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We study galaxy mergers using a high-resolution cosmological hydro/N-bodysimulation with star formation, and compare the measured merger timescales withtheoretical predictions based on the Chandrasekhar formula. In contrast toNavarro et al., our numerical results indicate, that the commonly used equationfor the merger timescale given by Lacey and Cole, systematically underestimatesthe merger timescales for minor mergers and overestimates those for majormergers. This behavior is partly explained by the poor performance of theirexpression for the Coulomb logarithm, \ln (m_pri/m_sat). The two alternativeforms \ln (1+m_pri/m_sat) and 1/2\ln [1+(m_pri/m_sat)^2] for the Coulomblogarithm can account for the mass dependence of merger timescale successfully,but both of them underestimate the merger time scale by a factor 2. Since \ln(1+m_pri/m_sat) represents the mass dependence slightly better we adopt thisexpression for the Coulomb logarithm. Furthermore, we find that the dependenceof the merger timescale on the circularity parameter \epsilon is much weakerthan the widely adopted power-law \epsilon^{0.78}, whereas0.94*{\epsilon}^{0.60}+0.60 provides a good match to the data. Based on thesefindings, we present an accurate and convenient fitting formula for the mergertimescale of galaxies in cold dark matter models.

A fitting formula for the merger timescale of galaxies in hierarchical clustering