NGC 4546, the Double-Spin SB0

Abstract

NGC 4546 is a barred S0 seen almost edge-on whose stellar disc is cut on the equatorial plane by a disc of gas rotating in the opposite direction and with relative circular velocities as great as 400 km sec⁻¹. Due to the high rotational velocities involved, the counterrotation between gas and stars is evident from a simple inspection of the spectra (Galletta 1986, 1987). Despite to this peculiarity, the morphology of NGC 4546 is quite normal: CCD images in the V band taken at the 1.52-m telescope at La Silla indicate the presence of a bright bulge, including ∼50% of the total light, perfectly aligned with a stellar disc of constant flattening and with 71° of inclination with respect to the plane of the sky. The bar is quite small, extending ∼ 10″ on either side of the nucleus, and ends with two pseudo-spiral arms. A knotted dark lane crosses the bar and the pseudo-arms. In Hα, the image of the galaxy is also quite normal: a slightly irregular disc of gas is visible, extending for ∼ 50″ from the nucleus and with about the same ellipticity as the stellar disc.