The Origin of the [C ii] Deficit in a Simulated Dwarf Galaxy Merger-driven Starburst

We present [C ii ] synthetic observations of smoothed particle hydrodynamics (SPH) simulations of a dwarf galaxy merger. The merging process varies the star formation rate (SFR) by more than three orders of magnitude. Several star clusters are formed, the feedback of which disperses and unbinds the dense gas through expanding H ii regions and supernova (SN) explosions. For galaxies with properties similar to the modeled ones, we find that the [C ii ] emission remains optically thin throughout the merging process. We identify the warm neutral medium ( 3 < log T gas < 4 with χ HI > 2 χ H2 ) to be the primary source of [C ii ] emission (∼58% contribution), although at stages when the H ii regions are young and dense (during star cluster formation or SNe in the form of ionized bubbles), they can contribute ≳50% to the total [C ii ] emission. We find that the [C ii ]/far-IR (FIR) ratio decreases owing to thermal saturation of the [C ii ] emission caused by strong far-UV radiation fields emitted by the massive star clusters, leading to a [C ii ] deficit medium. We investigate the [C ii ]−SFR relation and find an approximately linear correlation that agrees well with observations, particularly those from the Dwarf Galaxy Survey. Our simulation reproduces the observed trends of [C ii ]/FIR versus Σ SFR and Σ FIR , and it agrees well with the Kennicutt relation of SFR−FIR luminosity. We propose that local peaks of [C ii ] in resolved observations may provide evidence for ongoing massive cluster formation.