Gas mass derived from dust mass vs. gas mass for this sample and the z = 3.62 source in Jin et al. (2019). Left panels: comparison of gas mass from optically thin dust mass to gas mass from dynamical and [CI] estimations (M_gas, dyn, M_gas, CI), respectively. Right panel: gas mass derived from optically thick dust to M_gas, dyn (red dots) and M_gas, CI (blue squares). Solid line shows the 1:1 identity relation and the dashed line is three times the identity line. Optically thin dust models significantly and systematically overestimate the dust mass of galaxies in this work, indicating that they have optically thick dust.