Stellar and gas kinematics of NGC 4546, the double-spin SB0*

Photometric, spectroscopic, optical, and neutral hydrogen observations are presented of NGC 4546, a barred S0 galaxy possessing a disk of gas rotating in the opposite direction compared to the stellar disc. The relative circular velocities exceed 350 km s^–1. The stellar velocity dispersion and the width of the emission lines within the galaxy are analyzed. Hα imaging reveals a bright condensation close to the center of the galaxy, possibly a remnant of the center of a small galaxy captured by NGC 4546. Dust lanes, pseudo spiral arms and faint objects similar to globular clusters are also revealed after a model for the light distribution of the disc and the bulge has been subtracted from the image. On the basis of optical spectroscopy and H I observations, it is shown that the gas disc is more dense on the NE side of the galaxy. Large irregular motions are detected in the gas disc. The intensity ratios between [N II], [S II], and [O III] lines along the major axis indicate values intermediate between galaxy nuclei and galactic H II regions. To first order the gas rotates in a disc, but large irregular motions are superimposed. The gas disc appears to be a structure not yet relaxed but in the process of being shaped by the galaxy potential. An external origin of the gas, by means of capture of a small galaxy or a gas cloud more recent than $$\sim {10}^{9}$$ yr is suggested.