Making Spirals with Counter-rotating Disks

A single merger scenario for making galaxies such as NGC 4550 possessing equal coplanar counter-rotating stellar disks is investigated by collisionless N-body technique. The scenario is successful in producing an axisymmetric disk made of two almost equal counter-rotating populations. The final disk shows a clear bimodal line profile in the outer part, which demonstrates that disk-disk mergers do not always produce ellipticals. 1. The Puzzle of NGC 4550 The galaxy NGC 4550, an E7/S0 lenticular galaxy in the Virgo Cluster, was discovered by Rubin et al. (1992) to be a galaxy consisting of two coeval, coplanar, and counter-rotating stellar disks. Many other cases of counter-rotation are now known in ellipticals and also in spirals, but what makes the case of NGC 4550 particular (Kuijken, Fisher, & Merrifield 1996) is that the mass ratio of the counter-rotating disks is nearly 1/1. In the case of ellipticals counter-rotation is so frequent (e.g. Schweizer 1998) that the merger or accretion origins seem the most likely explanations for such kinematic misalignments. The difficulty with a merger scenario for NGC 4550 is that strong disk mergers usually lead to ellipticals and the destruction of the disks, a “truth” often believed to be general since the seminal paper of Toomre & Toomre (1972). Thakar & Ryden (1996) have shown that over several Gyr a series of well correlated small gaseous merger events can lead to a massive counterrotating gaseous disk. But then this scenario requires to preserve special correlations over several Gyr, contrary to a single event. Thus, it remains open whether a single merger of two equal mass spirals can result in some circumstances to a galaxy like NGC 4550.