Search

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.

Ionized gas and stellar kinematical parameters have been measured along the major axis of 20 nearby disc galaxies. We discuss the properties of each sample galaxy, distinguishing between those characterized by regular or peculiar kinematics. In early-type disc galaxies, ionized gas tends to rotate faster than stars and to have a lower velocity dispersion ($V_{\rm g} > V_\star$and $\sigma_{\rm g} < $ß), whereas in late-type spirals, gas and stars show almost the same rotation velocities and velocity dispersions ($V_{\rm g} \simeq V_\star$and $\sigma_{\rm g} \simeq $ß). Incorporating the early-type disc galaxies studied by Bertola et al. (1995), Fisher (1997) and Corsini et al. (1999), we have compiled a sample of some 40 galaxies for which the major-axis radial profiles of both the stellar and gaseous components have been measured. The value of ßmeasured at $R_{\rm e}/4$turns out to be strongly correlated with the galaxy morphological type, while $\sigma_{\rm g}$is not and sometimes takes values above the range expected from thermal motions or small-scale turbulence.