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It is now clear that the stars in the Solar neighbourhood display large-scalecoherent vertical breathing motions. At the same time, Milky Way-like galaxiesexperience tidal interactions with satellites/companions during theirevolution. While these tidal interactions can excite vertical oscillations, itis still not clear whether vertical breathing motions are excited\textit{directly} by the tidal encounters or are driven by the tidally-inducedspirals. We test whether excitation of breathing motions are directly linked totidal interactions by constructing a set of $N$-body models (with mass ratio5:1) of unbound, single fly-by interactions with varying orbitalconfigurations. We first reproduce the well-known result that such fly-byinteractions can excite strong transient spirals (lasting for $\sim 2.9-4.2$Gyr) in the outer disc of the host galaxy. The generation and strength of thespirals are shown to vary with the orbital parameters (the angle ofinteraction, and the orbital spin vector). Furthermore, we demonstrate that ourfly-by models exhibit coherent breathing motions whose amplitude increases withheight. The amplitudes of breathing motions show characteristic modulationalong the azimuthal direction, with compressing breathing motions coincidingwith the peaks of the spirals and expanding breathing motions falling in theinter-arm regions -- a signature of a spiral-driven breathing motion. Thesebreathing motions in our models end when the strong tidally-induced spiral armsfade away. Thus, it is the tidally-induced spirals which drive the large-scalebreathing motions in our fly-by models, and the dynamical role of the tidalinteraction in this context is indirect.

Excitation of vertical breathing motion in disc galaxies by tidally-induced spirals in fly-by interactions