  We study galaxy mergers using a high-resolution cosmological hydro/N-body
simulation with star formation, and compare the measured merger timescales with
theoretical predictions based on the Chandrasekhar formula. In contrast to
Navarro et al., our numerical results indicate, that the commonly used equation
for the merger timescale given by Lacey and Cole, systematically underestimates
the merger timescales for minor mergers and overestimates those for major
mergers. This behavior is partly explained by the poor performance of their
expression for the Coulomb logarithm, \ln (m_pri/m_sat). The two alternative
forms \ln (1+m_pri/m_sat) and 1/2\ln [1+(m_pri/m_sat)^2] for the Coulomb
logarithm 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 this
expression for the Coulomb logarithm. Furthermore, we find that the dependence
of the merger timescale on the circularity parameter \epsilon is much weaker
than the widely adopted power-law \epsilon^{0.78}, whereas
0.94*{\epsilon}^{0.60}+0.60 provides a good match to the data. Based on these
findings, we present an accurate and convenient fitting formula for the merger
timescale of galaxies in cold dark matter models.
