Abstract
The star formation efficiency per free-fall time (ϵff) quantifies how efficiently giant molecular clouds (GMCs) convert gas into stars and is often observed to be orders of magnitude lower than expected for free-fall collapse. Observers typically estimate ϵff by mapping far-infrared (FIR) dust emission to infer gas surface densities (Σgas) and counting embedded protostars, assuming simplified lifetimes and masses. Using the fiducial starforge radiation–magnetohydrodynamics simulation of an Mcl = 2 × 104 M⊙ GMC that self-consistently models star cluster formation and stellar feedback, we generate synthetic FIR observations and apply the same methodologies used in GMC surveys to investigate this discrepancy. We present the first ϵff − Σgas analysis in a fully feedback-regulated GMC simulation that resolves the formation of stellar systems. Our synthetic measurements reproduce the low observationally inferred efficiencies, showing that feedback-regulated star formation naturally produces ϵff ∼ 1%–3% without requiring extreme initial conditions. We also find that ϵff varies strongly over a GMC’s lifetime, suggesting that much of the observed scatter reflects evolutionary sampling rather than intrinsic cloud-to-cloud differences. Finally, by comparing observational and simulation definitions, we show that methodological assumptions introduce systematic biases, with a transition near that depends on resolution. Above this threshold, the smoothing of dense structure increases both the inferred free-fall time and enclosed gas, with the latter dominating and suppressing ϵff. Below the threshold, the discrepancies are primarily driven by star formation rate assumptions.