Stellar feedback SPICEs up [C II] emission in the first galaxies

Abstract

The bright [C II] 158 micron line is widely used to trace star-forming gas and feedback-driven outflows in high-redshift galaxies. Using the SPICE simulations, we investigate how bursty versus smooth stellar feedback shapes galaxy properties at z > 5 as traced by [C II] emission. All models exhibit a tight correlation between [C II] luminosity (L_[CII]) and star formation rate (SFR), though bursty feedback yields systematically lower L_[CII] at fixed SFR and larger intrinsic scatter. [C II] emission is more extended than rest-frame UV light by factors of ~2-4, consistent with ALMA observations. While outflows are ubiquitous and mass outflow rates scale with L_[CII] (reaching ~10 Msun/yr), the net mass flux remains inflow-dominated in [C II]-bright systems. The emission is dominated by low-velocity (|v_rad| < 200 km/s) cold gas, with fast outflows contributing little, causing [C II]-based kinematics to overestimate cold gas velocities and underestimate fast components by factors of ~2-5. Consequently, inferred mass-loading factors and wind energetics are biased low, and large line widths primarily reflect the gravitational potential rather than outflow speeds. Although [C II] spatial and spectral properties alone do not distinguish feedback models, gas kinematics provides a strong diagnostic: smooth feedback promotes earlier disk settling, yielding higher V/sigma and disk fractions (~48% vs. ~28% for bursty feedback at z = 5). Overall, [C II] robustly traces star formation but is a biased tracer of feedback-driven outflows, highlighting the need for multiwavelength constraints on the multiphase ISM.