The two faces of tides: gravitational instability during galaxy mergers

Abstract

Gravitational instability is a fundamental mechanism driving collapse of interstellar gas, yet in dynamically evolving environments such as galaxy mergers, external tidal fields can significantly alter the conditions for collapse. In this work, we develop an analytical framework to investigate how the tidal field of a companion galaxy modifies the classical Jeans instability by incorporating its anisotropic and time-dependent nature into the dispersion relation. We find that the tidal field introduces a critical angle between the cloud's position vector and the merger axis, $\theta_c \approx 54.7^\circ$, separating disruptive and compressive regimes of the radial tidal component. Disruptive tides suppress instability by increasing the characteristic length scale and restricting the range of unstable modes, whereas compressive tides enhance collapse by extending the unstable spectrum and increasing the growth rate of perturbations. The impact of tidal fields depends sensitively on gas density, being significant in diffuse media but negligible in dense molecular environments where the classical Jeans limit is recovered. All tidal effects peak near pericentric passage due to the strong dependence on galaxy separation. Since the free-fall time of diffuse gas is comparable to the duration of the close passage, tidally assisted collapse in the diffuse medium lags pericentre by roughly a free-fall time, whereas dense gas responds essentially instantaneously. These results demonstrate that companion-induced tidal fields play a key role in regulating the scale and efficiency of gravitational instability of diffuse interstellar gas in galaxy mergers. The code underlying this work is publicly available.