Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550

We present observations of the CO(1-0) emission in the central 750 pc (10 arcsec) of the counter-rotating disc galaxy NGC 4550, obtained at the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer. Very little molecular gas is detected, only 1×10⁷M_solar, and its distribution is lopsided, with twice as much molecular gas observed at positive relative velocities than at negative relative velocities. The velocity gradient in the CO(1-0) emission shows that the molecular gas rotates like the thicker of the two stellar discs, which is an unexpected alignment of rotations if the thinner disc was formed by a major gas accretion event. However, a simulation shows that the gas rotating like the thicker disc naturally results from the coplanar merger of two counter-rotating disc galaxies, demonstrating the feasibility of this scenario for the formation of NGC 4550. We investigate various star formation tracers to determine whether the molecular gas in NGC 4550 is currently forming stars. Ultraviolet (UV) imaging data and optical absorption line strengths both suggest a recent star formation episode; the best-fitting two-population model to the UV-optical colours yields a mass of young stars of 5.9×10⁷M_solar with an age of 280 Myr. The best information on the current star formation rate is a far-infrared-based upper limit of only 0.02M_solaryr^-1. We are thus witnessing NGC 4550 either in a dip within a bursty star formation period or during a more continuous low-level star formation episode.