RIGEL: Feedback regulated cloud-scale star formation efficiency in a simulated dwarf galaxy merger

Abstract: Major mergers of galaxies are likely to trigger bursty star formation activities. The accumulation of dense gas and the boost of star formation efficiency (SFE) are considered to be the two main drivers of the starbursts. However, it is still unclear how each process operates on the scale of individual star-forming clouds. Here, we present a high-resolution (2 Msun) RHD simulation of a gas-rich dwarf galaxy merger using the RIGEL model to investigate how mergers affect the properties of the structure of dense star-forming gas and the cloud-scale SFE. We track the evolution of sub-virial dense clouds in the simulation by mapping them across successive snapshots taken at intervals of 0.2 Myr. We found that the merger triggers a 130 times higher SFR and shortens the galaxy-wide gas-depletion time by two orders of magnitude compared to those of two isolated galaxies. However, the depletion time of individual clouds and their lifetime distribution remained unchanged over the simulation period. The cloud life cycles and cloud-scale SFE are determined by the local stellar feedback rather than environmental factors regardless of the merger process, and the integrated SFE ($\epsilon_{\rm int}$) of clouds in complex environments is still well described by an $\epsilon_{\rm int}-\Sigma_{\rm tot}$ relation found in idealized isolated-cloud experiments. During the peak of the starburst, the median integrated SFE only changed by 0.17-0.33 dex lower compared to the value when the galaxies are not interacting. The merger boosts the SFR primarily through the accumulation and compression of dense gas fueling star formation. Strong tidal torques assemble $>10^ 5$ Msun clouds that seed massive star clusters. The average separation between star-forming clouds decreases during the merger, which in turn decreases the cloud--cluster spatial decorrelation from >1 kpc to 0.1 kpc depicted by tuning fork diagrams.