Minor-axis velocity gradients in disk galaxies

Abstract

We present the ionized-gas kinematics and photometry of a sample of 4 spiral galaxies which are characterized by a zero-velocity plateau along the major axis and a velocity gradient along the minor axis, respectively. By combining these new kinematical data with those available in the literature for the ionized-gas component of the S0s and spirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies we realized that about 50% of unbarred galaxies show a remarkable gas velocity gradient along the optical minor axis. This fraction rises to about 60% if we include unbarred galaxies with an irregular velocity profile along the minor axis. This phenomenon is observed all along the Hubble sequence of disk galaxies, and it is particularly frequent in early-type spirals. Since minor-axis velocity gradients are unexpected if the gas is moving onto circular orbits in a disk coplanar to the stellar one, we conclude that non-circular and off-plane gas motions are not rare in the inner regions of disk galaxies.