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

Abstract

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 (${\sim }30{{\ \rm per\ cent}}$, merger-suite sample average) and elevate their cold-dense gas content (${\sim }18{{\ \rm per\ cent}}$). This is accompanied by a decrease in warm gas (${\sim }11{{\ \rm per\ cent}}$), a negligible change in cool gas (${\sim }4{{\ \rm per\ cent}}$ increase), and a substantial increase in hot gas (${\sim }400{{\ \rm per\ cent}}$). The amount of cold-dense gas with densities above 1000 cm-3 (the cold ultra-dense regime) is elevated significantly (${\sim }240{{\ \rm per\ cent}}$), but only accounts for ${\sim }0.15{{\ \rm per\ cent}}$ (on average) of the cold-dense gas budget.