The Luminosity Dependence of the Galaxy Merger Rate

We measure the number of companions per galaxy (N_c) as a function of r-band absolute magnitude for both the Sloan Digital Sky Survey and the Croton and coworkers semianalytic catalog applied to the Millennium Run simulation. For close pairs with projected separations of 5-20 h⁻¹ kpc, velocity differences less than 500 km s⁻¹, and luminosity ratios between 1:2 and 2:1, we find good agreement between the observations and simulations, with N_c consistently close to 0.02 over the range -22 < M_r < − 18. For larger pair separations, N_c(M_r) instead becomes increasingly steep toward the faint end, implying that luminosity-dependent clustering plays an important role on small scales. Using the simulations to assess and correct for projection effects, we infer that the real-space N_c(M_r) for close pairs peaks at about M^* and declines by at least a factor of 2 as M_r becomes fainter. Conversely, by measuring the number density of close companions, we estimate that at least 90% of all major mergers occur between galaxies which are fainter than L^*. Finally, measurements of the luminosity density of close companions indicate that L^* galaxies likely dominate in terms of the overall importance of major mergers in the evolution of galaxy populations at low redshift.