Tidal-induced lopsidedness in Magellanic-type galaxies

Abstract

We investigate the tidally-induced conversion of barred late-type spirals to Magellanic-type discs with numerical simulations, to establish how the lifetime of lopsidedness (asymmetry) varies with numerical parametrizations. Using a reference model based on observed properties of the Large Magellanic Cloud (LMC), we show that its off-centre bar, one-arm spiral and one-sided star formation can be induced by a brief tidal interaction. We thereafter perform a detailed parameter study, and find that at the dynamical mass of LMC-type discs ($\sim$10$^{\rm 10}$ \md{}), stellar lopsidedness (as quantified by the m$=$1 Fourier mode) and bar off-centredness can vary widely in amplitude, but are generally short-lived ($\sim$Gyr). Tidal interactions induce more persistent lopsidedness in lower mass galaxies (several Gyr), in particular those with large halo-to-disc mass ratios as implied by recent halo occupation models. We suggest that the tidal interactions play a larger role in the observed ubiquity of lopsidedness than the presently favoured gas accretion mechanism for lower mass galaxies. Other characteristics of Magellanic-type galaxies, such as one-sided star formation, are quantified and tend to arise more prominently in discs with later-type spiral structure (more halo dominated inner disc, weaker bars) following retrograde orbital encounters.