Gas-rich galaxy mergers are more easily identified by their disturbed morphologies than mergers with less gas .
Because the typical gas fraction of galaxy mergers is expected to increase with redshift , the under-counting of low gas-fraction mergers may bias morphological estimates of the evolution of galaxy merger rate .
To understand the magnitude of this bias , we explore the effect of gas fraction on the morphologies of a series of simulated disc galaxy mergers .
With the resulting g -band images , we determine how the time-scale for identifying major and minor galaxy mergers via close projected pairs and quantitative morphology ( the Gini coefficient G , the second-order moment of the brightest 20 % of the light M _ { 20 } , and asymmetry A ) depends on baryonic gas fraction f _ { gas } .
Strong asymmetries last significantly longer in high gas-fraction mergers of all mass ratios , with time-scales ranging from \leq 300 Myr for f _ { gas } \sim 20 % to \geq 1 Gyr for f _ { gas } \sim 50 % .
Therefore the strong evolution with redshift observed in the fraction of asymmetric galaxies may reflect evolution in the gas properties of galaxies rather than the global galaxy merger rate .
On the other hand , the time-scale for identifying a galaxy merger via G - M _ { 20 } is weakly dependent on gas-fraction ( \sim 200-400 Myr ) , consistent with the weak evolution observed for G - M _ { 20 } mergers .