuation of the CO bar and we suggest that it may play an important role in the fuelling of the AGN. From our H i observations we deduce a heliocentric systemic velocity of 1135 ± 10 km s _ 1 for NGC 3227. We derive a value of 56° for the inclination of the galactic disc and 158° ± 2° for the PA of the major axis. Neutral hydrogen masses of 5.7 x 10 8 M 0 for the disc, 2.1 x 10 8 M 0 for the cloud, 1.3 x 10 8 M 0 in the southern plume and 1.8 x 10 8 M 0 in the northern plume were found, resulting in a total H i mass for NGC 3227 of 1.1 x 10 9 M 0 .