The effect of gas fraction on the morphology and time-scales of disc galaxy mergers

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 per cent 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 < 300 Myr for f(gas) similar to 20 per cent to >= 1 Gyr for f(gas) similar to 50 per cent. 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 (similar to 200-400 Myr), consistent with the weak evolution observed for G-M(20) mergers.