Interacting galaxies on FIRE-2: the connection between enhanced star formation and interstellar gas content

We present a comprehensive suite of high-resolution (parsec-scale), idealized (non-cosmological) galaxy merger simulations (24 runs, stellar mass ratio ∼2.5:1) to investigate the connection between interaction-induced star formation and the evolution of the interstellar medium (ISM) in various temperature–density regimes. We use the GIZMO code and the second version of the 'Feedback in Realistic Environments' model (FIRE-2), which captures the multiphase structure of the ISM. Our simulations are designed to represent galaxy mergers in the local Universe. In this work, we focus on the 'galaxy-pair period' between first and second pericentric passage. We split the ISM into four regimes: hot, warm, cool, and cold-dense, motivated by the hot, ionized, atomic and molecular gas phases observed in real galaxies. We find that, on average, interactions enhance the star formation rate of the pair (⁠∼30 per cent, merger-suite sample average) and elevate their cold-dense gas content (⁠∼18 per cent⁠). This is accompanied by a decrease in warm gas (⁠∼11 per cent), a negligible change in cool gas (⁠∼4 per cent increase), and a substantial increase in hot gas (⁠∼400 per cent⁠). The amount of cold-dense gas with densities above 1000 cm^(−3) (the cold ultra-dense regime) is elevated significantly (⁠∼240 per cent⁠), but only accounts for ∼0.15 per cent (on average) of the cold-dense gas budget.