Tidal tails in CDM cosmologies

Abstract

We study the formation of tidal tails in pairs of merging disk galaxies with structural properties motivated by current theories of cold dark matter (CDM) cosmologies. In a recent study, Dubinski, Mihos & Hernquist (1996) showed that the formation of prominent tidal tails can be strongly suppressed by massive and extended dark haloes. For the large halo-to-disk mass ratio expected in CDM cosmologies their sequence of models failed to produce strong tails like those observed in many well-known pairs of interacting galaxies. In order to test whether this effect can constrain the viability of CDM cosmologies, we construct N-body models of disk galaxies with structural properties derived in analogy to the analytical work of Mo, Mao & White (1998). With a series of self-consistent collisionless simulations of galaxy-galaxy mergers we demonstrate that even the disks of very massive dark haloes have no problems developing long tidal tails, provided the halo spin parameter is large enough. We show that the halo-to-disk mass ratio is a poor indicator for the ability to produce tails. Instead, the relative size of disk and halo, or alternatively, the ratio of circular velocity to local escape speed at the half mass radius of the disk are more useful criteria. This result holds in all CDM cosmologies. The length of tidal tails is thus unlikely to provide useful constraints on such models.